Chemistry

Key unit competence: Compare and contrast the chemical properties of the Group 1 elements and their compounds in relation to their position in the Periodic Table.

Learning objectives

By the end of this unit, I will be able to:

•Describe and explain the physical properties of Group 1 elements in terms of metallic character and strength of metallic bond

•Describe and explain the reactivity of Group 1 elements with oxygen, water and halogens•State and explain the properties of Group 1 oxides and hydroxides

•Explain the trends in the solubility of Group 1 compounds

•State the uses of Group 1 elements and their compounds

•Compare the reactivity of Group 1 elements•Interpret the trends in the thermal decomposition of Group 1 carbonates and nitrates

•Perform experiments to test the alkalinity of Group 1 hydroxides•Carry out flame test for the presence of Group 1 metal cations in solution.

6.1. Occurrence and physical properties of group 1 elements, physical state, metallic character, physical appearance and melting point

Activity 6.1.a

1. Study the following table of data and answer the questions that follow

Data

a) What’s meant by 1st ionization energy ?

b) Explain the trend in 1st ionization energy for group 1 metals down the group.

c) What is the relationship between melting point and atomic radius in group 1 element? Explain this trend.

2. What do you know about Group 1 elements?

3. Do you know any compound of a group 1 element? If yes, what is its application in our daily life?

Group 1 elements are: lithium (3Li), sodium(11Na), potassium(19K), rubidium(37Rb), cesium(55Cs) and Francium(87Fr); they are characterised by valence electronic structure ns1 and that is why they are called s-block and Group 1 elements.

Although hydrogen has the same valence of ns1, it behaves differently from the others in many aspects; for example: hydrogen is a gas at room temperature whereas all other members are solid. Hydrogen behaves as a metal in some compounds when it combines with non-metals, and as a non-metal when it combine with very active metals. For this reason, the study of hydrogen is presented separately from the other members of group 1 elements.

Francium is exceptionally rare. It is formed by the radioactive decay of heavier elements.

Because Francium is both rare and highly radioactive, few of its properties have been determined and we are not going to talk much about it in this Unit.

Physical Properties of Alkali Metals

Activity 6.1.b

In groups, learners make research in libraries / internet and discuss on the physical properties of group 1 elements and or explain the following statements

a) Group 1 elements show weak metallic bonding

b) Atomic radius of Na is smaller than the corresponding ionic radius of Na+

c) The shining appearance of metals disappears after a certain period of time.

Group 1 elements are grey metals, soft, and can be easily cut with a knife to expose a shiny surface which turns dull on reaction with oxygen in air.

1. They have low melting and boiling points. They show relatively weak me-tallic bonding as only one valence electron is attracted by the nucleus. Also they have a big atomic radius and the attraction of nucleus toward the va-lence electron is weak.

2. They are good conductors of heat and electricity

3. They have a low ionisation energies that decreases down the group

4. They have low density compared to other metals and Li, Na and K are less dense than waterGroup 1 metals color flames: When alkali metals are put in a flame they produce characteristic colors.

Table 1: Physical properties of group 1 elements

Checking up 6.1

1. Discuss how the ionization energies vary in function of atomic radius for group 1 elements.

2. Is there any relationship between the atomic radius and the melting point in group 1 elements? Yes or No. Justify your answer

3. Why group 1 elements are said to be good conductors of electricity? Illustrate your answer

4. The following table shows 3 unknown group 1 metals X, Y, Z and some of their physical properties. Predict among the alkalis metal (Cs, Li, K), which one should correspond to X,Y,Z. Justify your answer

6.2. Reactivity of group 1 elements with oxygen, water and halogens

Activity 6.2 (a)

Analyse the case study below and answer the related question:1. In groups learners discuss about the following scenario and compare their findings to the reactivity of a chemical element in term of the variation of atomic radius

Suppose that a hen walking in the garden with its chicks. Some of the chicks are feeding themselves just near the hen (mother). Other chicks are feeding themselves far away from the mother. Which ones of the two groups of chicks will be an easy prey of a predator? Explain.

Now extend your reasoning to the behavior of group 1 element and explain the following statement

Group 1 elements react by losing their single electron in outermost shell. Arrange them in order to show which one loses easily the single electron and which one loses electron with difficult.

2. Consider a case of people who are warming around a fire; some are near, other are a little bit far; who will feel the heat of the fire more than the other?

These activities show the distance between two object affect interactions between them. The hen mother cannot protect the chicks that are far from her. In the other case, people who are far from the fire feel less the heat of the fire.

This resembles to the interactions between the nucleus and the valence electrons. The attraction between the nucleus and the valence electron decreases with increasing distance between the nucleus and the valence electrons.

When the atomic radius and volume increase, the distance between the nucleus and the valence electrons increases, the attraction between the nucleus and the valence electrons decreases, and it becomes easier to remove the valence electrons.

This explains the origin of properties of group 1 metals: they have low ionization energy; lose easily the only one valence electron to form a mono-positive cation (M+) with a rare gas electronic structure, and consequently are very active metals.

Activity6.2 (b)

a) Given the following element of group 1(Na (Z=11), Li(Z=3), Cs(Z=55), K(Z=19) , Rb (Z=37). Arrange them according to their increasing reactivity and justify why.

b)Establish the electronic structure for the following species and explain how you get it Na+,Na2+,Na-.Which one is stable and why?

6.2.1. Reactions with Oxygen

Activity 6.2.1

Experiment: burning an alkali metal in air/oxygen

Apparatuses: deflagrating spoon, Bunsen burner, glass beaker, filter paper,

Chemicals: lithium, sodium, potassium, water, and red litmus paper

Other requirements: knife, match box and petroleum gas

Procedure:

1. Cut a small piece of lithium and wrap it in piece of filter paper to remove the oil.

2. Place it on to a deflagrating spoon and heat it in a non luminous flame.

3. Observe what happens

4. When combustion is complete, dip the deflagrating spoon into a beaker of 100 ml filled up to 50 ml of water.

5. Stir the water with the spoon and then drop a piece of litmus papers into the solution in the beaker. Observe

6. Repeat the experiment with sodium and potassium

Task on the experiment:

a) Write the equations of reactions that take place when each metal is burnt in air

b) Name the product that was formed in each case.

c) What are the color changes when the aqueous solutions above are tested with litmus paper? Explain why?

d) Write the equations of reaction between each product in (b) with water

Alkali metals form oxides when they burn in air or in oxygen. The nature of the oxides varies among the elements. For example lithium forms normal oxide (Li2O), sodium forms peroxide (Na2O2) and the remaining elements mainly form superoxides (MO2) with very little amounts of peroxides:

The reason for formation of peroxides and superoxide as we move down the group is due to increased reactivity of Group 1 metals down the group.

Oxides formed by group 1 metals are highly soluble in water giving strongly alkaline solutions:

Hence Group1 metals form basic oxides.

All group 1 metals are active metals, but there are differences within the group. The reactivity increases down the group due to the increase of atomic radius which results in the decrease of attraction between the nucleus and the valence electron, which requires less energy to remove the electron down the group.

6.2.2. Reaction with water

Activity 6.2.2

Experiment to investigate the reaction of alkali metals with water

Procedure:Cut a small piece of sodium metal and put it on water in wide beaker and observe.Test the obtained solution with red and blue litmus papers and observe.

Questions

1. Why sodium is so easy to cut?

2. What are the observations when sodium is placed on water in the beaker

3. Explain the observation when tests with red and blue litmus papers are performed on the resulting solution and write the equation of the reaction that takes place to explain you answer

Alkali metals react with water and form hydroxides along with liberation of hydrogen:

Where M represents any group 1 metal, MOH represents the corresponding hydroxide such as sodium hydroxide. This reaction explains why it is said that group 1 metals reduce water (where the ion H+ from dissociation of water is reduced to H2molecule). This reaction where group 1 metals react with water to form an alkaline solution is at the origin of their name of “alkali metals”.

As said previously, the reactivity of Group 1 metals increases as we move down the group.

The reaction with water can be used to illustrate the increasing reactivity on descending the Group. Li reacts slowly with water, with effervescence; sodium reacts violently fizzing (a hissing sound is heard) and skating about on the water surface, a colorless gas is evolved; K reacts more violently and ignites on contact with water; Cs sinks in water, and the rapid generation of hydrogen gas under water produces a shock wave that can shatter a glass container.

The chemistry of Li shows some anomalies, as the cation Li+ is so small that it forms covalent compounds.

6.2.3. Reaction with halogens

Activity 6.2.3

a) In terms of s, p, d, f orbitals write the electronic configuration of chlorine (Z=17), bromine (Z=35) and iodine (Z=53)

b) Deduce the valency of each element.

c) Write molecular equations, complete and balance them, when

Sodium reacts with bromine        Potassium reacts with iodine            Lithium reacts with chlorine

Alkali metals react with halogens very easily forming halides. The reactivity of alkali metals with halogen increases from Li to Cs.

Elements of group 1 are highly reactive and form a variety of compounds. Two of the most important are Oxides and Hydroxides.

6.3.1 Oxides

Alkali metals form oxides when they burn in air or in oxygen as seen in point (6.2.1):

They are also formed by thermal decomposition of corresponding hydroxides

Checking up 6.2

1. An element J has 19 as atomic number while the element A has 35 as atomic number.

a) Write their electronic structures in term of s, p, d, f orbitals and deduce their respective valencies.

b) The element J is able to react with oxygen gas by forming two types of oxides

i) Write the formula of the 2 oxides that can be formed between the ele-ment J and oxygen

ii) Write the formula of the compound formed between J and A

iii) What type of bond does exist between J and A. Justify your answer.

c) Show how you would write the equation of reaction between J and water supposing that J stands for the real symbol of the element.

d) When the reaction stated in (c) takes place a colorless solution and a colorless gas are formed.

i) Which test would you use to identify each product of the reaction, by stating the reagent and related observations?

Oxides of Group 1 metals dissolve in water to give strong alkaline solutions; that is why they are said to form basic oxides.

6.3.2. Hydroxides

As said above hydroxides are formed when the metals or metals oxides are dissolved in water. In solid state, these hydroxides dissolve very easily in water and in alcohol. They dissociate completely in water to form alkaline solution; hence they are strong bases. The basic character of the hydroxides increases as we move down the group.

Checking up 6.3

1. Complete and balance the reactions when water is reacted with the following:

a) Potassium metal

b) Potassium oxide

2. Lithium hydroxide decomposes on heating. White powder X and a colorless gas Y is released and condenses in a colorless liquid.

i) Write the chemical formula of X.

ii) Propose a chemical test to identify Y

3. Explain why Group 1 metals form ionic compounds?

6.4. The effect of heat on Group 1 carbonates and nitrates

6.4.1. Heating the nitrates

Activity 6.4.1

Experiment: effect of heat on nitrates.

In groups learners perform the following experiment, discuss and make conclusions by explaining the observed phenomena, and write involved chemical reactions

.Apparatus: glass test tubes, pair of tongs, wooden splint/match stick, Bunsen burner/heat source and spatula.

Chemicals: Lithium nitrate, potassium nitrate Other requirements: match box

Procedure:

I.1.Take two spatula end full of lithium nitrate into a test tube and heat it strongly until there is no further change.

2. Test the gases evolved with a damp blue litmus paper and a glowing splint.

3.Observe and make conclusions on your observations.

II .Repeat the procedure but using potassium nitrate/sodium nitrate

Group 1 compounds are generally stable on heating.

Group 1 nitrates (except lithium nitrate) decompose on heating in Bunsen flame to form their corresponding nitrites (which are stable) and oxygen

All nitrates from sodium to cesium decompose in this same way.

Lithium nitrate behaves differently, producing lithium oxide, nitrogen dioxide and oxygen due tothe small size of Lithium leading to high polarizing power and formation of compounds with covalent properties.

Checking up 6.4.1

a) Differentiate between KNO3 and LiNO3 in terms of their thermal decomposition.

b) One of the two compounds decomposes releasing a colored gas that turns damp blue litmus paper to red.

i) State which one.

ii) Give the name, the formula and the color of that gas

6.4.2. Heating the carbonates

Activity 6.4.

2Experiment: effect of heat on carbonates of group 1 elements

In groups learners perform the following experiment, discuss and make conclusions by explaining the observed phenomena, and write chemical reactions.

Apparatus: glass test tubes, pair of tongs, Bunsen burner/heat source and spatula.

Chemicals: lithium carbonate, calcium carbonate, potassium carbonate and sodium carbonate, lime waterOther requirements: match box.

Procedure

I.1 Take a spatula end full of lithium carbonate into a test tube and heat it strongly until there is no further change.

2. Test the gases evolved with a damp blue litmus paper and lime water into an-other glass test tube as shown in the figure

3. Observe and make conclusions on your observations.

II .Repeat the procedure but using calcium carbonate, potassium carbonate /sodium carbonate

Most carbonates of group 1 resist to heat.

However in Group 1, lithium carbonate decomposes on heating, producing lithium oxide and carbon dioxide.

The remaining of the Group 1 carbonates does not decompose at Bunsen flame temperatures.

Checking up.6.4.2

Li2CO3 decomposes on heat releasing a colorless gas.

a) Write the equation of thermal decomposition of Li2CO3

b) What do you observe when the evolved gas is tested by lime water and damp blue litmus paper? Explain

6.5. Solubility of group 1 compounds

Activity 6.5

a) Group 1 elements form ionic compounds.

(i) Explain why.

(ii)State the properties of ionic compounds

b) Explain why lithium forms components with a covalent character contrarily to other component of the same group. State the properties of covalent com-pounds

c) Both hydroxides and carbonates of lithium are less soluble than other hydrox-ides and carbonates of group 1.Why?

All group 1 salts and hydroxides are soluble in water.However LiCO3, LiOH are less soluble of group 1 compounds due to their high covalent properties.

6.5.1. Carbonates

The carbonates of Group 1 metals are all very soluble - increasing to an astonishing 261.5 g per 100 g of water at 20oC temperature for cesium carbonate.

The least soluble Group 1 carbonate is lithium carbonate. A saturated solution of

Li2CO3 has a concentration of about 1.3 g per 100 g of water at 20°C.

Solubility of the carbonates increases as you go down the group.

6.5.2. Hydroxides

Hydroxides of Group 1 are even more soluble.

The least soluble hydroxide in Group 1 is lithium hydroxide - but it is still possible to make a solution with a concentration of 12.8 g per 100 g of water at 20°C. Solubility of the hydroxides increases as you go down Group 1.

6.5.3. Sulphates

The solubility of sulphates behaves in different way from carbonates and hydroxides. The solubility of group 1 sulphates decreases as you go down the group.

Checking up 6.5

Arrange the following compounds in ascending order of their solubility and explain why:

a) RbCl, NaCl, LiCl, KCl, CsCl

b) Cs2SO4, Li2SO4, Na2SO4, Rb2SO4, K2SO4

6.6. Flame Test for Li+, Na+ and K+

Activity 6.6:

Experiment: Flame test of alkalis metals

Materials: Mortar and pestle, beakers, Lithium carbonate, potassium sulphate, sodium sulphate

Procedure: Flame test wire /magnesia rod

NB: Wear your safety glasses.

Dip the flame test wire/magnesia rod in the salt to be tested. Some of the salt should stick to flame test wire/magnesia. Gently wave the flame test wire/magnesia rod in the flame of the Bunsen burner and note the color given of

Examples of flame test

Repeat the experiment for each of the other salts. To avoid cross contamination, use a separate splint/flame test wire for each salt. Again, note the color in each case. If you are given an unknown salt, you should be able to identify the metal in the salt from the results of your experiment. The following are the flame colors for respectively K, Na, Li

Below are examples of flame tests for Na, K, and Li.

Student Questions

What color is observed in each case when the following salts are heated in a flame: lithium carbonate, sodium sulfate, , potassium sulfate?

Alkali metals color flames

When the element or its compound isput in a flame, the heat provides sufficient energy to promote the electron of the metal (or metal ion) to a higher energy level. On returning to the ground level, energy is emitted in form of light that has a wavelength in the visible region that is seen as color.

Table 2: Group 1 metals and their corresponding flame colors

Checking up 6.6

Describe a chemical test to distinguish between 2 salt solutions below and predict expected results for:

a) Solutions of CsCl and KCl

b) Solutions of NaCl and LiCl

6.7. Uses of group 1 elements and their compounds

Basically, the use or application of any elements is determined by the chemistry and the physical properties of the particular element. Hereafter are some of the uses and applications of group 1 metals.

Activity 6.7

Make research on internet and in library about the use of group 1 elements and present your findings

•Lithium: Used for making alloys especially with aluminium to make aircraft parts, which are light and strong.

•Due to its size and electropositivity (opposite of electronegativity), lithium is used in both primary and secondary lithium batteries. Lithium is used in light weight electrical batteries of the type found in clocksand watches, heart pacemakers (a small piece of electronic equipment connected to someone’s heart to help the heart muscles move regularly.

•Lithium carbonate, Li2CO3, is used to toughen glass.

•Caustic soda, NaOH, and soda ash, Na2CO3 are the most important alkali used in industry. Both find applications in paper making, alumina, soap, and rayon.Na2CO3 (soda ash) is used in water treatment.

•NaOCl is used as bleaching agent and disinfectant

•NaCl is used in seasoning food, preparing hydrogen chloride gas, in soap production, manufacture of sodium, chlorine, sodium hydroxide and sodium carbonate.

•Molten sodium is used as a coolant in 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 purpose.

•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.

•Sodium vapor lamps are used for street lighting; the yellow light is characteristic of sodium emission.

•Sodium amalgam (alloy with mercury) and sodium tetrahydridoborate, NaBH4, are used as reducing agents.

•Sodium cyanide is used in the extraction of silver and gold.

•Lithium: Used for making alloys especially with aluminium to make aircraft parts, which are light and strong.

•Due to its size and electropositivity (opposite of electronegativity), lithium is used in both primary and secondary lithium batteries. Lithium is used in light weight electrical batteries of the type found in clocksand watches, heart pacemakers (a small piece of electronic equipment connected to someone’s heart to help the heart muscles move regularly.

•Lithium carbonate, Li2CO3, is used to toughen glass.

•Caustic soda, NaOH, and soda ash, Na2CO3 are the most important alkali used in industry. Both find applications in paper making, alumina, soap, and rayon. Na2CO3 (soda ash) is used in water treatment.

•NaOCl is used as bleaching agent and disinfectant

•NaCl is used in seasoning food, preparing hydrogen chloride gas, in soap production, manufacture of sodium, chlorine, sodium hydroxide and sodium carbonate.

•Molten sodium is used as a coolant in 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 purpose.

•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.

•Sodium vapor lamps are used for street lighting; the yellow light is characteristic of sodium emission.

•Sodium amalgam (alloy with mercury) and sodium tetrahydridoborate, NaBH4, are used as reducing agents

•Sodium cyanide is used in the extraction of silver and gold.

6.8 Hydrogen

Although hydrogen has the same valance electronic structure as Group 1 elements, H[1s1], it is not generally studied with the other elements of the group. Even some Periodic Tables do not put H in Group 1 column.

This is due the fact hydrogen has specific properties that distinguish it from the other elements of Group1. Hydrogen is the smallest chemical element, with only 1 electron in its electronic structure. Hydrogen is a gas, whereas other Group 1 elements are metals, all solid at room temperature. In some compounds with non-metals, hydrogen tends to lose electron and form polar compounds such as Hδ+Clδ-,

but when it reacts with active metals, it behaves as a non-metal and captures electron to form hydride such as Na+H-

.All this justifies why the study of hydrogen is separated from the study of other Group 1 elements.

6.8.1. Properties of hydrogen atom

Hydrogen is the first element in the periodic table. It is the simplest and smallest element of the chemical elements.

It is formed by only one electron and one proton for the common hydrogen(11H), with the simplest nucleus. Its electron configuration is 1s1(similar to the electron configurations of group 1 elements).Hydrogen is colorless, odorless, and tasteless gas.

Table 3: Properties of hydrogen

Hydrogen exists as a diatomic molecule in its gaseous state, H2.

H2 is small and non-polar, so H2 molecules can only attract each other through weak van der Waals forces.Hydrogen is the most abundant element in the Universe and accounts for 89% of all atoms. There is little free hydrogen on Earth because H2 gas is so light that it moves very fast and can escape the Earth’s gravitational pull

6.8.2. Production of hydrogen

a) From methane gas

Example:

b) Electrolysis of water

c) Laboratory preparation

Metals are reacted with dilute acids. Group 1 metals are not used because the reaction with acid is violent and dangerous.

6.8.3. Uses of hydrogen

One third of the hydrogen produced is used for hydrometallurgical extractions of copper and other materials since hydrogen is a reducing agent. Half of the hydrogen is used in manufacturing of ammonia.

Hydrogen is also used in the manufacture of saturated oil (solid), such as margarine, from unsaturated oil (liquid) by hydrogenation reaction.

Hydrogen is used as fuel in space rocket/space shuttles.

6.8.4. Chemical properties of hydrogen

Hydrogen can form both cations (H+) and anions (H-). It has an intermediate electronegativity (2.1). Hydrogen forms covalent bonds with nonmetals where it bears a partial positive charge, Hδ+, and ionic hydrides with very active metals where it forms a hydride ion H-.

Examples: (a) Polar covalent compounds: NH3, H2O, HF, and SiH4

(b) Ionic compounds: CaH2, NaH, KH

When bonded to very electronegative elements, O, F, N, hydrogen is responsible of hydrogen bonding, a strong force between very polar covalent groups having O-H, F-H, and N-H present in a molecule.

6.9. End unit assessment

1. a)Arrange the following salts from the least soluble in water to the most soluble and justify your choice: KCl,NaCl, CsCl ,LiCl, RbCl.

b) As student in research you would like to distinguish between the above salts. Which quick chemical test will you carry out and what observations do you expect from that experiment?

2. Sodium is reacted with water:

a) What will happen?

b) Write the equation of the reaction between Na and water

c) How would you test for the presence of the products formed?

3. You are provided with the following chemical compounds KNO3, Li2CO3, LiNO3, and K2CO3.Identify which one is described as follows:

a) A soluble compound in water, but which does not decompose on heat

b) A soluble compound in water, but which decomposes on heat releasing a gas that relights a glowing splint.

c) A soluble compound in water, but which decomposes on heat releasing brown fumes and a colorless gas that supports combustion.

d) A slightly soluble compound in water, but which decomposes on heat and releases a colorless gas that turns lime water milky.

4. Group 1 consists of the elements Li, Na, K, Rb, and Cs.

a. The first member of the group often shows anomalous properties. Give two properties in which the behavior of Lithium is abnormal and explain why.

b. How does each of the following properties of the elements in Group 1 change with the increasing atomic number? Explain why.

i. Atomic radius

ii. Ionization energy

iii. Reducing properties

iv. Reactivity with water

v. Electronegativity

c. How will successive ionization energies of Na vary?

d. Why is the Na+ ion formed in normal chemical reaction rather than Na2+?

e. How are ionization energies related to the reactivity of these elements?