- UNIT 1: PROPERTIES AND USES OF TRANSITION
Key unit competence:
The learner should be able to explain the properties and uses of transition metals.
At the end of this unit , students will be able to:
• Discuss qualitatively the propertie of transition elements;
• Explain the principle of ligan exchange;
• State the rules of naming complex ions and stereoisomers;
• Describe reactions of transition metals;
• State the use of transition metals;
• Relate the electronic configurations to special properties of transition metals;
• Predict the shape of the complex compounds of transition metal cations;
• Perform the confirmatory tests for transition metal ions.Introductory Activity
The following photos show how some elements play a big role in our daily lives.
Observe these objects carefully.
Most of the metals in the periodic table belong in the d-block of transition metals.
They are hard and strong, and many of them are very familiar to us. For instance, zinc
is in brass instruments like trumpets and tubas. Have you ever heard of the element
“scandium” before? But you’ve interacted with it if you have ever ridden a bicycle.1.1. Definition and electronic configuration of transition metals.
1. Write the electronic configuration of the following atoms and ions:
a. Ca(Z=20) b. Ca 2+ c. Na(Z=11) d. Na+
2. Referring to the portion of periodic table in this book,
a. Write the electronic configuration of the elements from Sc to Zn.
b. Point out any difference between the electronic configuration of the
above elements and that of other elements in s and p blocks
3. Define the term transition metal.According to IUPAC system, a transition metal is “an element whose atom has a
partially filled d sub-shell, or which can give rise to cations with an incomplete
Transition metals are located between groups 1& 2 (s-block) and group 13 (p-block)
on the periodic table. The elements are also called d-block elements because their
valence electrons are in d-orbitals.
The properties of transition elements are between the highly reactive metallic
elements of the s-block which generally form ionic compounds and the less reactive
elements of the p-block which form covalent compounds. Transition metals form
ionic compounds as well as covalent compounds.
The first 3 rows, i.e. period 4, period 5 and period 6, are called first transition series,
second transition series and third transition series respectively. The metals of the
first series are all hard and dense, good conductors of heat and electricity.
This block is known as the transition metals because some of their properties show a
gradual change between the active metals in s-block and p-block where non-metals
Electronic configuration is the arrangement of electrons in orbitals around the
nucleus. The electronic structure of the first transition series is shown in the table
When building electronic structure of transition metals, 4s orbital is filled before 3d
The transition elements are stable when their d-orbitals are filled (d10) or when their
d-orbitals are half filled (d5).This explains the electronic structure of copper, [Ar]
4s13d10 instead of
[Ar] 4s23d9. The same applies for Cr: [Ar] 4s13d5 and not [Ar] 4s23d4.
In order to attain that stability an electron can jump from 4s orbital to 3d orbital
because those two orbitals are close in energy.
This also explains why Fe2+ with 3d6 is easily oxidized to Fe3+ with 3d5 and Mn2+ with
3d5 is resistant to oxidation to Mn3+ with 3d4.
Transition metals form ions by losing electrons first from the 4s sub-shell rather than
the 3d sub-shell. Hence electronic configuration of Fe, Fe2+ and Fe3+ are the following:
are inner while the 4s electrons are outer therefore the outer electrons (4s) have to
be removed before the inner electrons.
Checking up 1.1
1. Explain the difference between the electronic configuration of transition
elements and that of main group elements.
2. 2.Why d-block metals are so called transition metals?
1.2. Properties of the transition metals
1.2.1. Melting and boiling points
Activity 1.2 (a)
Experiment: Investigation of the melting point of transition metals compared
with s-block elements
Materials: Potassium or Rubidium metal and copper or iron metal, pair of tongs,
spatulas, bunsen burner and match box.
1. Take a half filled spatula of
a. Potassium (K) or Na, Rb, or Cs
b. Iron turnings or very small piece of copper sheet (which can fit on a
2. Heat both spatulas on the Bunsen burner flame
3. Write down the observations
4. What can you conclude about your findings?
The melting points and the molar enthalpies of fusion of the transition metals are
both high in comparison to main group elements. Most of the transition metals have
melting points above 1000oC; mercury is liquid at room temperature.
This is due to the high number of valence electrons that increases the electrostatic
attraction force between those electrons and the metallic cations, hence increasing
the strength of the metallic bond and the melting point.
Table 1.1: Melting and boiling points of the 1st series of Transition Metals
Checking up 1.2 (a)
Compare and comment on the melting points of transition metals and those of
s-block metals.1.2.2. Densities and atomic/metallic radii
Activity 1.2 (b)
Procedure for practical:
1. a. Take a magnesium ribbon and a copper foil of the same size (if
possible you may use their turnings)
b. You weigh those two samples using an electronic balance. And record
2. a. Take aluminum foil and copper foil of the same size (if possible you
may use their turnings)
b. You weigh those two samples using an electronic balance. Record their
3. Comment on your observations by explaining why their masses are
different and yet they have the same size.
4. Use the internet or any book or even this one to interpret the data
given about metallic radii of the first series transition metals. From your
research, compare metallic radii of transition metals and those of main
group elements.The transition elements are much denser than the s-block elements and show in
general a gradual increase in density from left to right in a period as you can see
below from scandium to copper. This trend in density can be explained by a decrease
in metallic radii coupled with the relative increase in atomic mass
Table 1.2: Density/g cm -3 of the first transition series
Table1.3: Metallic radii of the first transition series
Checking up 1.2 (b)
The metallic radius of vanadium is smaller than that of titanium. Explain this
statement.1.2.3. Ionization energies
Activity 1.2 (c)
Use this book or any other source (textbook or search engine) to interpret/ analyze
the summary about ionization energies of the transition metals (first series). From
your findings, compare
a. Ionization energies of those transition metals.
b. Ionization energies of transition metals and those of main group elements
The ionization energy of transition metals is related to the energies of its d orbitals,
its ease of oxidation, and its basicity.In simplest terms, the greater the ionization
energy of a metal, the harder it is to pull an electron from it.
As the number of protons increases across a period (or row) from left to right of
the periodic table, the first ionization energies of the transition-metal elements are
relatively the same, while that for the main-group elements increases.
In moving across the series of metals from scandium to zinc, a small change in the
values of the first and second ionization energies is observed. This is due to the buildup
of electrons in the immediately underlying d-sub-shells that efficiently shields
the 4s electrons from the nucleus and minimizing the increase in effective nuclear
charge from element to element.Table 1.4: First, second and third ionization energies of 1st Series Transition
metals /kJ mol-1
The figure 1.3 below shows the first iazonisation energies for transition metals of 1st,
2nd and 3rd rows (series).
In general, ionization energy increases as we move from left to right across the
period. Notable dips occur at row 1, group 10 (Ni) and row 3, group 7 (Re).
Checking up 1.2 (c)
Briefly explain the following observations:
a. The first ionization energy of cobalt is only slightly larger than the first
ionization energy of iron.
b. The third ionization energy of iron is much lower than the 3rd ionization
energy of Mn.1.2.4. Transition elements have variable oxidation states
Activity 1.2 (d)
Use this book or any other source (textbook or search engine) to
a. Explain the term oxidation number
b. Compare the oxidation numbers of transition metals (first series) and those
of main group elements.
c. Analyze the stability of ions formed by transition metals (first series).
Oxidation state is a number assigned to an element in chemical combination which
represents the number of electrons lost or gained.The transition elements from
Titanium to Copper all form ions with two or more oxidation states. In most cases,
this is the result of losing the two electrons of 4s orbital and electeons in 3d orbitals.
The 4s electrons are lost first because they are in the highest energy level. However,
because the 3d and 4s energy level are so close in energy, the 3d electrons can also
be lost when an atom forms a stable ion. The common oxidation states shown by the
first transition series are:
Table 1.5: The oxidation states shown by the transition metals (series)
• The common stable oxidation states for those transition metals with variable
oxidation states are bolded and underlined.
• The oxidation state corresponding to a full or half-filled d-orbital is energetically
stable. For example, Fe3+ is more stable than Fe2+and Mn2+ is more stable than
• However, in most compounds and solutions, copper exist as Cu2+ ion rather
than Cu+ ion. Meaning that the former is more stable than the latter. The
explanation of this is beyond this level.Checking up 1.2 (d)
Which gaseous ion is more stable, Mn2+ or Mn3+? Explain why.1.2.5. Most transition metals and their compounds have high ability of being
Activity 1.2 (e)
1. Preparation of oxygen using hydrogen peroxide, H2O2, without a catalyst
a. Put 10 mL of H2O2 in a conical flask (Pyrex preferably)
b. Heat the conical flask for about 5 min
c. Write down the observation in A.
2. Preparation of oxygen using hydrogen peroxide, H2O2, with MnO2 as a
a. Put10 mL of H2O2 in a conical flask(Pyrex preferably)
b. Put a very small amount of MnO2 in the conical flask
c. Heat the conical flask for about 5 min
d. Write down the observation in (B)
Question: What is the role of MnO2 in the above experiment?A catalyst is a substance that can speed up (positive catalyst) or that can slow down
(negative catalyst) the rate of reaction and is found unchanged at the end of the
reaction. But generally the term catalyst is used for the substance that helps in
accelerating the rate of the reaction. A catalyst that speeds up the reaction provides
another pathway with lower activation energy.
In some catalytic process, transition metal ions undergo changes in their oxidation
states but are regenerated at the end of the reaction.
The reasons for transition metals to work as catalysts:
• Presence of empty d orbitals which enable transition metal ions (or atoms) to
form temporary bonds with reactant molecules at the surface of a catalyst and
weakens the bond in the reactant molecules
• Variable oxidation states which allow them to work as catalysts in the reactions
involving the transfer of electrons.
Table 1.6: Reactions catalysed by transition metals
Checking up 1.2 (e)
Explain why s-block metals and their compounds are not used as catalysts1.2.6. Most transition metal ions are paramagnetic
Given the following materials:
1. Organize yourself in to group to find the objects shown in the photo
2. Using a magnet, classify the above materials into two groups as shown in
the table below.
Objects attracted by a magnet
Objects not attracted by a magnet
3. Research, using any relevant source (textbook or internet), to identify in
which metal the objects A to E are made
4. Research to know why some objects are attracted by a magnet while
others are not
Paramagnetism is a property of substances to be attracted in a magnetic field.
Substances which are not attracted (i.e slightly repelled) in a magnetic field are
said to be diamagnetic.Transition metal ions show paramagnetism because of the
presence of unpaired electrons in their 3d arbitrals.
The greater the number of unpaired electrons, the stronger the paramagnetism;
that is the reason why:
• Fe3+ is more paramagnetic than Fe2+ because Fe3+ has five unpaired electrons
while Fe2+ has four unpaired.
• Sc3+ and Zn2+ are not paramagnetic, they are diamagnetic because they do not
have unpaired electrons.
Other examples of paramagnetic substances are: Cr, Mn, CuSO4, Fe, Co, Ni, Pt.
Examples of diamagnetic substances are: Zn, Cu+, Au+, TiO2.
Checking up 1.2 (f)
Predict whether the following substances are paramagnetic or not. Explain
d. Cr1.2.7. Formation of alloys
Activity 1.2 (g)
Observe the trophies/or other objects made in the materials below and compare
their appearances with the elements from which they are derived.
a. Bronze with copper
b. Stainless steel with iron
c. Pewter and copper
You can use the internet, books (including this one) or any other relevant source
to find the figures of the above objects, the elements they are made from and
their uses.An alloy is a homogenious solid mixture (solid solution) made by combining two or
more elements where at least one is a metal.
Importance of alloying:
• Increase of the strength of a metal,
• Resistance to corrosion,
• Gives to the metal a good appearance
Generally, alloys are needed and used to improve the quality of the required
material. For example, brass (alloy of zinc and copper) is much stronger than either
pure copper or pure zinc. Pure gold is too soft to be used in some applications.Table 1.7: The properties and uses of some common alloys formed by transition
metals (first series)
Checking up 1.2 (g)
1. Explain why alloys are said to be solid solutions.
2. Give the importance of alloying1.2.8. Formation of complex ions
Use this book or any other source (library textbook or internet) to analyze and
discuss on the following. You have to take note on what to be presented to share
with your colleagues and teacher.
a. What is a ligand?
b. State the types of ligands
c. The geometry of complexes
A complex or coordination compound is a chemical species made of a central metal
(cation or neutral) bonded to other chemical species called ligands by coordination
or dative bonds. A complex may be neutral, positively or negatively charged.
Transition metal form complexes because of:
• Their small and highly charged ions,
• The presence of vacant (empty) d-orbitals which can accommodate lone pair
of electrons donated by other groups (ligands)The general formula of a complex is: [MLn]y
• M-metal ion or atom
• n-the number of ligands surrounding the metal
• y-the charge of the complex; [MLn] indicates a neutral complex.
-Coordination number of a complex: is the number of coordinate bonds on the
central metal in a complex.-Ligand: It is a species (anion or a molecule) that is bonded to a central metal ion
or atom in a complex. A ligand should have at least one lone pair of electrons
to form a coordinate bond.
Ligands are classified depending on the number of sites at which one molecule
of a ligand is coordinated to the central metallic atom; the ligands are classified
as monodentate (or unidentate), ambidentate and polydentate (or multidentate)
a. Monodentate ligands
The ligands which have only one donor atom or are coordinated through one
electron pair are called monodentate ligands because they have only one tooth with
which to attach themselves to the central cation or atom. Such ligands are coordinated
to the central metal at one site or by one metal-ligand bond only. These ligands may
be neutral molecules or in anionic form.The table below provides examples of some monodentate ligands.
Table 1.8: some monodentate ligands
Ligands that can use different sites to coordinate to the central metal are called
“ambidentate”: e.g. CN- and NC-(see table above).
Notice that a ligand with a donor atom that possesses 2 lone pairs of electrons, such
as H2Ö:, is not bidentate, since it cannot use both lone pairs simultaneously to bind
to the metal because of the steric effect.
b. Polydentate ligands
These may be bidentate, tridentate, tetradentate, pentadentate, and hexadentate
ligands if the number of donor atoms present in one molecule of the ligand attached
with the central metallic atom is 2, 3, 4, 5, and 6 respectively. Thus one molecule of
these ligands is coordinated to the central metallic atom at 2, 3, 4, 5, and 6 sitesrespectively. In other words, we can say that one molecule of these ligands makes 2,
3, 4, 5, and 6 metal-ligand coordinate bonds respectively.
The structure shows that it has two neutral N- atoms and four negatively charged Oatoms
as its donor atoms which can form coordinate bonds with a transition metal
as chelates or chelated complexes.
In general, polydentate ligands form more stable complexes than monodentate
ligands. The stability of complex is much enhanced by chelation. A polydentate
ligand can hold the central cation more strongly.Examples of complexes:
• Copper (II) ions have a coordination number of four in most of its complexes:
[Cu(H2O)4]2+, [Cu(NH3)4]2+, [CuCl4]2+, [Cu(NH2-(CH2)2-NH2)2]2+, …
• Most of ions have coordination number of 6.
[Cr(H2O)6]3+ , [Cr(NH3)6]3+ , [Cr(H2O)4Cl2]- , …
• Very few ions have a coordination number of 2: [Ag(NH3)2]+, [Ag(CN)2]-, [CuCl2]-,
Geometry of complexes
Complexes have a variety of geometries or shapes, but the most common geometries
are the following:
• Complexes with coordination number 2 adopt a linear shape. Example:
The complexes having coordination number of 2 are linear since minimises ligand
• Complexes with coordination number 4 generall adopt a tetrahedral shape.
But few of them can form a square planar shape.Examples:
[Zn(NH3)4]2+, [NiCl4]2- and some few others adopt a square planar shapes, examples:
[Cu(NH3)4]2+ , [Ni(CN)4]2-,[CuCl4]2-,[CoCl4]2-,…
The square plannar geometry is characteristic of transition metal ions with eight d
electrons in the valence shell, such as platinum(II)and gold(III).
Copper (II) and cobalt (II) ions have four chloride ions bonded to them rather than
six, because the chloride ions are too big to fit any more around the central metal
• Complexes with coordination number 6 adopt an octahedral shape.
These ions have four of the ligands in one plane, with the fifth one above the plane,
and the sixth one below the plane.
Checking up 1.2
1. What do you understand by :
a. Coordination number.
2. Give the main types of ligands and give an example for each
3. Say if the following statement is correct or wrong and justify: The
coordination number equals the number of ligands bonded to the central
1.2.9. Many transition metal ions and their compounds are coloured
Activity 1.2 (i)
Experiment 1: Observation of the colors of transition elements
Apparatus: Test tubes, droppers, spatula, test tube holders.
Chemicals: NaCl, CaCl2, FeSO4, Fe2 (SO4)3, KMnO4, K2Cr2O7 ,distilled water, Cr2(SO4)3.
1. What are the colours of the compounds above?
2. Determine the oxidation states of each metal in the above compounds?
3. a. Take an endful spatula of each product given above and put each in a test
b. Put 10 mL of distilled water in each test tube.
c. Write down the colours of solutions formed and conclude.
Experiment 2: Investigation of ligand exchange reactions involving copper (II) ions, Cu2+
Apparatus: Test tubes, droppers, spatula, test tube holders.
Chemicals: Copper (II) sulphate, concentrated hydrochloric acid, concentrated ammonia
solution and distilled water.
1. Use a spatula to place a small amount of anhydrous copper (II) sulphate in a test
2. Add 10 drops of distilled water to the anhydrous copper (II) sulphate and shake
3. To the test tube in step 2, add concentrated ammonia solution drop by
drop while shaking the test tube until there is no further change. Record all
4. Repeat steps 1 and 2
5. To the test tube from step 4, add concentrated hydrochloric acid drop by drop
while shaking until there is no further change. Record all observations.
Points for discussion:
1. What happens when anhydrous copper (II) sulphate is dissolved in water?
2. Describe what is observed when concentrated ammonia is added dropwise to
an aqueous solution of copper (II) sulphate.Write balanced equations for each
observation if possible
3. Describe what happens when concentrated hydrochloric acid is added to an
aqueous solution of copper (II) sulphate. Write balanced equation(s) for the
4. State any other possible observation(s) for this experiment.
The formation of colored ions by transition elements is associated with the presence
of incompletely filled 3d orbitals.
This property has its origin in the excitation of d electrons from lower energy
d-orbitals to higher energy. In fact, when the central metal is surrounded by ligands,
these cause d orbitals to be split into groups of higher and lower energy orbitals.
When electrons fill d-orbitals, they fill first of all the lower energy orbitals; if there is
free space in higher energy d-orbitals, an electron can be excited from lower energy
d-orbitals to higher energy d-orbitals by absorbing a portion of light corresponding
to a given colour, the remaining color light is the white light minus the absorbed
When a coloured object is hit by white light, the object absorbs some colour and
the colour transmitted or reflected by the object is the colour which has not been
absorbed. The observed colour is called complementary colour.
When a metal cation has full d-orbitals, such as Cu+or Zn2+or no electron in d orbital,
such as Sc3+.
Table1.9: Complementarities of colors observed and absorbed when light is
The colour of a particular transition metal ion depends upon two factors:
• The nature of the ligand
The principle of ligand exchange
Complexing reactions involve competitions between different ligands for central
metal. A more powerful ligand displaces a less powerful ligand from a complex.
During the process there is a change in colour.
Here below is a list of some ligands in increasing order of strength.
The above series are called the spectrochemical series and shows that cynide ion
and carbon monoxide are very strong ligands
The stability of a complex ion is measured by its stability constant. The higher the
stability constant of a complex, the more stable is the complex.Checking up 1.2 (i)
Predict whether each of the following ion forms coloured compounds and explain
your reasoning: Fe2+, Mn7+, K+
1.3. The anomalous properties of Zinc and Scandiu
From the information you have learnt about the properties of transition metals,
Suggest the difference between the properties of Zn and Sc and other transition
metals. You can consult different sources (books or internet) to provide enough
On the basis of the properties of transition metals, zinc and scandium are not
considered as typical transition metals even though they are members of the d-block.
• It has a complete d-orbital.
• Zinc forms only the colourless Zn2+ ion, isoelectronic with the Ga3+ion, with 10
electrons in the 3d subshell.
• Zinc and its compounds are not paramagnetic
• Has one oxidation state,+3
• Sc forms only the colourless Sc3+ion, isoelectronic with the Ca2+ ion, with no
electrons in the 3d subshell.
• Its compounds are diamagnetic
• It forms compounds containing ions with a completely empty 3d subshell.
Checking up 1.3
Give any one property by which Zn differs from Sc
1.4. Naming of complex ions and isomerism in of transition
1.4.1. Naming of complex ions
Activity 1.4 (a):
1. Name the following molecules and explain the basis /principle used to
2. Analyze the IUPAC rules for naming complex ions in the summary in this
book or using any other source (textbook or search engine) and apply
them by naming the following:
Naming molecules requires the knowledge of certain rules, such as how to name
cations, anions, where to start from when both a cation and an anion are combined
in an ionic molecule or when two non metals are combined in a covalent molecule.
Like other compounds, complex compounds/ions are named by following a set of
rules. You are familiar with some of them and the new ones can be understood and
1. In simple metal compounds, the metal is named first then the anion.
Example: CaCl2: calcium chloride
2. In naming the complex:
a. Name the ligands first, in alphabetical order, then the metal atom or cation,
followed by its oxidation state written between brackets as Roman number,
though the metal atom or cation is written before the ligands in the chemical
Example: [CuBr4]2-: Tetrabromocuprate (II) ion
The names of some common ligands are listed in the table below:
Table 1.10: Names of common ligands
b. Greek prefixes are used to indicate the number of each type of ligand in the
The numerical greek prefixes are listed in the following table:
Table 1.11: Greek numerical prefixes
c. After naming the ligands, name the central metal.
• If the complex bears a positive charge (cationic complex), the metal is named
by its usual name.
Example: Cu: Copper Pt: Platinum
If the complex bears a negative charge (anionic complex), the name of the metal
ends with the suffix –ate
Example: Co in a complex anion is called cobaltate and Pt is called platinate.
For some metals, the Latin names are used in the complex anions e.g. Fe is called
ferrate (not ironate). See table below:
Table 1.12: Latin names of some transition metals in anionic complexes
1. For historic reasons, some coordination compounds are called by their common
Example: Fe(CN)63- and Fe(CN)64- are named ferricyanide and ferrocyanide
respectively, and Fe(CO)5 is called iron carbonyl.
2. To name a neutral complex molecule, follow the rules of naming a complex
cation. Example: [Cr(NH3)3Cl3]: triamminetrichlorochromium (III)
You can have a compound where both the cation and the anion are complex ions.
Notice how the name of the metal differs even though they are the same metal ions.
Remember: Name the cation before the anion.
Example: [Ag(NH3)2][Ag(CN)2] is diamminesilver(I)dicyanoargentate(I)
• The names are written as a one word: Tetraamminecopper (II), not Tetraammine
• Complex ions formula is written between square brackets and the charge of the
ion as superscript outside the brackets: [Cu(NH3)4]2+. When oppositely charged
ions approach the complex ion, a neutral molecule can be obtained:
[Cu(NH3)4]2+2Cl- or simply, [Cu(NH3)4]Cl2: tetraamminecopper(II)chloride.
The ions outside the square brackets are known as “counter ions”.
Checking Up 1.4 (a):
1. Complete the table below using the names of the given metals when they
are in anionic complexes
Name in an anionic complex
2. Give the systematic names for the following complex ions/compounds:
d. Fe(CO)51.4.2. Isomerism in complexes
Activity 1.4 (b):
1. Discuss on the following questions:
a. What do you understand by the term “isomerism”?
b. Is there any relationship between isomers and isomerism?
c. Give examples of molecules that can exist as isomers and explain their
2. Read and discuss the summary below to understand how complex ions/
compounds exhibit isomers
3. Present your findings to your colleagues and teacher to share your
Isomers are chemical species that have the same molecular formulal, but different
molecular structures or different arrangements of atoms or groups of atoms in
space. Isomerism among transition metal complexes arises as a result of different
arrangements of their constituent ligands around the metal.
The diagram below shows the different categories of isomerism in transition metal
In this unit, we are specifically concerned with ‘stereoisomerism’ which gives rise to
isomers known as “stereoisomers”. Stereoisomers have the same structural formulal
but different arrangements of ligands in space.
They are classified in two categories: geometrical isomers and optical isomers.
1. Geometrical isomers
Coordination complexes, with two different ligands in the cis and trans positions
from a ligand of interest, form isomers.
For example, the square planar, diammine dichloroplatinum (II) Pt(NH3)2Cl2),can be
presented as follows:
The octahedral [Co(NH3)4Cl2]+ ion can also have geometrical isomers.
Different geometrical isomers are different chemical compounds. They exhibit
different properties, even though they have the same formula. For example, the two
isomers of [Co(NH3)4Cl2]NO3 differ in color; the cis form is violet, and the trans form is
green. Furthermore, these isomers have different dipole moments, solubilities and
2. Optical isomers (enantiomers)
Optical isomers are non-superimposable mirror images of each other. A classic
example of this is your two hands (left and right); hold them face-to-face: one is the
mirror image of the other. Now try to superimpose them one over another: they
are non-superimposable (only the middle fingers superimpose one over the other.
Chemical compounds that behave like the hands are called “chiral”, in reference to
the Greek word for hands.
Optical isomers are very important in organic and biochemistry because living
systems often incorporate one specific optical isomer and not the other.
Unlike geometric isomers, optical isomers have identical physical properties (boiling
point, polarity, solubility, etc.). Optical isomers differ only in the way they affect
polarized light and how they react with other optical isomers.
1. For coordination complexes, many coordination compounds such as
[M(en)3]n+ [in which Mn+ is a central metal ion such as iron(III) or cobalt(II)]
form enantiomers, as shown in figure below.These two isomers will react
differently with other optical isomers. For example, DNA helices are optical
isomers, and the form that occurs in nature (right-handed DNA) will bind
to only one isomer of [M(en)3]n+ and not the other.
Checking up 1.4 (b):
1. The geometric isomer of [Pt(NH3)2Cl2] is shown in the figure below. Draw
the other geometric isomer and give its full name.
3. Sketch the arrangement of bonds in the complexes
a. Hexaaquacobalt(III) ion
b. Hexacyanoferrate (III) ion
c. Diamminesilver (I) ion
d. The complex compound tetracarbonylnickel (0).
4. The compound [NiCl2(NH3)2] has cis-trans isomers. These have a complex
a. Does [NiCl2(NH3)2] have a tetrahedral or a square-planar structure?
Explain your answer.
b. Draw the cis and trans isomers for [NiCl2(NH3)2].
5. Early in the 20th century, the German scientist Werner succeeded in
clarifying the situation concerning the five compounds of PtCl4- and
ammonia. The properties of these compounds are listed in the table
a. What is the oxidation state of Pt in each of the compounds A-E?
b. The co-ordination number of Pt in each compound is six. Write a right formula for each
of the five compounds. Show the complex ion and the other ions and/or molecules
c. Each of the compounds forms an octahedral complex ion. Draw the structures for the
complex ions in A, B, C, and D.
d. Which of the complex ions in (c) have isomers?1.5. The Chemistry of individual transition metals
Using the library and internet or other textbooks, make your own research and make
presentation of the results of your research:
1. On how each of the transition metals (first series) reacts with each of the
c. Hydrochloric acid
d. Sodium hydroxide
2. On the uses and their corresponding properties for each of the above
transition metals.1.5.1. Scandium
Scandium is a silvery-white solid. It melts at 1539oC and boils at 2748oC. Its density
is about 3.0.
1. Chemical reactions
a. Reaction of scandium with air
Scandium tarnishes in air, and burns readily, forming scandium (III) oxide, Sc2O3.
4 Sc(s) + 3 O2(g) ——→ 2 Sc2O3(s)
b. Reaction of scandium with water
When finely divided, or heated, scandium dissolves in water, forming Sc (III)
hydroxide and hydrogen gas, H2.
2 Sc(s) + 6 H2O(l) ——→2 Sc(OH)3(aq) + 3 H2(g)
c. Reaction of scandium with acids
Scandium dissolves readily in dilute hydrochloric acid, forming Sc(III) ions and
hydrogen gas, H2.
2 Sc(s) + 6 HCl(aq) ——→2 Sc3+(aq) + 6 Cl−(aq) + 3 H2(g)
d. Reaction of scandium with halogens
Scandium reacts with the halogens, forming the corresponding Sc(III) halides
2 Sc(s) + 3 F2(g)——→ 2 ScF3(s)
2 Sc(s) + 3 Cl2(g) ——→2 ScCl3(s)
2 Sc(s) + 3 Br2(g)——→ 2 ScBr3(s)
2 Sc(s) + 3 I2(g)——→ 2 ScI3(s)
• Scandium has as low density (2.99 g/cm3) asaluminium (2.7 g/cm3) but a much
higher melting point.
• An aluminium-scandium alloy has been used in fighter planes, high-end
bicycle frames and baseball bats.
• Scandium iodide is added to mercury vapour lamps to produce a highly
efficient light source resembling sunlight. These lamps help TV cameras to
reproduce colour well when filming indoors or at night-time.
Titanium is a gray, solid with a density of about 4.50. It melts at 1667oC and boils at
3285oC.1. Chemical reactions
a. Reaction of titanium with air
Titanium does not react with air under normal conditions. If brought to burn,
titanium will react with both oxygen, O2, and nitrogen, N2.
Titanium does not react with water, under normal conditions. If the water is heated
to steam, it will react with titanium, forming titanium(IV) oxide, TiO2, and hydrogen,
c. Reaction of titanium with acids
Titanium does not react with most acids, under normal conditions. It will react with
hot hydrochloric acid, and it reacts with HF, forming Ti(III) complexes and hydrogen
d. Reaction of titanium with bases
Titanium does not appear to react with alkalis, under normal conditions, even when
e. Reaction of titanium with halogens
Titanium reacts with halogens, when heated, forming the corresponding titanium(IV)
• Titanium is as strong as steel but much less dense. It is therefore important as
an alloying agent with many metals including aluminium, molybdenum and
iron. These alloys are mainly used in aircraft, spacecraft and missiles because
of their low density and ability to withstand extremes of temperature. They are
also used in golf clubs, laptops, bicycles and crutches.
• Power plant condensers use titanium pipes because of their resistance to
corrosion. Because titanium has excellent resistance to corrosion in seawater,
it is used in desalination plants and to protect the hulls of ships, submarines
and other structures exposed to seawater.
• Titanium metal connects well with bone, so it has found surgical applications
such as in joint replacements (especially hip joints) and tooth implants.
• The largest use of titanium is in the form of titanium (IV) oxide. It is extensively
used as a pigment in house paint, artists’ paint, plastics, enamels and paper.
It is a bright white pigment with excellent covering power. It is also a good
reflector of infrared radiation and so is used in solar observatories where heat
causes poor visibility.
Vanadium is a grey, solid with a density of about 6.11. It melts at 1915oC and boils at
3350oC. It is insoluble in water at room temperature.
1. Chemical reactions
a. Reaction of vanadium with air
Vanadium metal reacts with excess oxygen, O2, upon heating to form vanadium (V)
oxide, V2O5. When prepared in this way, V2O5 is sometimes contaminated by other
b. Reaction of vanadium with water
Vanadium does not react with water, under normal conditions.
c. Reaction of vanadium with bases
Vanadium metal is resistant to attack by molten alkali.
In strong alkaline solutions (pH > 13), Vanadium (V) exists as colourless
orthovanadate ions, VO43−.
d. Reaction of vanadium with halogens
Vanadium reacts with fluorine, F2, when heated, forming vanadium (V) fluoride
• About 80% of the vanadium produced is used as a steel additive. Vanadium-
steel alloys are very tough and are used for spanners, armour plate, axles,
piston rods and crankshafts. Less than 1% of vanadium, and as little chromium,
makes steel shock resistant and vibration resistant. Vanadium alloys are used
in nuclear reactors because of vanadium’s low neutron-absorbing properties.
• Vanadium (V) oxide is used as a pigment for ceramics and glass, as a catalyst
and in producing superconducting magnets.
Chromium is a silver gray metal with density of about 7.14. It melts at 1900oC and
boils at 2690oC. Chromium is insoluble in water at room temperature.
1. Chemical reactions
a. Reaction of chromium with air
Chromium metal does not react with air at room temperature. Heated clean
chromium is oxidized superficially in air to green solid, chromium (II) oxide.
b. Reaction of chromium with water
Normally, Chromium metal does not react with water at room temperature. When
red hot, it reacts with steam to form chromium (II) oxide.
c. Reaction of chromium with acids
Metallic chromium dissolves in dilute hydrochloric acid forming Cr(II) and hydrogen
gas, H2. In aqueous solution, Cr(II) is present as the complex ion [Cr(OH2)6]2+.
Similar results are seen for sulphuric acid but pure samples of chromium may be
resistant to attack.
Chromium metal is not dissolved by nitric acid, HNO3 but is passivated instead.
d. Reaction of chromium with hydroxide ions
Chromium dissolves rapidly in hot concentrated aqueous alkali forming a blue
solution containing chromium (II) ion and hydrogen gas is evolved.
Similar results are seen for sulphuric acid but pure samples of chromium may be
resistant to attack.
Chromium metal is not dissolved by nitric acid, HNO3 but is passivated instead.
d. Reaction of chromium with hydroxide ions
Chromium dissolves rapidly in hot concentrated aqueous alkali forming a blue
solution containing chromium (II) ion and hydrogen gas is evolved.
e. Reaction of chromium with halogens
Chromium reacts directly with fluorine, F2, at 400°C and 200-300 atmospheres to
form chromium (VI) fluoride, CrF6.
Under milder conditions, chromium (V) fluoride, CrF5, is formed.
Under milder conditions, chromium metal reacts with the halogens to form
chromium tri halides or chromium (III) halides:
• Chromium is used to harden steel, to manufacture stainless steel (resists to
corrosion) and to produce several alloys.
• Chromium plating can be used to give a polished mirror finish to steel.
Chromium-plated car and lorry parts, such as bumpers, were once very
common. It is also possible to chromium plate plastics, which are often used
in bathroom fittings.
• About 90% of all leather is tanned using chromium. However, the waste
effluent is toxic so alternatives are being investigated.
• Chromium compounds are used as industrial catalysts and pigments (in
bright green, yellow, red and orange colours). Rubies get their red colour from
chromium, and glass treated with chromium has an emerald green colour.
• Chromium (IV) oxide is used in magnetic tapes for sound/video recording.
• Chromium is used in the control of cholesterol and help insulin sugar control
Manganese is a grey-white solid with a slightly red colour. Its density is about
7.44oC. Manganese melts at 1244oC and boils at 2060oC. It is insoluble in water but
soluble in diluted acids, at room temperature.
1. Chemical reactions
a. Reaction of manganese with air
Manganese is not very reactive with air. The surface of manganese lumps oxidizes
a little. Finely divided manganese metal burns in air. In oxygen the oxide Mn3O4 is
formed and in nitrogen the nitride Mn3N2 is formed.
b. Reaction of manganese with water
Manganese reacts slowly with water to form manganese (IV) oxide:
c. Reaction of manganese with acids
Manganese dissolves readily in dilute sulphuric acid, forming a colorless solution of
Mn(II) ions and hydrogen gas, H2.
d. Reaction of manganese with halogens
Manganese reacts with the halogens, forming the corresponding manganese (II)
halides. For fluoride, manganese (III) fluoride is also formed.
• Manganese is too brittle to be of much use as a pure metal. It is mainly used
in alloys, such as steel. Steel contains about 1% manganese, to increase the
strength and also improve workability and resistance to wear. Manganese
steel contains about 13% manganese. This is extremely strong and is used for
railway tracks, safes, rifle barrels and prison bars.
• Drinks cans are made of an alloy of aluminium with 1.5% manganese, to improve
resistance to corrosion. With aluminium, antimony and copper it forms
highly magnetic alloys.
• Manganese (IV) oxide is used as a catalyst, a rubber additive and to decolourise
glass that is coloured green by iron impurities. Manganese (IV) oxide is a powerful
oxidising agent and is used in quantitative analysis. It is also used to make
Fertilizers and ceramics.
• Manganese sulphate is used to make a fungicide.
Iron is a grey to black, odourless metal with density 7.874. It melts at 1535 oC and
boils at 2750 oC.1. Chemical reactions
a. Reaction of iron with air
Iron reacts with oxygen, O2, forming Fe (II) and Fe(III) oxides. The oxide layer does not
passivate the surface. Finely divided iron, e.g. powder or iron wool, can burn:
b. Reaction of iron with water
Air-free water has little effect upon iron metal. However, iron metal reacts in moist
air by oxidation to give a hydrated iron oxide. This does not protect the iron surface
to further reaction since it flakes off, exposing more iron metal to oxidation. This
process is called rusting.
c. Reaction of iron with acids
Iron metal dissolves readily in dilute sulphuric acid in the absence of oxygen forming
Fe(II) ions and H2. In aqueous solution Fe(II) is present as the complex [Fe(H2O)6]2+.
Concentrated nitric acid, HNO3, reacts on the surface of iron and passivates the
surface (makes it unreactive).d. Reaction of iron with halogens
Iron reacts with excess of the halogens, F2, Cl2, and Br2, to form Fe(III) halides.
• Iron is an enigma – it rusts easily, yet it is the most important of all metals. 90%
of all metal that is refined today is iron. Most is used to manufacture steel, used
in civil engineering (reinforced concrete, girders etc) and in manufacturing.
• Alloy steels are carbon steels with other additives such as nickel, chromium,
vanadium, tungsten and manganese. These are stronger and tougher than
carbon steels and have a huge variety of applications including bridges, electricity
pylons, bicycle chains, cutting tools and rifle barrels.
• Stainless steel is very resistant to corrosion. It contains at least 10.5% chromi
um. Other metals such as nickel, molybdenum, titanium and copper are added
to enhance its strength and workability. It is used in architecture, bearings,
cutlery, surgical instruments and jewellery.
• Cast iron contains 3–5% carbon. It is used for pipes, valves and pumps. It is not
as tough as steel but it is cheaper.
• Magnets can be made of iron and its alloys and compounds.
• Iron catalysts are used in the Haber process for producing ammonia, and in the
Fischer–Tropsch process for converting syngas (hydrogen and carbon monoxide)
into liquid fuels.
• Iron plays an important role in the transfer of oxygen by hemoglobin. Each
hemoglobin binds four iron atoms. Iron in hemoglobin binds with oxygen as
it passes through the blood vessels in the lungs and releases it in the tissues.
Cobalt is a dark grey metal with a density of 8.90. It is insoluble in water at room
1. Chemical reactions
a. Reaction of cobalt with air
Cobalt does not react readily with air. Upon heating the oxide Co3O4 is formed, and if
the reaction is carried out above 900°C, the result is cobalt (II) oxide, CoO.
Cobalt does not react directly with nitrogen, N2.
b. Reaction of cobalt with water
Cold water has little effect upon cobalt metal. The reaction between red hot cobalt
metal and steam produces cobalt (II) oxide, CoO.
c. Reaction of cobalt with acids
Cobalt metal dissolves slowly in dilute sulphuric acid to form solutions containing
the hydrated Co(II) ion together with hydrogen gas, H2. The actual occurrence of Co
(II) in aqueous solution is as the complex ion [Co(OH2)6]2+.
It also dissolves in dilute nitric acid to form cobalt (II) nitrate and oxides of nitrogen.
(where NOx stands for any oxide of nitrogen, i.e, NO, NO2, …)
Concentrated nitric acid renders it passive due to the formation of oxide layer Co3O4
which is insoluble in the acid.d. Reaction of cobalt with halogens
Metallic cobalt reacts with halogens, forming cobalt (II) halides.
• Cobalt, like iron, can be magnetized and so is used to make magnets. It is alloyed
with aluminium and nickel to make particularly powerful magnets.
• Other alloys of cobalt are used in jet turbines and gas turbine generators at
• Cobalt metal is sometimes used in electroplating because of its attractive appearance,
hardness and resistance to corrosion.
• Cobalt salts have been used for centuries to produce brilliant blue colours in
paint, porcelain, glass, pottery and enamels.
• Cobalt is an essential trace element and found at the centre of the vitamin
B12 (cobalmin, C63H88CoN14O14P). It contains a cobalt(III) ion and is necessary for
the prevention of pernicious anaemia and the formation of red blood corpuscles,
but it is involved many other functions too.
Nickel is a grey solid metal with density of about 8.9. It melts at 1455oC and boils at
1. Chemical reactions
a. Reaction of nickel with air
Nickel does not react with oxygen, O2, at room temperature, under normal conditions.
Finely divided nickel can burn in oxygen, forming nickel (II) oxide, NiO.
b. Reaction of nickel with water
Nickel metal does not react with water under normal conditions. Nickel (II) ion
complexes with water under acidic and neutral conditions forming a light green
hexaqua nickel ion: [Ni(H2O)6]2+(aq)
In basic condition, nickel hydroxide precipitates:
c. Reaction of nickel with acids
Nickel metal dissolves slowly in dilute sulphuric acid to form the aquated Ni(II) ion
and hydrogen, H2.
The strongly oxidizing concentrated nitric acid, HNO3, reacts on the surface of
nickel and passivates the surface.
d. Reaction of nickel with hydroxide ions
Metallic nickel does not react with aqueous sodium hydroxide.
e. eaction of nickel with halogens
Nickel reacts slowly with halogens, forming the corresponding dihalides.
• Nickel resists corrosion and is used to plate other metals to protect them. It is,
however, mainly used in making alloys such as stainless steel. Nichrome is an
alloy of nickel and chromium with small amounts of silicon, manganese and
iron. It resists corrosion, even when red hot, so is used in toasters and electric
ovens. A copper-nickel alloy is commonly used in desalination plants, which
convert seawater into fresh water. Nickel steel is used for armour plating. Other
alloys of nickel are used in boat propeller shafts and turbine blades.
• Nickel is used in batteries, including rechargeable nickel-cadmium batteries
and nickel-metal hydride batteries used in hybrid vehicles.
• Nickel has a long history of being used in coins. The US five-cent piece (known
as a ‘nickel’) is 25% nickel and 75% copper.
• Finely divided nickel is used as a catalyst for hydrogenating vegetable oils.
Adding nickel to glass gives it a green colour.
Copper is a light pink to red (shiny-reddish) metal of density 8.95 g/cm3. It melts at
1083oC and boils at 2570oC.
1. Chemical reactions
a. Reaction of copper with air
Copper metal is stable in air under normal conditions. When heated until red hot,
copper metal and oxygen react to form Cu2O.
b. Reaction of copper with water
Copper does not react with water in all conditions.
c. Reaction of copper with acids
Copper is not dissolved by non-oxidizing dilute acids such as dilute
H2SO4 and HCl to produce hydrogen gas. This is why it is called a
‘noble metal’. Other noble metals include gold, silver and platinum.
But copper metal dissolves in dilute and concentrated nitric acid, HNO3 to form
copper (II) nitrate and oxides of nitrogen. Here nitric acid acts as an oxidising agent.
It also reacts with hot concentrated sulphuric acid to form copper (II) sulfate, sulphur
dioxide gas and water. But normally, sulphuric acid is not an oxidising acid!
d. Reaction of copper with halogens
Metallic copper metal reacts with the halogens forming corresponding dihalides.
• Historically, copper was the first metal to be worked by people. The discovery
that it could be hardened with a little tin to form the alloy bronze gave the
name to the Bronze Age.
• Traditionally it has been one of the metals used to make coins, along with silver
and gold. However, it is the most common of the three and therefore the
least valued. All US coins are now copper alloys, and gun metals also contain
• Most copper is used in electrical equipment such as wiring and motors. This is
because it conducts both heat and electricity very well, and can be drawn into
wires. It also has uses in construction (for example roofing and plumbing), and
industrial machinery (such as heat exchangers).
• Copper sulphate is used widely as an agricultural poison and as an algaecide
in water purification.
• Copper compounds, such as Fehling’s solution, are used in chemical tests for
• Copper helps in storing iron, is involved in production of pigments for colouring
hair, skin and eyes.
Zinc is a grey solid with a density of 7.14 g/cm3. It melts at 419.5 oC and boils at 907
1. Chemical reactions
a. Reaction of zinc with air
Zinc reacts with oxygen in moist air. The metal burns in air with a blue-green flame to
form zinc (II) oxide, a material that goes from white to yellow on prolonged heating.
b. Reaction of zinc with water
Zinc is unaffected with cold water. However, elemental zinc will reduce steam at
c. Reaction of zinc with acids
Zinc metal dissolves slowly in dilute sulphuric acid to form Zn(II) ions and hydrogen,
H2. In aqueous solution the Zn (II) ion is present as the complex ion [Zn(H2O)6]2+.
When zinc reacts with oxidizing acids like HNO3, no hydrogen gas is evolved.
• Mostly, zinc is used to galvanise other metals, such as ironsheets (amabati), to
prevent corrosion. Galvanised steel is used for car bodies, street lamp posts,
safety barriers and suspension bridges.Many houses in Rwanda are covered by
galvanized iron sheets (amabati).
• Large quantities of zinc are used to produce die-castings, which are important
in the automobile, electrical and hardware industries.
• Zinc is also used in alloys such as brass, nickel silver and aluminium solder.
• Zinc oxide is widely used in the manufacture of many products such as paints,
rubber, cosmetics, pharmaceuticals, plastics, inks, soaps, batteries, textiles
and electrical equipment. Zinc sulphide is used in making luminous paints,
fluorescent lights and x-ray screens.
• It is a component of insulin.
Checking up 1.5
1. State what is observed and write an equation, for the reaction that would
take place when
a. Copper is added to hot concentrated sulphuric acid.
b. Chromium is dissolved rapidly in hot concentrated aqueous alkali
c. Nickel (II) ions complexes react with water under acidic and neutral
d. Powdered zinc is dissolved in hot aqueous alkali.
2. State at least one property that makes that:
a. An aluminum - scandium alloy be used in fighter planes, high-end bicycle
frames and baseball bats.
b. Many alloys of titanium with aluminium, molybdenum and iron be mainly
used in aircraft, spacecraft and missiles.
c. Vanadium-steel alloys be used for armour plate, axles, piston rods and
d. Alternatives of tanning leather using chromium be investigated.
e. Manganese steel be used for railway tracks, safes, rifle barrels and prison
f. Iron be considered as an enigma.
g. Cobalt be necessary for the prevention of pernicious anaemia and the
formation of red blood corpuscles.
h. Nichrome be used in toasters and electric ovens.
i. Most copper be used in electrical equipment such as wiring and motors.
j. Galvanised steel be used for car bodies, street lamp posts, safety barriers
and suspension bridges.
Question 3 is given to you as an assignment. You can use any source to carry
out research in order to gain and provide relevant information to be presented
3. The following figures show objects made in different transition metals.
Observe them and complete the table with the main transition metal
which forms the objects, its two important properties and other two uses
(apart from that shown by the figure).
1.6. Identification of transition metal ions
Given a substance Y which contains one cation (from transition metal) and one
anion,identify the cation and anion in Y. Carry out the following tests on Y , record
your observations and deductions in the table below. Identify any gas evolves.
• The cation in Y is …………
• The anion in Y is ……………
• Write the ionic equations for the reactions in test (i) and test (ii)
Different transition metals have different colors. Also, different charges, or cations
of one transition metal can give different colors. Another factor is the natural of
the ligand. The same cations of a transition metal can produce a different color
depending on the ligand it binds to.
Many compounds containing transition metals have certain characteristic colours
and thus, by observing a compound, we can not identify it.
• Appearance or colour of different solid compounds containing transition
metalsTable 1.13: Colours of different solid compounds containing transition metals
• Colours of aqueous solutions of some transition metal ions
In aqueous solutions where water molecules are the ligands, the colours of some
metal ions observed are listed in the table below:
Table 1.14: Colours of different aqueous solutions containing some transition
metals (first series)
• Action of heat on solid compounds containing transition metal ionsTable 1.15: Colours of different solid compounds containing transition metals
(first series) due to action of heat.
Note: On heating the following temporary colour changes may also occur:
• Effect of aqueous sodium hydroxide and aqueous ammonia on solutions
containing transition metal ions
The hydroxides of transition metals are precipitated from solutions of the metal ions
by the addition of hydroxide ions or ammonia. The colour of the precipitate can
often be used to identify the metal present. The precipitates formed are gelatinous
and often coloured and some form soluble complex ions with excess ammonia.a. To about 1cm3 of the solution containing the positive ion (cation), add
2M aqueous sodium hydroxide dropwise until in excess
b. To about 1cm3 of the solution containing the positive ion (cation), add
2M aqueous ammonia dropwise until in excess
Confirmatory tests for some transition metal ions
Confirmatory tests are the tests required to confirm the analysis. Generally, a
confirmatory test is used after other tests have been carried out to isolate/identify
the ion. In order to confirm the ion without any dought
a. Zinc ions
i. Addition of little solid ammonium chloride followed by disodium
hydrogen phosphate solution to a solution of zinc ions gives a white
precipitate. The precipitate dissolves in excess ammonia or dilute
ii. Addition of potassium ferrocyanide solution to a solution of zinc ions
gives a white precipitate.
b. Chromium ions
To a solution of chromium (III) ions, add excess aqueous sodium hydroxide followed
by little hydrogen peroxide and boil the resultant mixture. A yellow solution of a
chromate is formed.
Treatment of the yellow solution with:
i. Lead (II) ethanoate or Lead(II)nitrate solution gives a yellow precipitate of
Lead(II) chromate. Pb2+(aq) + CrO42-(aq) →PbCrO4(s)
ii. Barium nitrate (or chloride) solution gives a yellow precipitate of barium
c. Manganese (II) ions
To the solution of manganese (II) ions, add little concentrated nitric acid followed
by little solid lead(IV) oxide or solid sodium bismuthate(V) and boil the mixture. A
purple solution is formed due to MnO4- ion.
d. Iron (II) ions
i. Addition of potassium hexacyanoferrate (III) solution to a solution of iron
(II) ions gives a dark blue precipitate .
ii. Addition of few drops of concentrated nitric acid to a solution of iron (II)
ions gives a yellowish solution due to iron (III) ions formed. The solution
gives positive test for iron (III) ions.
e. Iron(III) ions
i. Addition of potassium hexacyanoferrate (II) solution to a solution of
iron(III) ions gives a dark blue precipitate
ii. Addition of potassium thiocyanate or ammonium thiocyanate
solution to a solution of iron (III) ions gives a blood red coloration.
f. Cobalt (II) ions
Addition of potassium thiocyanate or ammonium thiocyanate solution to a solution
of cobalt(II) ions gives a blue colored product of potassium cobalt(II) tetrathiocyanate.
g. Nickel (II) ions
i. Addition of potassium cyanide solution gives a yellow-green
precipitate of Nickel(II) cyanide. The precipitate dissolves in excess
reagent to form a dark yellow solution tetracyanonickel (II) ion.
ii. Addition of aqueous ammonia followed by 2 to 3 drops of
dimethylglyoxime solution to a solution of nickel (II) ions gives a red
precipitate. The formation of this precipitate may sometimes require
that the solution mixture would be warmed.
h. Copper (II) ions
In addition to use of aqueous ammonia, the copper(II) ions can be confirmed by
addition of the following reagents to an aqueous solution of copper(II) ions:
i. Potassium iodide solution: A white precipitate of copper (I) iodide
stained brown with free iodine.
Brown color fades on addition of sodium thiosulphate solution due to the reaction
Checking up 1.6 (a)
Given a substance K which contains one cation and one anion, carry out the
following tests on K and record your observations and deductions in the table
below. Identify any gas evolved.
• The cation present in the compound K is ……………
• The anion present in the compound K is ……………Checking up 1.6 (b)
You are provided with substance D which contains one cation and one anion.
You are required to identify the cation and anion in D. Carry out the following
tests, record your observations and deductions in the table below. Identify any
Checking up 1.6 (c)
Aqueous sodium hydroxide is added separately to solutions of salts of the
transition metals A, B and C. Identify A, B and C from the following observations.
A: The white precipitate which appears is soluble in an excess of aqueous sodium
hydroxide and also in aqueous ammonia.
B: The blue precipitate which appears is insoluble in an excess of aqueous sodium
hydroxide but dissolves in aqueous ammonia to form a deep blue solution.
C: The green precipitate which appears is insoluble in an excess of aqueous
sodium hydroxide and also in aqueous ammonia.END UNIT ASSESSMENT
a. Multiple choice questions: Write the Roman number corresponding to the
1. Which of the following elements is not a transition metal?
2. Which of the following complexes is linear?
3. Which of the following ions does not form coloured solutions?
4. Which of the following reactions of Cu2+ is an example of a chelation
i. [Cu(H2O)6]2+ + 2OH- → [Cu(H2O)4(OH)2] + 2H2O
2. What is the characteristic of electron configurations of transition metals?
3. Which electrons, 3d or 4s, have the lowest ionization energies in a
4. a. Name any three transition metals that are essential to the biological
b. Why do you think transition metals form coordination compounds that
have covalent bonds?
5. Name the following coordination compounds using systematic
6. a. (i) What is meant by the term co-ordinate bond?
(ii) Explain why co-ordinate bonds can be formed between transition
metal ions and water molecules.
b. What name is given to any ligand that can form two co-ordinate bonds
to one metal ion? Give an example of such a ligand.
7. In order to determine the concentration of a solution of cobalt(II) chloride,
a 25.0 cm3 sample was titrated with a 0.0168 M solution of EDTA4-; 36.2
cm3 were required to reach the end-point. The reaction occurring in the
[Co(H2O)6]2+ + EDTA4–——→ [Co(EDTA)]2– + 6 H2O
a. What type of ligand is EDTA4–?
b. Calculate the molar concentration of the cobalt (II) chloride solution.
8. The ethanedioate (oxalate) ion,C2O42 , acts as a bidentate ligand. This ligand
forms an octahedral complex with iron (III) ions.
a. Deduce the formula of this complex and draw its structure showing all
the coordinate bonds present.
b. Give the name of a naturally-occurring in human body complex
compound which contains iron.
c. What is theimportant function of this complex compound?
9. The compound [Co(NH3)4Cl2]Cl contains both chloride ions and ammonia
molecules as ligands.
a. State why chloride ions and ammonia molecules can behave as
b. What is the oxidation state and the co-ordination number of cobalt in
this complex compound?
10. a. Suggest why the compound [Co(NH3)6]Cl3 has a different colour from
that of [Co(NH3)4Cl2]Cl.
b. Name and give the formula of an ammonia complex used to distinguish
between aldehydes and ketones.
11. Chloride ions form the tetrahedral complex ion [AlCl4]– but fluoride
ions form the octahedral complex ion [AlF6]3-. Suggest a reason for this
difference.URLs: 3Files: 4Labels: 2
UNIT 2: EXTRACTION OF METALS
Key unit competence
To be able to: Relate the properties of metals to their methods of extraction and uses
and suggest preventive measures to dangers associated with their extraction.
At the end of this unit , students will be able to:
• Describe the extraction of copper, iron, sodium, tantalum, zinc, wolfram and
• Outline and explain the uses of copper, iron, tantalum, zinc, wolfram, and tin
• Explain the issues associated with the extraction of metals and preventive
• Relate the properties of metals to their methods of extraction.
1. a. Do you know any metal and mineral extracted in Rwanda?
b. If yes, name them?
c. Give the applications of those metal and mineral .
2. Most of the metals are found in nature, not as pure metals, but as
compounds, i.e. combined with other chemical elements. Such metals are
extracted from their compounds using chemical reactions. The following
setup shows one example of a laboratory chemical reaction. Analyse it and
follow the procedure to be able to interpret the results.
1. Transfer one spatula measure of copper (II) oxide to a hard-glass test-tube.
2. Carefully add one spatula of charcoal powder on top of the copper (II)
oxide without any mixing.
3. Strongly heat these two layers for 5 minutes in a Bunsen flame.
4. Allow the tube to cool and then look closely at where the two powders
meet in the test tube.
1. Describe the solid copper (II) oxide before heating.
2. Name the gas formed in the reaction.
3. Describe the solid remaining after heating.
4. Name the solid formed in the reaction.
5. Write the word and symbol equations for the reactions.
6. What does this reaction tell you about the relative reactivities of carbon
7. Explain why this reaction is a redox reaction.
The chemical substances in the earth’s crust obtained by mining are called
minerals. Minerals, which are source of metal economically needed, are called
“Ore”. The unwanted impurities present in ore are called gangue. A native metal is
any metal that is found in its metallic form, either pure or alloyed, in nature (example:
Gold). The entire process of extraction of metal from its ore is called metallurgy.
Many metals are found in the Earth’s crust as ores. An ore is usually a compound
of the metal mixed with impurities. An ore is any naturally-occurring source of a
metal that you can economically extract the metal from. Most metals exist in in
nature as compounds, usually oxides or sulphides.
Sulphide ores cannot be converted directly into the metal. Instead they must be
converted to the oxide. This is achieved by roasting them in air. Roasting involves
heating of ore in presence of air below melting point of metal in reverberatory
Reverberatory furnace, in zinc, copper, tin and nickel production, is a furnace used
for smelting or refining in which the fuel is not in direct contact with the ore but
heats it by a flame blown over it from another chamber. In steel making, this process,
now largely obsolete, is called the open-hearth process.
In this process, volatile impurities escape leaving behind metal oxide and sulphur
dioxide (metal sulphide is converted to metal oxide).
This process causes problems because of the large quantity of sulphur dioxide
produced. Sulphur dioxide is one of the principal causes of acid rain; hence SO2 is
one of the air pollutants.
However if the sulphur dioxide can be collected before being released into the
atmosphere, it can be used to make sulphuric acid.
Metals in their compounds are in oxidized form, i.e. have lost electron(s) and bear
a positive charge. In order to get them as metals, they need to gain electrons: this
process or reaction of gaining electrons is called “reduction”. Reduction of metals
can be carried out using a chemical reducing agent such as carbon (coke, charcoal)
When the mineral is dug up, a method must be used to separate the metal from the
rest of the ore. This is called extraction of the metal.
2.1. Relating the properties of metals to their methods of extraction
Make a research (in books or internet) in order .to:
1. Find different methods of extraction of metals referring to the reactivity.
2. Explain why the ores have to be concentrated and different ways that can
2.1.1. Methods of metal extraction according to their properties
A number of methods are used to extract metals from their ores. The best method
to use depends on a number of factors. These factors are based on the properties of the metal.
The main factor here is the reactivity of the metal (the position it takes in the reactivity
series). Let us use the following question method to clarify these factors.
• Will the method successfully extract the metal? This depends on the reactivity
of the metal.
• How much do the reactants cost? Raw materials vary widely in cost.
• What purity is needed, and are the purification methods expensive? Some
metals are not useful unless very pure, others are useful impure.
• How much energy does the process use? High temperatures and electrolysis
use a lot of energy.
• How efficiently, and in what quantities, can the metal be made? Continuous
processes are more efficient than batch processes.
• Are there any environmental considerations? Some processes produce a lot
Different methods of metal extraction will be considered in this unit. Some examples
are given below:
• Reduction of metal oxides with carbon
Carbon (as coke or charcoal) is cheap. It not only acts as a reducing agent, but it
also acts as the fuel to provide heat for the process. However, in some cases (for
example with aluminium) the temperature needed for carbon reduction is too high
to be economic - so a different method has to be used. Carbon may also be left in
the metal as an impurity. Sometimes this can be removed afterwards (for example,
in the extraction of iron); sometimes it cannot (for example in producing titanium),
and a different method would have to be used in cases like this.
• Electrolysis of the metal ore
This method is used to extract metals which are difficult to reduce by chemical
agents.These metals include aluminium or metals which are difficult to reduce such
as Group 1 and 2 metals.
Reduction of the metal oxide with hydrogen
Hydrogen can be used as a reducing agent, it is also used in purification of cpper.
This is the main method for the extraction of tungsten from its oxide.
Sustainable use of natural resources: As these extraction processes are expensive
and the supply of ore is not infinite, it is essential to recycle the metals as much
as possible. Recycling of metals is one way of conservation and sustainable use of
Metallurgical operations are sources of pollution, water and air pollution. Measures
must be taken to eliminate or at least to minimise that kind of pollution.
2.1.2. Concentrating the ore
Concentrating the ore is also called “Dressing” or “Benefaction of ore”. The ore from
which the metal will be extracted has to be prepared before extraction. Concentrating
the ore is getting rid of as much of the unwanted rocky material as possible before
the ore is converted into the metal.This simply means “Removal of gangue from
ore”. This may be done by chemical or physical means.
• By physical processes
Physical operations use physical techniques that do not change the chemical nature
of the minerals; these techniques are based on physical properties such as: density,
magnetic properties, etc… In many cases, it is possible to separate the metal
compound from unwanted rocky material by physical means.
- Mechanical sorting (Hand picking): this involves use of hands to pick the
gangue and breaking away the gangue using a hammer.
- Magnetic separation: the ore is crushed and a very strong magnet is used to
sort out magnetic materials from non-magnetic materials. This method is for
example used to separate wolframite from cassiterite where cassiterite being
non-magnetic is not attracted by the magnet.
- Washing: the ores are usually denser than the gangue, which may be washed
away in a stream of water on an inclined table. Examples of ores separated in
this way are galena and limestone; cassiterite from silicates.
- Froth flotation method: this process is important in treating many sulphide
ores like galena, zinc blende (zinc sulphide), copper pyrites, etc. Separation is
based on different abilities of mineral and gangue particles to be moistened
by water. During this process, the ore is first powdered and fed into a large
concentration tank containing water and a suitable oil (frothing agents).
The mixture is agitated by blowing air through at a high pressure. The sulphide ores
rise to the surface in the froth while the gangue sinks to the bottom. The froth is
skimmed off the surface, and an acid is added to break up the froth. The concentrated
ore is filtered and dried.
• By chemical processes.
For example, pure aluminium oxide is obtained from bauxite by a process involving
a reaction with sodium hydroxide solution. Some copper ores can be converted
into copper (II) sulphate solution by leaving the crushed ore in contact with dilute
sulphuric acid for a long time and then copper can be extracted from the copper (II)
- Leaching with aqueous solvents: in this method the finely powdered ore is
treated with a suitable reagent that dissolves the ore but not impurities, for
1. Bauxite is crushed and digested with sodium hydroxide solution
2. Zinc ores can be leached with dilute sulphuric acid and electrolysed
- Roasting in air and Calcination: Here the ore is powdered and roasted in air
to drive off the water (for the hydrated ores) and other volatile substances. For
Carbonates decompose to release carbon dioxide.
Roasting: Sulphides release sulphur dioxide gas:
2ZnS(s) + 3O2(g) → 2ZnO(s) + 2SO2(g)
- Calcination: It is a process heating the ore strongly either in limited supply of air
ih the absence in air
ZnCO3(s) → ZnO(s) + CO2(g)
- Smelting is the process by which a metal is obtained, either as the element or
as a simple compound, from its oxide ore by heating beyond the melting point,
ordinarily in the presence of reducing agents, such as coke.
CuO + C → Cu +CO
Checking up 2.1
1. What is an ore?
2. What is the difference between an ore and a mineral?
3. Are ores a finite resource?
4. Are ores renewable?
5. When is carbon used for extraction?
6. Name a metal that could be extracted from its ore using carbon.
7. When is electrolysis used for extraction?
8. What do you understand with dressing of ore, smelting, froth flotation and
9. Name two metals that can only be extracted by electrolysis.
10. Suggest a reason why iron is extracted using carbon rather than by
11. State three factors which determine the choice of reduction method
used for the extraction of metals from their ores.
12. Complete with the following words: iron, extracted, crust, gold, native,
“Metals come from the Earth’s …….. Some metals like ……. are very unreactive
and are found as ………. in their ………state. Metals such as zinc, lead and
………are found combined with oxygen in compounds. These metals can be
……… using chemical reactions. The metal oxides are ………as oxygen is
removed from the compound”.
13. Why most of metals are produced by reduction method ?
2.2. Methods of extraction of Copper
Use the search engine and the library:
1. To find out the name of the two main ores of Copper.
2. To describe the full process in copper metal is extracted from its ores.
3. To demonstrate how copper is useful in our daily life.
We are going to deal with the extraction of copper from its ores, its purification by
electrolysis, and some of its uses.
2.2.1. Extracting copper from its ores
There are two main copper ore types of interest, copper oxide ores and copper
sulphide ores. Both ore types can be economically mined, however, the most
common source of copper ore is the sulphide ore mineral chalcopyrite also known
as copper pyrites, which accounts for about 50 percent of copper production.
Table 2.1: Some main ores of copper
The ores typically contain low percentages of copper and have to be concentrated
by, for example, froth flotation before processing.
The method used to extract copper from its ores depends on the nature of the ore.
Sulphide ores such as chalcopyrite are converted to copper by a different method
from silicate, carbonate or sulphate ores. Copper, Cu, is mainly extracted from the
ore chalcopyrite, CuFeS2, in a three stage process.
• In the first stage, chalcopyrite is heated with silicon dioxide and oxygen
2 CuFeS2 + 2 SiO2 + 4 O2→ Cu2S + 2 FeSiO3 + 3 SO2
• In the second stage, the copper (I) sulphide is roasted with oxygen at a high
temperature in a reverberatory furnace giving copper (II) oxide.
Cu2S + 2 O2→2 CuO + SO2
• In the third stage, the copper (II) oxide is reduced by heating with carbon.
CuO + C → Cu +CO
The end product of this is called blister copper - a porous brittle form of copper,
about 98 - 99.5% pure.
2.2.2. Purification of copper
When copper is made from sulphide ores by the first method above, it is impure.
The blister copper is first treated to remove any remaining sulphur (trapped as
bubbles of sulphur dioxide in the copper - hence “blister copper”) and then cast into
anodes for refining using electrolysis(electrolytic refining).The purification uses an
electrolyte of copper (II) sulphate solution, impure copper anodes, and strips of high purity copper for the cathodes.
The diagram shows a very simplified view of a cell.
• Any metal in the impure anode which is below copper in the electrochemical
series (reactivity series) does not go into solution as ions. It stays as a metal and
falls to the bottom of the cell as “anode sludge” together with any unreactive
material left over from the ore. The anode sludge may contain valuable metals
such as silver and gold.
• Metals above copper in the electrochemical series (like zinc) will form ions at
the anode and go into solution. However, they will not get discharged at the
cathode provided their concentration does not get too high.
Extracting copper from other ores
Copper can be extracted from non-sulphide ores by a different process involving
three separate stages:
Step 1: Reaction of the ore (over quite a long time and on a huge scale) with a
dilute acid such as dilute sulphuric acid to produce a very dilute copper (II) sulphate
Step 2: Concentration of the copper (II) sulphate solution by solvent extraction.
The very dilute solution is brought into contact with a relatively small amount of an
organic solvent containing a substance which will bind with copper (II) ions so that
they are removed from the dilute solution. The solvent must not mix with water.
Copper (II) ions are removed again from the organic solvent by reaction with fresh
sulphuric acid, producing a much more concentrated copper (II) sulphate solution
Step 3: Electrolysis of the new solution. Copper (II) ions are deposited as copper
on the cathode. The anodes for this process were traditionally lead-based alloys, but
newer methods use titanium or stainless steel. The cathode is either a strip of very
pure copper or stainless steel.
2.3. Methods of extraction of Iron
Using your own research, use the available resources (chemistry books, notebooks,
internet...) to make a succinct summary of:
1. The main ores of iron.
2. The full process involved in the blast furnace while the iron is being
extracted (Include the diagram, the raw materials introduced and the
role of each and the equations of the main reactions occurring).
3. The main different forms of iron, its properties and their respective uses.
The common ores of iron are iron oxides, and these can be reduced to iron by
heating them with carbon in the form of coke. Coke is produced by heating coal
in the absence of air. Coke is cheap and provides both the reducing agent for the
reaction and also the heat source - as you will see below.
Three substances are needed to enable extraction of iron from its ore. The combined
mixture is called the “charge”:
• Iron ore
• Limestone (calcium carbonate).
• Coke - mainly carbon.
The charge is placed into the blast furnace. Hot air is blasted through the bottom.
Several reactions take place before the iron is finally produced. This is a continuous
process that needs a high temperature. The high temperature is produced by
burning the carbon in a blast of hot air.
Oxygen in the air reacts with coke to give carbon dioxide.
2.3.5. Different forms of iron
• Pig iron (Cast iron after removing some impurities), an alloy of iron that contains
2 to 4 percent carbon, along with varying amounts of silicon and manganese
and traces of impurities such as sulfur and phosphorus.
- It is made by reducing iron ore in a blast furnace.
- It has a low tensile strength
- It is used in making gates, pipes, lamp posts where high strength is not needed.
• Wrought iron is a soft, ductile, fibrous variety that is produced from a semifused
mass of relatively pure iron globules (haematite) partially surrounded by
slag. It usually contains less than 0.1 percent carbon and 1 or 2 percent slag.
- It is purer than cast iron.
- It is fibrous and tough
- It can be welded (joined by hammering when red hot)
- It is malleable and ductile
- It is used for making sheets, wire and nails.
- Steel is an alloy of iron, carbon, manganese, nickel and vanadium.
2.3.6. Alloys of iron and their uses
Checking up 2.3
1. Iron is extracted using the Blast Furnace
a. What is introduced into the top of the blast furnace?
b. What is the source of heat used in the blast furnace? Write the involved
c. What is the main reducing agent and how is it produced? (give an equation)
d. How is the iron oxide reduced by this reducing agent? (give an equation)
e. What is the other reducing agent and how does it reduce the iron oxide?
(give an equation)
2. The blast furnace is a continuous process. What does this mean and why
is it advantageous?
2.4. Methods of extraction of tin
Make a research using the appropriate books and internet to:
1. Find out the main tin ore and state 2 regions in Rwanda where it is mined.
2. Describe the process taken to extract tin from its principal ore.
3. State three properties and uses of tin.
prevent corrosion. In modern times tin is used in many alloys. The first alloy,
used in large scale since 3000 BC, was bronze, an alloy of tin and copper.
Most notably tin/lead soft solders, typically containing 60% or more of tin.
• Another large application for tin is corrosion-resistant tin plating of steel.
• Because of its low toxicity, tin-plated metal is also used for food packaging,
giving the name to tin cans, which are made mostly of steel.
• Due to its resistance to atmospheric corrosion and low melting point, it can be
used to make sheet glass.
Checking up 2.4
1. Name the main ore of tin.
2. Explain the extraction of tin from tin oxide.
3. Tin metal is obtained by removing oxygen from the metal oxide. What
name do we give to this chemical reaction?
4. Explain why tin is said to be very useful.
2.5. Methods of extraction of Zinc
Use the library or other search engine to
1. Find out the main ores of zinc.
2. Describe all steps followed to obtain the purified zinc from its sulphide
3. Demonstrate how zinc is so useful.
concentrated zinc sulphide ores settles on the surface leaving behind the impurities
b. Process of roasting
The concentrated ore is then treated at 900oC in the presence of excess air, on the
base of a reverberatory furnace.This process of heating is called roasting. During
this process, zinc oxide is obtained from the zinc sulphide ore. The equation for this
Checking up 2.5
1. Recall 2 uses of zinc (refer to unit 1)
2. Describe the extraction of zinc from zinc oxide.
3. Zinc is extracted by removing oxygen from zinc oxide.
a. What name is given to a reaction in which oxygen is removed from a
b. Explain how oxygen can be removed from zinc oxide to make zinc.
4. Describe different methods by which the impurities in zinc extracted can
2.6. Methods of extraction of Sodium
You are requested to use all resources about sodium (books, internet, textbooks).
1. Discuss on the importance of sodium and its compound in Chemistry
and in everyday life.
2. Sodium metal does not occur in Free State in nature. Explain why these
3. Use the available resources to describe the way this valuable metal can be
obtained at large scale (include the mining process, separation methods
from its compounds, reaction equations where necessary, the main
drawbacks that can be encountered during this process and the way to
On industrial scale sodium metal is extracted by “Down’s Process”. Down’s Process is
based on the electrolysis of fused salt (NaCl). This compound is found all around
the world, dissolved in sea water. It is also mined from the ore called rock salt, which
is made up mainly of sodium chloride (NaCl).
2.6.1. Extraction using Down’s Cell
The salt is found mixed with insoluble impurities of sand and bits of rock. Once the
dissolved rock salt has been filtered, we are left with a solution of sodium chloride
in water. You might think that this could be electrolysed to extract the sodium.
However, because sodium is more reactive than the hydrogen in the water,
hydrogen would be produced at the cathode instead of sodium. Instead, sodium
chloride is crystallized out of the solution and melted. The molten sodium chloride
is then electrolysed to extract the sodium metal. This is carried out in a Down’s cell as
shown in figure below.
2.6.4. Uses of sodium
• Molten sodium is used as a coolant in some types of nuclear reactor. Its high
thermal conductivity and low melting temperature and the fact that its boiling
temperature is much higher than that of water make sodium suitable for this
• Sodium wire is used in electrical circuits for special applications. It is very
flexible and has a high electrical conductivity. The wire is coated with plastics
to exclude moisture.
• In countries that experience very cold winter, NaCl is spread on the roads to
de-ice the roads.
Checking up 2.6
1. Why is sodium not extracted by carbon reduction process? What is the
2. What difficulties arise in the extraction of sodium from its ore?
3. Choose the suitable answer. In sodium extraction, fused (molten) NaCl is
used rather than the dissolved (aqueous) NaCl because:
a. Obtaining molten NaCl is easy than obtaining aqueous NaCl.
b. In molten NaCl, the ions are free to move and not in aqueous NaCl.
c. Molten sodium NaCl is an electrolyte and not the aqueous NaCl.
2.7. Methods of extraction of Aluminium
Research in library textbook or search engine about aluminium and its extraction
and make a good summary containing the following:
1. The main ores of aluminium.
2. Why aluminium extraction is suitable to be done by electrolytic method
rather than reduction with carbon.
3. The way the main ore is purified to have the pure aluminium oxide
4. The process by which aluminium is extracted from the purified ore
by electrolytic method (include the diagram, reaction equations at
electrodes and the way to solve some difficulties which may arise during
this extraction process)
5. The properties of aluminium which makes it to be very important in our
daily uses and the uses of aluminium.
min (Al, Cu, Mg and Mn). Aluminium alloys are used in aircraft and other transportation
vehicles because of its low density.
• It is used as jewellery because it is a shiny metal, it has a good appearance.
• Resists corrosion because of the strong thin layer of aluminium oxide on its
surface. This layer can be strengthened further by anodising the aluminium.
• It is used for making window frames.
• Aluminium foil is used for wrapping cigarettes, food, etc because it reflects
• Drink cans because it is not toxic.
• Aluminium utensils are extensively used for household purposes.
• Aluminium is used to produce metals such as chromium and manganese from
their ores (aluminothermic process).
2.7.4. Recycling of aluminium
Aluminum is one of the most recycled materials in the world. This is due to high
cost of new aluminium that involves high cost of electricity needed to produce
aluminium. Recycled aluminium cost less in production and constitutes another
way of sustainable management of our natural resources.
• The consumer throws aluminium cans and foil into a recycle bin.
• The aluminium is then collected and taken to a treatment plant.
• In the treatment plant the aluminium is sorted and cleaned ready for
• It then goes through a re-melt process and turns into molten aluminium, this
removes the coatings and inks that may be present on the aluminium.
• The aluminium is then made into large blocks called ingots.
• The ingots are sent to mills where they are rolled out, this gives the aluminium
greater flexibility and strength.
• This is then made into aluminium products such as cans, chocolate wrapping
and ready meal packaging.
• In as little as 6 weeks, the recycled aluminium products are then sent back to
the shops ready to be used again.
Checking up 2.7
Consider the electrolytic extraction of aluminium.
1. How is aluminium ore called?
2. Explain why cryolithe is added to aluminium oxide
3. Describe the process by which the ore of aluminium is purified.
4. Write half-equations for the reactions at each electrode, and write an
overall equation for the reaction.
5. State what each electrode is made of.
6. Explain why
a. The anodes need to be regularly replaced.
b. The electrolysis of aluminium oxide is expensive.
c. Aluminium is recycled.
7. Give three uses of aluminium and the properties responsible for each use.
2.8. Methods of extraction of Wolfram (Tungsten)
1. Visit the nearby mining sites of wolfram and make a field report.
2. Research using internet and some books and make a summary about the
a. The ores of wolfram.
b. Where in Rwanda do we find wolfram?
c. How wolfram extraction differs from that of zinc in terms of reduction.
d. Full description of the extraction process of tungsten from its ore.
e. The main uses of tungsten.
Tungsten ore is a rock from which the element tungsten can be economically
2.8.2. The extraction process
2.8.3. Advantages and disadvantages of the process
• It produces very pure tungsten
• Hydrogen is a cheap reagent
• The energy cost are high
• Using a flammable gas such as hydrogen at high temperatures is very dangerous
• Tungsten is mostly used in light bulb filaments which heat up to 2000ºC, when
many other metals would vaporise, particularly at the pressures found inside
light bulbs. This is because it has a very high melting point.
• Tungsten has the highest melting point of all metals and is alloyed with
other metals to strengthen them. Tungsten and its alloys are used in many
high-temperature applications, such as arc-welding electrodes and heating
elements in high-temperature furnaces.
• Tungsten carbide (WC) is extremely hard and is very important to the metalworking,
mining and petroleum industries. It is made by mixing tungsten
powder and carbon powder and heating to 2200°C. It makes excellent cutting
and drilling tools, including a new ‘painless’ dental drill which spins at ultrahigh
• Calcium and magnesium tungstates are widely used in fluorescent lighting.
Tungsten mill products are either tungsten metal products, such as lighting
filaments, electrodes, electrical and electronic contacts, wires, sheets, rods etc
or tungsten alloys.
• Due to tungsten’s ability to keep its shape at high temperatures, tungsten
filaments are now also used in a variety of household applications, including
lamps, floodlights, heating elements in electrical furnaces, microwave ovens,
x-ray tubes and cathode-ray tubes (CRTs) in computer monitors and television
• The metal tolerance to intense heat also makes it ideal for thermocouples and
electrical contacts in electric arc furnaces and welding equipment.
Checking up 2.8
1. Nowadays, there is a special reduction method used to extract Tungsten
from its ores.
a. State 2 main ores of tungsten.
b. Write the balanced equation of the reduction reaction of tungsten (II)
2. Suggest the reason why ore concentrating plants are always located in
close proximity to the mine.
3. Give 2 widely known physical properties of tungsten and the uses
associated to these properties.
2.9. Methods of extraction of tantalum
1. Visit the nearby mining sites of tantalum and make a field report.
2. The picture below shows the miners at work in Rwanda. The metal being
extracted here is very important in modern area. According to Merchant
and Consulting Ltd (2018), Rwanda is the world’s largest producer of this
metal which is called “tantalum”.
Research using any relevant source of information about tantalum:
a. To find out the physical properties of tantalum metal and its main uses.
b. To make a description of the extraction of tantalum from its main ore
Tantalum is a hard, heavy, shiny, grayish-blue metal that is very stable, almost
impervious (impermeable) to air, water and all but a few acids. It has the third highest
melting point of all elements (over 3000 oC), and its primary use is in capacitors for
electronic applications, and for vacuum furnace parts.
It is classified as a “refractory” metal, which means it can sustain high temperatures
and resist corrosion. It is a good conductor of heat and electricity, which makes it
useful in various electronics. Pure tantalum can be drawn into fine wire filament,
which is used to evaporate other metals.
2.9.1. Where tantalum is found
Tantalum is found in hard rock deposits such as granites, carbonites and pegmatites
(igneous rock that consists of coarse granite). The chief tantalum ores are tantalite[(Fe, Mn)
Tantalum is an important component in many modern technologies, and is used in
capacitors for everything from computers to mobile phones.
Despite its importance in the world today, tantalum mining takes place in very few
countries and mining it is difficult. Only four countries produced tantalum in 2016,
and most was mined in the Democratic Republic of Congo (DRC). Rwanda, Brazil
and China were the other top countries for tantalum mining in that year. Sites are
being identified for future development, and existing sites are being evaluated for
2.9.2. How tantalum is mined
Tantalum comes from the processing and refining of tantalite. Tantalite is the
common name for any mineral ore containing tantalum. Most tantalum mines are
open pit; some are underground.
The process of mining tantalum involves blasting, crushing and transporting the
resulting ore to begin the process of freeing the tantalum. Before transportation, the
ore is concentrated at or near the mine site, to increase the percentage (by weight)
of tantalum oxide and niobium. The material is concentrated through wet gravity
techniques, gravity, electrostatic and electromagnetic processes.
2.9.3. How tantalum is processed
The tantalum concentrate is transported to the processor for chemical processing.
The concentrate is then treated with a mixture of hydrofluoric and sulphuric acids
at high temperatures. This causes the tantalum and niobium to dissolve as fluorides.
Numerous impurities are also dissolved. Other elements, such as silicon, iron,
manganese, titanium, zirconium, uranium and thorium, are generally present and
processed for other uses.
The concentrate is broken down into a slurry (A slurry is a watery mixture of insoluble
matter such as mud, lime, or plaster of Paris). The slurry is filtered and further
processed by solvent extraction. Using methyl isobutyl ketone (MIBK), or liquid ion
exchange using an amine extract in kerosenes, produces highly purified solutions of
tantalum and niobium. In this way, the tantalum oxide is obtained which is finally
reduced with molten sodium to produce tantalum metal in powder form.
It can then be compacted (as it is for capacitors) to final shape, or may be melted
(and refined) in an electron beam furnace.
2.9.4. Uses for tantalum
• Tantalum is used to make electrolytic conductors, aircraft engines, vacuum
furnace parts, nuclear reactors and missile parts.
• Tantalum is unaffected by body fluids, and is non-irritating, which makes it
useful for surgical appliances.
• It is common in the production of cell phones, personal computers, igniter
chips in car air bags, cutting tools, drill bits, teeth for excavators, bullets and
• Because the metal is an electrical conductor, it is useful in many consumer
electronics, such as microprocessors for plasma televisions.
Checking up 2.9
In pairs, answer the following questions:
1. Where tantalum is found in Rwanda?
2. How tantalum is mined.
3. How tantalum is processed
4. Give 3 uses of tantalum.
5. What is the role of methyl isobutyl ketone (MIBK) in tantalum processing?
2.10. Dangers associated with extraction of metals
If you have ever visited a mining site, remember all the processes involved and
suggest all possible common dangers associated with the extraction of metals.
The following are some of the dangers associated with metals extraction. Especially,
these hazards are found in smelting and refining and in addition mining.
Metal extraction industry has a higher rate of injuries than most other industries.
Sources of these injuries include: splattering and spills of molten metaland slag
resulting in burns; gas explosions and explosions from contact of molten metal
with water; collisions with moving vehicles; falls of heavy objects; falls from a
height,slipping and tripping injuries from obstruction of floors and passageways.
Adequate training, appropriate personal protective equipment (PPE) [for example,
hard hats, safety shoes, work gloves and protective clothing]; good storage,
housekeeping and equipment maintenance; traffic rules for moving equipment
(including defined routes and an effective signal and warning system); and a fall
2. Heat illnesses
Heat stress illnesses such as heat stroke are a common hazard, primarily due to
infrared radiation from furnaces and molten metal. This is especially a problem when
strenuous work must be done in hot environments.
Prevention of heat illnesses
Water screens or air curtains in front of furnaces, spot cooling, enclosed airconditioned
booths, heat-protective clothing and air-cooled suits, allowing
sufficient time for acclimatization, work breaks in cool areas and an adequate supply
of beverages for frequent drinking.
Mining operations are major contributors to the pollution of our environment such
as: air pollution, water pollution, degradation of landscape, etc.
Exposure to a wide variety of hazardous dusts, fumes, gases and other chemicals
can occur during smelting and refining operations. Crushing and grinding ore in
particular can result in high exposures to silica and toxic metal dusts (containing
lead, arsenic and cadmium, for example). There can also be dust exposures during
furnace maintenance operations. During smelting operations, metal fumes can
be a major problem especially risk of developing respiratory system illness. In
metallurgical operations, the carbon dioxide released has more dangers especially
the global warming because it is a greenhouse gas.
Dust and fume emissions can be controlled by enclosure, automation of processes,
local and dilution exhaust ventilation, wetting down of materials, reduced handling
of materials and other process changes. Where these are not adequate, respiratory
protection would be needed.
Many smelting operations involve the production of large amounts of sulphur
dioxide from sulphide ores and carbon monoxide from combustion processes.
Dilution and local exhaust ventilation (LEV) are essential.
Sulphuric acid is produced as a by-product of smelting operations and is used in
electrolytic refining and leaching of metals. Exposure can occur both to the liquid
and to sulphuric acid mists. Skin and eye protection and LEV are needed.
The extraction of some metals can have special dangers. Examples include fluorides
in aluminium smelting, arsenic in copper, etc.
These processes require their own special precautions
• Glare and infrared radiation from furnaces and molten metal can cause eye
damage including cataracts. Proper goggles and face shields should be worn.
High levels of infrared radiation may also cause skin burns unless protective
clothing is worn.
• High noise levels from crushing and grinding ore, gas discharge blowers and
high-power electric furnaces can cause hearing loss. If the source of the noise
cannot be enclosed or isolated, then hearing protectors should be worn. A
hearing conservation program including audiometric testing and training
should be instituted.
• Electrical hazards can occur during electrolytic processes. Precautions include
proper electrical maintenance with lockout/tagout procedures; insulated
gloves, clothing and tools; and ground fault circuit interrupters where
• Manual lifting and handling of materials can cause back and upper extremity
injuries. Mechanical lifting aids and proper training in lifting methods can
reduce this problem.
CASE OF RWANDA
According to Rwanda Environment Management Authority (REMA), mining
activities often impact significantly on the environment. For instance, sand
collecting and quarrying are already shown some significant environmental
impacts, including resource depletion, energy consumption, waste generation
and emissions of air pollutants. The dangers to human life and health associated
with mining include the displacement of people, land use changes, dust and
In fact, the preparation of ores which uses a lot of water constitutes a major
pollutant of stream water in Rwanda. For example, the waters draining the
mining sectors of Rutongo and Gatumba pollute the rivers of Nyabarongo
and Nyabugogo by sediments of clay and sand which they transport over long
distances. It is this considerable mineral load which partly gives them the brown
colour that is characteristic of the rivers in Rwanda. Mining and quarrying
produce massive rejects which appear in nature in the form of enormous lots of
earth and rocks. Erosion from rain water transports the mineral residue towards
the valleys where streams are filled and covered by the residue which may be
toxic to biodiversity.
Checking up 2.10
1. State 5 sources of injuries associated with the extractions of metals
2. Give 3 ways you can prevent from heat illnesses.
3. Describe 2 ways you can control the dangers associated with the
chemicals in extraction of metals.
4. Suggest a way of protecting mining workers against the risk of lung
disease due to dust at their working place?
2.11. Preventive measures associated with metal extractions
We already know that there are many dangers associated with the extraction of
Recommend measures that must be taken to prevent from these dangers (risks).
Effort should be made by mining componies or planned for the future, to eliminate
or minimize the environmental problems associated with metal extractions include:
1. The potential sources of air contaminants should be enclosed and isolated
2. Brief, for any operation related to metal extraction, measures must be
adopted to protect the workers in particular and the environment in
• Elimination or reduction of air polluting gases.
• Avoiding water and soil pollution.
• Protect the landscape.
• Using cleaner production techniques i.e Minimize sources of pollution and use
3. Adopt technology that minimizes wastes produced through process-reengineering/
Checking up 2.11
1. 1.In extraction of metals, the best and least costly preventive measures
are those taken at the planning stage of a new process of extraction.
Explain the main aspects that should be taken into account.
2. 2.what are the main sources of pollution in metallurgy?
END UNIT ASSESSMENT
1. Which metal is extracted from Bauxite?
2. Brass is
a. An Element
b. A Compound
c. A Mixture
d. An Alloy
3. Bronze is an alloy of
a. Copper and Zinc
b. Lead and Copper
c. Copper and Tin
d. Barium, Zinc and Iron
4. Which of the following metals is often found in pure state?
5. Which metal is extracted from Haematite?
6. Rocks rich in metals with economic value are known as
7. An alloy is a
a. Compound of three elements
b. Homogeneus mixture of two or more metals
c. Heterogeneous mixture
d. Element in impure form
8. If a metal ore is called "pyrites" then it most probably has
9. Often to prevent corrosion, metals are galvanized by covering them with a
10. What is not true about Tantalum?
a. It is classified as a "refractory" metal
b. Tantalum oxide is reduced with molten sodium to produce tantalum
metal in powder form.
c. Its ore minerals include scheelite
d. It is found in hard rock deposits such as granites, carbonites and
11. Complete with the terms applied in the Extraction of iron from haematite
in industry (Blast Furnace).
a. Raw materials: ___________________________________
A mixture of ___________, __________ and _____________ is added at
the top of the furnace. ________ air is blown into the furnace from the
bottom. A chain of chemical reactions occur:
b. Carbon reacts with oxygen in air to form ______________.
c. The hot carbon dioxide rises in the furnace and is reduced by _________
to form __________. Equation: ____________________________
d. Carbon monoxide is a ____________ agent. It ___________ iron(III) oxide
in haematite to form hot molten ___________.
The hot molten iron is then rum out from the bottom of the furnace.
e. The formula of limestone: ________________
Limestone breaks up into____________ and_______________ when
f. Calcium oxide helps to remove ___________ (the impurities) to form a
liquid ‘_______’. Equation: ________________________________
12. a. By giving reagents and conditions, state three different methods
of extracting metals starting from their oxides. In each case, write
equation(s) to illustrate the extraction of an appropriate metal.
b. i. Why are metals more usually extracted from their oxides rather than
from any other compound?
ii. State two environmental problems associated with the extraction of metals
from their oxides or sulphides and give the chemical responsible for each
13. a. Give the ores of iron.
b. Explain the extraction of iron from its ores.
14. a. Give 2 or 3 uses of aluminium, copper, zinc, and iron.
b. Give two reasons why the extraction of aluminium is expensive.
15. Tungsten is prepared in a pure form by high temperature reduction of
tungsten (VI) oxide with hydrogen.
a. Construct an equation for this reaction.
b. Suggest why carbon is not used as the reducing agent.
c. Suggest one advantage (other than purity of the product) and one
disadvantage of using hydrogen as the reducing agent on an industrial
16. Zinc and copper are extracted in the same way as iron (in blast furnace)
but exist as their sulphide ores.
a. How is the sulphide ore converted into an oxide and what is the problem
with this process? (give an equation)
b. Why can aluminium not be extracted in this way?
c. Why can tungsten not be extracted in this way?
17. Copper is a widely used metal. The main ore of copper contains copper
sulphide. Copper can be extracted from copper sulfide in a three stage
a. In the first stage of extraction the copper sulfide is heated in air.
i. Balance the symbol equation for the reaction. Cu2S + O2 → CuO +
ii. Explain why there would be an environmental problem if the gas from
this reaction were allowed to escape into the atmosphere.
b. In the second stage copper oxide, CuO, is reduced using carbon.
Describe and explain what happens during this reaction.
c. During the third stage the copper can be purified as shown in the
i. What is the name of the type of process used for this purification?
ii. Give one use of purified copper.
d. Copper-rich ores are running out. New ways of extracting copper from
low grade ores are being researched. Recycling of copper may be better
than extracting copper from its ores. Explain why.
Key unit competency:
To be able to analyze the components of quality Fertilizers and their benefits, effects of misuse and dangers associated with the substandard fertilizers.
At the end of this unit , students will be able to:
• State the major constituents of fertilizers;
• Identify the characteristics of good fertilizer;
• Briefly describe the manufacture of fertilizers;
• State the advantages and disadvantages of using fertilizers;
• Interpret the labels on the fertilizer containers;
• Classify the fertilizers in terms of composition.
A plot of land has been divided into two parts and in both irish potatoes have been cultivated by two cultivators.
One of them harvested 2000 kg of irish potatoes of big size and the other harvested 50kg of irish potatoes of small size.
Given that on both plots of land, the following work has been done at the same time• Cultivation,
• Hoeing (or weeding)
Suggest reason(s) which caused the difference in the harvest.
The total population in Rwanda was estimated at 11.3 million people in 2016, according to the latest census figures. Looking back, in the year of 1960, Rwanda had a population of 2.9 million people. Rwanda’s population will shoot to 18.2 million people by 2050 at an average growth rate of 2.3 %, the United Nations Population Fund (UNFPA) has projected. These statistics show that the population of Rwanda is going on increasing but as we know the area of Rwanda is not increasing. That is why Fertilizers and other agricultural techniques are needed for the population of Rwanda to be capable of feeding itself and even feed some other population in the region.
3.1. Types of Fertilizers
1. a. What is the role of fertilizers
b. Name any examples of Fertilizers you have ever heard
2. Using this book or any other book or internet, read and analyse the content about the types of fertilizers and make a summary to be presented to the class.
A fertilizer is any material, organic or inorganic, that is used to supply nutrients to the soil.
There exist types of Fertilizers:
1. Natural Fertilizers(or organic Fertilizers)
2. Artificial Fertilizers (or chemical Fertilizers)
3.1.1. Natural Fertilizers
The name organic fertilizer refers to materials used as fertilizer that occur regularly in nature, usually as a by-product or end product of a naturally occurring process. They are made from remains of dead plants, wastes from animals or they can be minerals. Examples include manures and minerals. Manure is an organic material that is used to fertilize land.
1. Farmyard manure: animal manure that consists of feces
2. Green manure: is a term used to describe specific plant or crop varieties that are grown and turned into the soil to improve its overall quality.
3. Compost manure: is organic matter that has been decomposed and recycled as a fertilizer and soil amendment.
4. Minerals: Mineral mined powdered limestone, rock phosphate and sodium nitrate, are inorganic compounds which are energetically intensive to harvest and are approved for usage in organic agriculture in minimal amount.
3.1.2. Artificial Fertilizers
They are fertilizers which are chemically synthesized which contain one or more of the major elements required by plants for good growth.
Examples: Urea, N.P.K, ammonium dihydrogen phosphate, NH4 (H2PO4),…
Checking up 3.1
Give the two main types of Fertilizers and discuss the pros and cons of using one or another type of fertilizer.
3.2. Components of a fertilizer
1. Name any nutrients you know that plants need in order to grow
2. Using this book, any other books or internet do a research and find out
a. The types of nutrients and give any three nutrients in each category that plants need for their growth and classify the nutrients depending on how plants need them
b. Any two roles for each nutrients for the plant growth.
First it is important to understand that all industrial Fertilizers, by convention, regardless of type and specific use, have something called a NPK ratio.The NPK ratio will be prominently labeled on the package and indicates the percentage of major (or primary) nutrients the fertilizer contains. Example: Urea is a fertilizer with an NPKratio of 46-00-00.
The nutrients of plants are classified into three types namely:
• Major nutrients
• Secondary nutrients
3.2.1. The major nutrients
The major nutrients for soil are nitrogen (N), phosphorus (P), and potassium (K). These major nutrients usually are lacking or insuffiscient in the soil because plants consume these nutrients in large amounts for their growth and survival.
The letter N represents the actual nitrogen content in the fertilizer by percentage mass while P and K represent the amount of oxide in the form of phosphorus (V) oxide (P2O5) and potassium oxide (K2O) respectively.
3.2.2. Secondary nutrients
Now, in the category of secondary nutrients, are calcium (Ca), magnesium (Mg), and Sulphur (S). As, these nutrients are generally enough in the soil, so fertilization is not always needed. Also, large amounts of Calcium are added when lime is applied to acidic soils. In fact, Sulphur is usually found in sufficient amounts from the slow decomposition of soil.
In fact, micronutrients are those elements essential for plant growth which are needed but in only very small (micro) quantities. These elements are even called minor elements or trace elements. The common micro nutrients are boron (B), copper (Cu), iron (Fe), chlorine (Cl), manganese (Mn), molybdenum (Mo) and zinc (Zn). In fact, recycling organic matter such as grass clippings and tree leaves is an excellent way of providing micro nutrients to growing plants.
3.3. The manufacture of Fertilizers
1. Write reactions for the formation of the following compounds
a. Ammonium sulphate
b. Potassium sulphate
c. Ammonium nitrate
e. Ammonium phosphate
2. Using this book or any other book or internet, read and analyse the content about the manufacture of the following Fertilizers and make a summary to be presented to the class;
Ammonium sulphate, potassium sulphate, ammonium nitrate, urea, and phosphates
3. Rwanda has a resources that can be used to produce an industrial fertilizer; name that resource.
supply phosphorus to the plants. These minerals are, therefore, converted into soluble materials, by reacting them with sulphuric acid, or phosphoric acid or nitric acid.
Characteristics of a good fertilizer
A good fertilizer should have the following characteristics:
It should contain the required nutrients, in such a form that they can be assimilated by the plants. It should be cheap.
It should be soluble in water.
It should be stable, so that it may be available for a long time for the growing plant.
It should not be injurious to the plants.
It should be able to correct the acidity of the soil.
3.4. Disadvantages of the use of organic and inorganic Fertilizers
“The use of fertilizers is a harm to humanity”, yes or not. Explain
3.4.1. Organic Fertilizers
The use of organic fertilizer may have many advantages but also it may have some disadvantages
1. The manures add organic matter (called humus) to the soil which restores the soil texture for better retention of water and for aeration of soil. For example, organic matter present in the manures increases the water holding capacity in sandy soils and drainage in clay soil.
2. The organic matter of manures provides food for the soil organisms (decomposers such as bacteria, fungi, etc.) which help in making nutrients available to plants.
3. Nutrient release: slow and consistent at a natural rate that plants are able to use. No danger of over concentration of any element, since microbes must break down the material.
4. Trace minerals: typically present in a broad range, providing more balanced nutrition to the plant.
5. They will not burn: safe for all plants with no danger of burning due to salt concentration.
6. Long lasting: does not leach out since the organic matter binds to the soil particles where the roots have access to it.
7. Fewer applications required: once a healthy soil condition is reached, it is easier to maintain that level with less work
8. Controlled growth: does not over-stimulate to exceptional growth which can cause problems and more work.
1. Many organic products produce inconsistent results.
2. The level of nutrients present in organic fertilizer is often low.
3. The time of their preparation is too long.
3.4.2. Inorganic Fertilizers
The use of inorganic fertilizers may have many advantages but also it may have some disadvantages
1. Chemical Fertilizers are made with synthetic ingredients designed to stimulate plant growth.
2. Commercial chemical Fertilizers have the advantage of predictability and reliability
3. Formulations are blended with accuracy and you can buy different blends for different types of plants; commercial formulated Fertilizers allow you to know exactly which nutrients you’re giving your plants, rather than guessing at the composition of organic formulas
1. They can burn plants
2. They require a specific timetable of application and watering because of fast release of nutrients
• Increased nitrate levels increase the risks of blue baby syndrome, a rare form of anaemia which affects babies below 6 months of age. The cause is the oxidation by nitrite ions of Fe2+ in haemoglobin to Fe3+. The oxidized hemoglobin cannot bind oxygen, and the baby turns blue from lack of oxygen. Conditions in the digestive tracks of young children are more favourable to the bacteria which reduce nitrates to nitrites than those in adults.
• Another hazard of chemical Fertilizers is that carcinogenic nitrosoamines (yellow oil substance) may be formed in the human digestive track by the conversion of nitrate into nitrite. The nitrite produced in the stomach it combines with HCl to produce nitrous acid. Nitrous acid can react with any secondary amine in foods to form nitrosoamines and the reaction of nitrite with amino acids.
4. Repeated use or excess use of the same fertilizer producing acidic ions (NH4+). Example of such a fertilizer is (NH4)2SO4.
5. Repeated use or excess use of the same fertilizer producing basic ions. Example of such a fertilizer is CaCO3.
6. Warm temperatures and high rain fall: Cations such as Ca++, Mg++, K+ which are essential to living organisms, are leached (dissolved) from the soil profile, leaving behind more stable materials rich in Fe and Al oxides. This natural weathering process makes soils acid.
• Man-made processes also contribute significantly to soil acidity. For example, Sulphur dioxide (SO2) and nitrogen oxides (NOx) released primarily by industrial activities react with water to form acid rain, which acidifies soils, particularly forest soils with.
• Organic acids from plants during decomposition;
• CO2 from root respiration and microbial respiration.
Effects of acid soil
• Major effects of extremes in pH levels include gaps in nutrient availability and the presence of high concentrations of minerals that are harmful to plants. In very alkaline soil, certain micronutrients such as zinc and copper become chemically unavailable to plants. In very acidic soil, macronutrients such as calcium, magnesium and phosphorous are not absorbed while others reach toxic levels,
• Acid soil, particularly in the subsurface, will also restrict root access to water and nutrients.
• In addition to affecting how nutrients are dispensed to growing plants, pH levels also influence microorganism activity that contributes to the decomposition of organic materials. A neutral pH is ideal for microbial action that produces chemical changes in soil, making nitrogen, sulfur and phosphorus more available. A pH that is either too high or too low may also interfere with the effectiveness of pesticides by changing their basic composition or weakening their ability to kill unwanted insects.
Plant growth and most soil processes, including nutrient availability and microbial activity, are favoured by a soil pH range of 5.5 – 8. Example: The optimal pH range for most plants is between 5.5 and 7.0. The optimal pH range for some plants is between is given in the table below.
For soils the pH should be maintained at above 5.5 in the topsoil and 4.8 in the subsurface.
The undesirable overgrowth of vegetation caused by high concentration of plants nutrients (Nitrogen and Phosphorous) in bodies of water (lakes, rivers,...)
As consequence, water plants (e.g: water hyacinth: amarebe) grow more vigorously and this prevents the sun light from reaching the water and stops photosynthesis of aquatic plants which provide oxygen in the water to animals needed then animals die, deposits of organic matter on the bottom of the lake build up.
When lake water is enriched with nutrients (e.g.: nitrates and phosphates), algal flourish, and produce an algae bloom, a green scum with an unpleasant smell. When algal die they are decomposed by aerobic bacteria. When the oxygen content falls too low to support aerobic bacteria, anaerobic bacteria take over. They convert the dead matter into unpleasant-smelling decay products and debris which falls to the bottom. Gradually, a layer of dead plant material builds up on the bottom of the lake. The lowering of the oxygen concentration leads to the death of aquatic animals (fish, crabs,…….)
Checking up 3.4
1. Ammonia itself can be used as a fertilizer but has some disadvantages. Explain the disadvantages of using ammonia as a fertilizer.
2. Give any two advantages of the use of
a. Natural Fertilizers
c. Artificial Fertilizers
3. Give any two causes of acid soils
3.5. Dangers of the use of substandard Fertilizers
Using books or internet find out the dangers of substandard fertilizers
Sub-standard fertilizer means any fertilizer which does not conform to the required NPK ratio.
Example: A fertilizer may be labelled 16-00-00, while the real NPK ratio is for example 25-00-00, 10-00-05, etc
Using these Fertilizers can lead to:
• Soil pollution (basic soil or acidic soil) due to accumulation of ions which are acidic or basic • Poor growth of plants
• Poor harvest
• Fertilizer burn: leaf scorch resulting from over-fertilization, usually referring to excess nitrogen salts. Fertilizer burn is the result of desiccation of plant tissues due to osmotic stress, creating a state of hypertonicity.
In order to reduce the effects of substandard fertilizers different measure can be taken;
• Standardization of the fertilizer before use
• Production of fertilizers in Rwanda, as this will help us to choose good minerals (where necessary) in producing fertilizers
• Use of chemical fertilizers with coated pellets so that nutrients are released slowly
• Regular watering
You provided with the following
1. A Solution prepared by mixing 5.0 g of a sample of ammonium sulphate fertilizer which were warmed with sodium hydroxide and the ammonia evolved was absorbed in 100 cm3 of 0.5moldm-3 sulphuric acid
2. 1M sodium hydroxide
a. Fill the burette with solution of sodium hydroxide
b. Pipette 20 cm3 of solutions of the prepared solution in (1), in conical flask. Add 2-3 drops of methyl orange indicator.
c. Titrate this solution with sodium hydroxide from the burette until the indicator changes colour (indicator changes from pink to yellow).
d. Record the results in the table.
UNIT 4: BENZENE
Key unit competence:
To be able to relate the chemistry and uses of benzene to its nature and structure
At the end of this unit , students will be able to:
• State the physical properties of benzene;
• Describe the uses of benzene;
• Outtline the preparations of benzene;
• Describe the chemical properties of benzene;
• State the conditions required for different reactions;
• Relate the conditions for the reactions of benzene to its chemical stability;
• Illutrate the mechanism of electrophilic substitutions on benzene.
From your prior studies in organic chemistry, it is known that carbon is tetravalent
while hydrogen is monovalent and compounds constituted by the two elements
are known as ‘hydrocarbons’. The structures and chemistry of the hydrocarbons
reflects to their uses as fuels and starting materials for many substances important
in life such as pharmaceutical drugs, solvents, packaging materials, clothes and
so on. In this activity you need to follow instructions given to explain how the
structure of a substance determines its chemical properties and uses.
1. Write down the molecular formulae for these five hydrocarbons
a. A molecule with 6 carbon atoms and 14 hydrogen atoms
b. A molecule with 6 carbon atoms and 12 hydrogen atoms
c. A molecule with 6 carbon atoms and 10 hydrogen atoms
d. A molecule of 6 carbon atoms with 8 hydrogen atoms
e. A molecule of 6 carbon atoms with 6 hydrogen atoms
2. From the molecules in 1) above, choose molecule(s) that fit(s) in the
description provided, and then draw its (their) structural formula (e).
a. Unsaturated hydrocarbon (s) that decolorize (s) bromine water and
alkaline potassium manganate (VII)
b. Saturated hydrocarbon (s)
c. Hydrocarbon (s) with empirical formula of CH
d. Unsaturated hydrocarbon (s) which do (es) not decolorize bromine
water and potassium manganate (VII).
e. Unsaturated hydrocarbon (s) that form(s) a white precipitate when
treated with ammoniacal silver nitrate and forms a reddish-brown
precipitate when treated with ammoniacal copper (I) chloride.
3. It is known that unsaturated hydrocarbons decolourise both bromine
water and alkaline potassium manganate (VII). Explain any assumption
you can suggest about the compound in question 2.d)
Some or all people are unique in their living attitudes and values. But being unique
does not mean to be isolated from others as people need each other in order to
complement and build a strong nation.
This is true for benzene. From the above activity question 3 you may have been stuck
while discussing on why this unsaturated compound has properties that are different
from other unsaturated hydrocarbons provided within the same activity. But this
does not mean that it is quite different from them. It will share some properties with
others but exhibit its identity or its unique properties from others.
In this unit, you will discover what makes benzene resistant towards some reactants
and its importance will be highlighted.
4.1. Structure of benzene
• Research in books or search engine about the structure of benzene.
• Read and make a summary on the historical development of benzene’s
Michael Faraday was the first to isolate benzene from coal. Benzene was found to
have the molecular formula of C6H6. However, its structural formula posed a problem
for many years.
For example, you can work out the structures of compounds whose molecular
formula is C6H6 and see how many you can find.
The structure of benzene must be only one, in which all the six hydrogen atoms
occupy equivalent positions. This was discovered by Friedrich August Kékulé Von
Stradonitz while daydreaming of a snake seizing its own tail. From this, he proposed a
ring structure of six carbon atoms with double bonds alternating with single bonds.
Furthermore, X-ray diffraction studies, first carried out by Kathleen Lonsdale, showed
that benzene is planar and all its C-C bonds are of the same length (0.139 nm which is
intermediate of C-C single bond and C=C double bond in alkenes) and bond angles
of the same size (120o).
By comparing benzene with alkenes, the following points are noticed:
• Benzene fixes 3 moles of hydrogen, thus it has 3 double bonds,
• Benzene does not decolourise bromine water or acidified potassium
manganate (VII) and does not turn green the acidified potassium dichromate,
• Benzene does not react with water and hydracids under normal conditions.
From the above points it can be easily noticed that benzene is not quite an alkene,
due to its double bonds which do not occupy fixed positions. This change of positions
of the double bonds is referred to as ‘resonance’.
The sp2 hybridized orbitals of carbon is involved in sigma bond formation with other
two carbon atoms and one hydrogen atom to make a hexagonal ring. The remaining
unhybrid p-orbital is involved in side-ways overlapping with a neighbor carbon
atom to form a pi-bond. Since there is an equal probability of making the pi-bond
with either neighbor carbon atom, pi-electron remains delocalized over six carbon
atoms of the ring.
Checking up 4.1
Discuss and provide appropriate answers for the following questions:
1. a. Benzene has the molecular formula C6H6. Draw the Kekulé structure for
this showing all the atoms.
b. Draw the skeletal structure showing the way the Kekulé structure is normally
2. How does the structure of benzene differ from the cyclohexane structure?
34.2. Physical properties, uses and toxicity of benzene
• Using the same resources (books or internet) as in activity 4.1, make a
research about the main points that should be talked about while discussing
the physical properties of any substance.
• Then, make a summary to be presented about properties, uses and toxicity
Benzene has the following physical properties:
• Benzene is a colourless volatile liquid with an aromatic (pleasant/sweet) smell.
• Benzene boils at 80.1 °C
• Benzene melts at 5.5°C.
• Like other aromatic hydrocarbons (arenes) benzene is insoluble in water.
• It is less dense than water (specific gravity or relative density is 0.88).. Describe the Structure and Bonding of Benzene
Benzene has many uses:
It has been used by chemists since 1800 because it is a good solvent for other organic
compounds. Benzene itself is an excellent solvent for certain elements, such as
sulphur, phosphorus, and iodine. It is found in crude oil. It is used to make plastics,
resins, synthetic fibers, rubber, lubricants, dyes, detergents, drugs and pesticides.
Benzene is highly toxic and is said to be carcinogenic.
A person exposed for long time to benzene (even at low levels), can develop anaemia
Benzene is formed in both natural and synthetic processes. Natural sources of
benzene include volcanoes and forest fires. It is a component of crude oil, petrol and
Checking up 4.2
1. Benzene is flammable and carcinogenic. What do you understand by
the term “carcinogenic”?
2. What advice can you give to your friend who smokes?
4.3. Preparation of benzene
• Some of the reaction of all alkanes and alkynes discussed in senior five lead
to the formation of benzene. Use the following examples to describe how
each of the following conversions can be carried out
1. From CH CH to C6H6
2. C6H6 from n-hexane
3. Ethanol to C6H6
• To add other methods used to prepare benzene and to be able to describe
them, use the same sources (books/search engines) as in previous activities
to discuss about all the methods that can be used to obtain benzene.
• Take a note to share with others.
• Some of the reactions of alkanes and alkynes discussed in senior five lead to
All the raw materials provided in the activity above are from the topics covered
in senior five, so hopefully you performed them very well. The methods used for
preparing benzene are based on reduction reaction and decomposition reaction and
even addition reaction.
1. Industrial preparation (on large scale)
a. From petroleum oils: By catalytic reformation of petroleum products
By fractional distillation followed by reforming. Fraction of naphtha is
heated over Cr2O3 – Al2O3 at 500-550oC and 15atm pressure (aromatisation).
When platinum is used at 15 atm pressure at 500oC, the process is called
b. By converting methylbenzene into benzene
Methylbenzene is much less commercially valuable than benzene. The
methyl group can be removed from the ring by a process known as
The methylbenzene is mixed with hydrogen at a temperature of between 550 and
650°C, and a pressure between 30 and 50 atmospheres, with a mixture of silicon
dioxide and aluminium oxide as catalyst.
c. From ethyne
When ethyne is heated in the presence of iron as catalyst or organo-Nickel, it undergoes
2. Laboratory preparation
a. From benzoic acid
In this method benzoic acid is heated with soda lime.
b. From benzenediazonium salt
In this method, the benzenediazonium salt formed by reacting phenylamine
with sodium nitrite and a mineral acid is treated with hyposphorous acid
(H3PO2) and water.
d. From cyclohexane
When cyclohexane is heated with Palladium or Platinum as catalyst and with
sulfur, it undergoes dehydrogenation forming benzene. When cyclohexane
is heated with sulphur, benzene is also produced.
Checking up 4.34.4. Chemical stability of benzene
Discuss and describe how you can obtain benzene starting with inorganic
reagents, showing necessary conditions at every step.
In chemical energetics (senior five), you learnt many forms of enthalpy changes
that take place when various reactions take place. In this activity, you have to
use some of the concepts of these enthalpy changes in order to understand the
stability of benzene. By following instructions provided as questions and using
the following data:
Enthalpy change of atomization of carbon, C(s): +715 kJ (mol of C atoms)-1
Enthalpy change of atomization of hydrogen, H2(g): +218 kJ mol-1
Bond energy of C=C (average): 610 kJ mol-1
Bond energy of C-C (average): 346 kJ mol-1
Bond energy of C-H (average): 413 kJ mol-1
Discuss and work out the enthalpy change of formation of benzene by the
following stages.1. Calculate the energy needed to produce
a. Six moles of gaseous carbon atoms from C(s)
b. Six moles of gaseous hydrogen atoms from H2(g)
2. Calculate the energy released when
a. Three moles of C-C bonds are formed from gaseous atoms
b. Three moles of C=C bonds are formed from gaseous atoms
c. Six moles of C-H bonds are formed from gaseous atoms.
3. Use your answers to  and  to calculate the total energy change when
a mole of gaseous benzene is formed from its elements.
4. Compare your answer with experimental value of +82 kJ mol-1.
5. Now, use the available resources (books or internet) to find out what you
can present about the stability of benzene.Benzene is an aromatic compound with molecular formula of C6H6. It is a planar
hexagonal ring with three pi-bonds in an alternate manner.The delocalization of pi-electrons in benzene molecule provides extra stability which
is known as ‘aromaticity’. Due to this aromaticity, benzene is more stable than
expected as compared to aliphatic alkenes or the cyclic alkenes with three double
bonds. Thus, it does not undergo addition reaction like alkenes do. In other words,
benzene is less reactive than alkenes for addition reactions as this type of reactions
can be responsible for loss of aromaticity (or resonance or stability). Benzene reacts
preferably through substitution reactions in which one of its bonded H-atoms is
replaced by an electrophile.Benzene is not the only aromatic molecule known (it is the smallest aromatic
molecule, others include naphthalene, anthracite,…). Thus, for a molecule to be
aromatic, it has to fulfill the following criteria:
It must be cyclic and flat
It must be conjugated (i.e, all atoms around the ring must be able to participate in
pi-bonding through resonance)
It must have pi-delocalised electrons (4n + 2), where n (number of benzene rings),
n= 0,1,2,3,4,5,6. This is known as Huckel’s rule.
The stability of benzene can be explained on the basis of resonance in the
molecule. There are two possible resonance structures (or forms) of benzene
molecule that are in equilibrium. Thus, an approaching reagent (such as bromine for
instance) can not be attracted to a double bond before the structures changes. The
resonance hybrid of benzene molecule is represented with a circle at the center of
hexagonal ring of carbon atoms as shown below:
Another measurement of stability of benzene is the tendency of benzene to undergo
electrophilic substitution reactions rather than electrophilic addition reactions as
alkenes. The regular-hexagonal planar ring of benzene is attributed to resonance
stabilization of this conjugated cyclic alkene. Two resonance structure of benzene is
responsible for the extra stability of molecule. The presence of the p electron cloud
makes a negative zone that could be attacked by electrophilic reagents, by giving
electrophilic substitution reactions.Thermochemical data show that benzene does not have true double bonds. The
theoretical heat of formation of gaseous benzene, taking into consideration 3
double bonds, is +252 kJ/mol while experimental value is +82 kJ/mol, therefore
the true structure is more stable by 170 kJ/mol than cyclohexa-1,3,5-triene (Kekulé
The enthalpy of hydrogenation of cyclohexane is -120 kJ/mol.
expected heat of hydrogenation is 3 times i.e. 3 x (-120) kJ/mol = -360 kJ/mol.
However the experimental enthalpy of hydrogenation of benzene is only -208 kJ/
mol! Therefore benzene is more stable by 152 kJ/mol than it would be if it was
cyclohexa-1,3,5-triene. This stabilization energy is called delocalisation energy or
Note: Because of this extra stability, benzene:
• Does not undergo reactions with halogens and halogen acids which are characteristic of alkene,
• Does not react with water in the presence of H+ and does not react with acidified KMnO4
• Cannot be represented by these structures because of its inertness
• Under drastic conditions, it however reacts with Cl2 or Br2 in the presence of ultraviolet light/light or halogen carrier,
• Reacts so fast with oxygen, by producing yellow luminous flame which is sooty.
Checking up 4.4
Refer to your results from the activity 4.4 to discuss and conclude on this:
Do your results support that real benzene is more or less stable than the Kekule structure? Explain your answer.
4.5. Reactions of Benzene
Activity 4.5 From the previous topics discussed in this unit, you have found that benzene has some uniqueness from aliphatic unsaturated compounds.
Use the same resources to find out
• How benzene reacts and
• Its reactions with different substances and their respective mechanisms
As seen in the previous discussions, since benzene contains carbon-carbon double bonds, it might be expected to undergo electrophilic addition reactions readily as it is the case for alkenes. This is not the case, however, and benzene does not decolourise bromine water. Neither does it readily undergo any other addition reactions.
The reason for this is that the delocalized system in benzene is stable, and addition reactions would break up this delocalization and lead to the formation of the products which are less stable than benzene itself. Benzene thus tends to undergo electrophilic substitution reactions rather than addition reactions.
4.5.1. Electrophilic aromatic substitution reactions
Aromatic compounds undergo substitution reactions with electrophiles in which one or more hydrogens of the benzene ring are substituted.
Since the reagents and conditions employed in these reactions are electrophilic, these reactions are commonly referred to as “Electrophilic Aromatic Substitution”. The catalysts and co-reagents serve to generate the strong electrophilic species needed to perform the initial step of the substitution.
Many substitution reactions of benzene have been observed and the five most useful are listed below.
The specific electrophile in each type of reaction is listed in the right hand column.
All electrophilic substitution reactions of benzene follow the same mechanism. After the formation of the electrophile, a two-step mechanism has been proposed for these electrophilic substitution reactions.
In the first, slow or rate-determining step the electrophile forms a sigma-bond to the benzene ring, generating a positively charged benzenonium intermediate. In the second, fast step, a proton is removed from this intermediate, yielding a substituted benzene ring.i
Briefly, electrophilic aromatic substitution reaction is realised in 3 steps:
1. Electrophile formation
2. Attack of the ring by electrophiles
3. Deprotonation = loss of H+
1. Halogenation Benzene reacts with chlorine or bromine in the presence of a catalyst, replacing one of the hydrogen atoms on the ring by a chlorine or bromine atom.
• The reactions happen at room temperature.
• The catalyst has to be a Lewis acid known as “halogen carrier”. The most commonly used catalysts are: aluminium (or iron) chloride, AlCl3/ FeCl3 or aluminium (or iron) bromide, AlBr3/ FeBr3 if you are reacting benzene with bromine.
Example: The reaction with chlorine ( Chlorination)
The reaction between benzene and chlorine in the presence of either aluminium chloride or iron gives chlorobenzene.
2. Friedel-craft-acylation Acylation involves substituting an acyl group, RCO-, into a benzene ring.
145Chemistry Senior Six Student Book
The most reactive substance containing an acyl group is an acyl chloride (also is known as an acid chloride). These have the general formula of RCOCl.
2. Friedel-craft-acylation Acylation involves substituting an acyl group, RCO-, into a benzene ring.
145Chemistry Senior Six Student Book
The most reactive substance containing an acyl group is an acyl chloride (also is known as an acid chloride). These have the general formula of RCOCl.
3. Friedel-Crafts Alkylation
This reaction involves substituting an alkyl group into a benzene ring. Hydrogen on the ring is replaced by a group like methyl or ethyl and so on.
a. Using haloalkanes
Benzene reacts with chloroalkanes in the presence of anhydrous AlCl3 or FeCl3 as a catalyst under reflux at 50oC to form alkylbenzenes
b. Using alkenes
Alkylbenzenes other than methylbenzene can be formed by reacting benzene with alkenes in the presence of HCl and AlCl3, under reflux at temperatures below 50oC. Mechanism:
Step 1: The alkene reacts with the HCl in the same way as in electrophilic addition reactions:
The carbocation behaves as the electrophile.
Step 2 and Step 3 proceed in the same way as in the alkylation reaction described above.
The overall reaction can be written as follows:
The more stable cation gives the major product, methylethylbenzene (or isopropylbenzene).
Nitration happens when one (or more) of the hydrogen atoms on the benzene ring is replaced by a nitro group, -NO2. Benzene is treated with a 50:50 mixture of concentrated nitric acid and concentrated sulphuric acid at a temperature not exceeding 50°C. The mixture is held at this temperature for about half an hour. Yellow oily nitrobenzene is formed.
C6H6 + HNO3 → C6H5NO2 + H2O
The concentrated sulphuric acid is acting as a catalyst and so is not written into the equations.
Step 1: Nitric acid is a less strong acid than sulphuric acid, and acts as a base as the electrophile is formed.
H2SO4 + HNO3→ H2O + NO2+ + HSO4
Step 2: The NO2+ is the electrophile and attacks the delocalised ring, breaking it temporarily:
Step 3: The delocalised system then reforms itself by pulling in the electrons from the C-H bond. The H+ recombines with the HSO4- to form H2SO4.
The overall reaction is C6H6 + HNO3→ C6H5NO2 + H2O
The sulphuric acid behaves as a catalyst. The product is known as nitrobenzene.
4.5.2. Some addition reactions and combustion reaction
The benzene ring can undergo addition reaction under drastic conditions, breaking down its resonance
3. Combustion reaction
As other hydrocarbons, benzene burns in air forming carbon dioxide (or carbon monoxide in a limited supply of air) and water.
Checking up 4.5
Discuss and find out the answers for the following questions: Benzene can be nitrated to form nitrobenzene, C6H5NO2.
a. Draw the structural formula for benzene and give its empirical formula
b. State the reagents needed for the nitration of benzene
c. An electrophile is formed during the nitration of benzene
i. What is the formula of this electrophile?
ii. Write an equation for the production of the electrophile iii. Use curly arrows to show the mechanism for the nitration of benzene
C6H6(l) + 15/2 O2(g) → 6 CO2(g) + 3 H2O(l) or C6H6(l) + 9/2O2(g) → 6 CO(g) + 3 H2O(l)
Checking up 4.5 Discuss and find out the answers for the following questions: Benzene can be nitrated to form nitrobenzene, C6H5NO2.
a. Draw the structural formula for benzene and give its empirical formula
b. State the reagents needed for the nitration of benzene
c. An electrophile is formed during the nitration of benzene
i. What is the formula of this electrophile?
ii. Write an equation for the production of the electrophile
iii. Use curly arrows to show the mechanism for the nitration of benzene
4.6. Nomenclature and positional isomerism in derivatives of benzene
Activity 4.6 1. Name the following molecules:
2. Discuss about rules for naming aromatic compounds in this book or any other source (textbook or internet). Then, make a summary to be presented.
As you have seen from the previous lessons of this unit, benzene and its derivatives are referred to as aromatic compounds. The following diagram provides the structures of some aromatic compounds starting with benzene with one ring and then others with more than one ring and their respective names:
Some benzene derivatives have their traditional or popular names such as the following:
• Di-substituted benzene derivatives with the prefixes “Ortho- or o-” for substituent groups on adjacent carbons (e.g, C1 and C2) in benzene ring. “Meta- or m-” for substituents separated by one carbon atom (e.g, C1 and C3). “Para- or p-” for substituent groups on carbons on opposite sides of ring (e.g, C1 and C4). The positions on the benzene ring are as follows:
Benzene derivatives consisting of two substituents attached to the ring could be distinguished among three positional isomers (ortho- , meta- and para- isomers).
These are named either by numbers or by using non numerical prefixes (ortho, meta and para).
Notice that there are 2 identical ortho positions (2, 6), and 2 identical meta positions (3,5).
Checking up 4.6
Discuss and provide appropriate answers to the following questions:
1. You are provided with C6H4Br2. Give three different structural formulae of isomers of C6H4Br2 and name them.
2. Provide all the structures and names of compounds having the same molecular formula as C6H5NO3.
Key unit competency
The learner should be able to relate aromatic ketones, aldehydes, carboxylic acids and amines to their chemical activity.
At the end of this unit , students will be able to:
• Explain the effects of substituent groups on the benzene ring;
• Give systematic names of aromatic compounds;
• Describe the preparation and reactions of phenol, benzoic acid, benzaldehyde, phenyl ethanone and phenylamine;
• State the uses of phenols;
• Describe the reactions of phenol, aromatic carbonyl compounds and carbox-ylic acids;
• Describe chemical properties of phenylamines;
• Explain the azo-coupling reactions of phenylamine in manufacture of dyes and indicators;
• Test and compare th acidity of phenol with alcohols and carboxylic acids;• Test for the presence of phenol in a given solution;
• Compare and contrast the alkalinity of phenylamines with aliphatic amines and ammonia.;
• Perform experiments on the reactions of phenol and phenylamine
The simplest and most important member of aromatic hydrocarbons is benzene (C6H6). The benzene ring is particular because of its stability and certain properties.Many important chemical compounds are derived from benzene by replacing one or more of its hydrogen atoms with another functional group. It is a typical compound from which many of compounds of common properties derive.
Some examples of derivatives of benzene are given below:
5.1. Effect of substituent groups on the benzene ring
The nature of a substituent already present in the benzene ring, not only determines the position of the next incoming group but also influences the rate of the second substitution reaction compared to the rate of substitution in benzene itself.A substituent might increase the rate of the second substitution, i.e. make the ring more reactive relative to benzene. Another group if present in benzene ring could decrease the rate of further substitution, i.e. make the ring less reactive compared to benzene.
Groups already on the ring affect both the rate of the reaction and the site of attack. We say, therefore, that substituent groups affect both reactivity and orientation in electrophilic aromatic substitutions.
5.1.1. Deactivating and activating substituents
We can divide substituent groups into two classes according to their influence on the reactivity of the ring. The substituents which cause the compounds to undergo second substitution faster than benzene are called Activating Substituents (electron-releasing groups); they increase the electronic density on the benzene ring.On the other side, substituents which retard the rate of further substitution are referred to as Deactivating Substituents (electron-withdrawing groups); they decrease the electronic density on the benzene ring.
5.1.2. Directing the incoming substituents
During the formation of monosubstituted products in benzene, the electrophile can be attached at any position on the benzene ring. But, when the monosubstituted product is to be converted into disubstituted one, the existing substituent present in the ring directs the incoming group to a particular position. This is referred to as directive influence of the group. Depending on their directive influence, various groups (substituents) can be divided into two categories:
• Ortho and Para Directors
• Meta Directors
a. Ortho and para directors
These direct the new substituents to enter the ring primarily in Ortho and Para positions to themselves. These groups increase the electron density at the ring. Thus the reactivity of benzene ring towards electrophilic substitution reactions increases. For example if we carry out nitration of toluene, the mixture of ortho and paranitrotoluenes is formed
These direct the new substituents to enter the ring primarily in Meta position to themselves. For example, the nitration of benzoic acid produces m-nitrobenzene.
These groups withdraw the electrons from benzene ring through resonance effect, reducing the electron density at the benzene ring. They decrease the reactivity of benzene ring towards electrophilic substitution reaction and make it less susceptible to the electrophilic attack.
It has been found experimentally that in general ortho-para directing substituents activate the benzene ring and thus enhance the rate of reaction with electrophiles. On the contrary, the meta directing substituents deactivate the ring and retard the rate of reaction as compared to unsubstituted benzene.
When the substituent present in the ring, has one or more lone pairs of electrons on the atom attached to the ring, it interacts with pi-electron system of the ring and it acts as electron donor (electron-donating substituent).The presence of an electron-donating group such as –OH or -NH2 causes further electrophilic substitution in ortho-para positions and also activates the ring to electrophilic attack.Let us take the example of phenol (C6H5-OH) and aniline (C6H5-NH2) which have available electron pairs on the atom directly attached to benzene ring. Thus phenol and aniline exhibit resonance and can be represented as hybrid of the following forms:
- Why activating substituents (Activators) have ortho and para directing properties?
In the above two examples, positions 2 and 4 are relatively richer in electrons than position 3 and this makes them susceptible to electrophilic attack. The electrophile would attack the ring preferentially at ortho and para positions where the electron density relatively is greater as compared to the meta positions. The second electrophile will be directed where sites are negatively charged, i.e ortho and para positions.
From the above considerations, we conclude that all groups which are electron-donating are ortho-para directing and facilitate electrophilic substitution in the benzene ring.
• Why Deactivating Substituents (Deactivators) have meta-directing prop-erties?
When the substituent has at least one strongly electronegative atom and a multiple bond in conjugation with benzene ring, the substituent acts as electron- withdrawing substituent.
Consider the nitrobenzene which contains –NO2 is able to exist as the following resonance forms:
In the above example, it may be noted that resonance causes the decrease of electron density in the ring of nitrobenzene, and specifically at the ortho and para positions.
In general, the electron withdrawing substituents decrease the electron density of benzene ring and thereby act as deactivators and meta-directors.
• Anomalous Behaviour of Halogen Substituents
The resonance effect enables the halogen substituents to act as ortho and para director. It is also expected to activate the ring to electrophilic attack, but on the contrary it is a ring deactivator. This is attributed to the very high electronegativity of the halogens due to which they withdraw electrons so strongly that they deactivate the benzene ring.
While the resonance effect accounts for the ability of halogen to donate electrons to ortho and para positions, the combination of the two effects makes the halogenated benzene deactivated.
The phenols are organic compounds with one or more -OH-OH- groups directly attached to a carbon atom in a benzene ring. The following are examples of phenols:
Phenols occupy an important position in the modern synthetic organic chemistry for the preparation of dyes, antioxidants, phenolic resins and certain pharmaceutical products.
The most important member in this family is phenol (hydroxybenzene):
Phenol (hydroxybenzene) is a colorless crystalline solid which melts at 43oC and boils at 182oC. On exposure to air or light, it becomes coloured due to oxidation.
Phenol is soluble in organic solvents and slightly soluble in water at room temperature, but infinitely soluble above 66 °C.
Phenol exhibits intermolecular hydrogen bonding and this makes its melting point higher than that of hydrocarbons of comparable molecular mass.
5.2.1. Sources and preparations of phenol
Phenols are common in nature; examples include tyrosine, one of the standard amino acids found in most proteins. Many of the more complex phenols used as flavourings and aromas are obtained from essential oils of plants. Other phenols obtained from plants include thymol, isolated from thyme, and eugenol, isolated from cloves.
Phenol, the cresols (methylphenols), and other simple alkylated phenols can be obtained from the distillation of coal tar or crude petroleum
Phenol can be prepared:
a. From benzenesulfonic acid
In this method, benzenesulphonic acid obtained from sulphonation of benzene reacts with sodium hydroxide to produce phenol.