Chemistry

Key unit competence: Compare and contrast the chemical properties of the Group 13 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:

•State the physical properties of Group 13 elements

•Explain the reactivity of Group 13 elements with oxygen, water, halogens, dilute acids and sodium hydroxide

•Describe the properties of oxides, hydroxides and chlorides of Group 13 elements•State the uses of Group 13 elements and their compounds

•Compare and contrast the reactivity of Group 13 elements with oxygen, water, halogens, dilute acids and sodium hydroxides

•Perform experiments to show the solubility of Group 13 compounds

•Practically illustrate the amphoteric properties of aluminium oxides and hydoxides

•Identify the anomalous properties of boron and its compounds

•Perform chemical tests for the presence of aliminium ion in the solution.

The elements of group 13 of the periodic table are boron (5B), aluminium (13Al), gallium (31Ga), indium (49In), and thallium (81Tl). Group 13 elements have a general outermost electronic configuration of ns2 np1 .This group marks the beginning of the p-block elements. It is sometimes called the Boron group.

8.1. Physical properties of group 13 elements

Activity 8.

1In groups learners make research in library or on internet, discuss and explain why

a. Group 13 elements have higher melting point than group 1and 2 elements.

b. Boron has higher ionization energy than other element of the same group.

With the exception of boron, group 13 elements are metals. Boron is a non-metal element with high melting point and low density.

Aluminium is a metal element and has a low density, it is a good conductor of heat and electricity, shiny, malleable, ductile and it has higher melting point than groups 1and 2 metals due to strong metallic bond resulting from 3 valency electrons involved in making metallic bonding in aluminium metal.

In small atoms electrons are held tightly and are difficult to remove, while in large ones they are less tightly held since they are far away from the nucleus and are easy to remove so that the ionization energy decreases down the group as the atomic radius increases.

The greater the forces of attraction and hence the boiling point and melting point decrease down the group as the atomic radius increases.

Table1: Physical properties of Group 13 elements

The 1st IE of boron is higher than the other member of the group due to its small size.

Checking up 8.1.

1a)Write the electronic structure of Al(Z=13).

b) Write the equation that shows how aluminium forms its ion.

c) Explain why aluminium forms Al3+ ion instead of Al1+2. Explain the cause of the strong bond in aluminium metal

3. Within the elements of group 13 elements down the group give and explain the relationship between atomic radius and:

i)the boiling point

ii) ionization energy

4.The following table shows 3 unkown group 13 elements A,B,C and some of their physical properties. Predict among the group 13 elements whichone should correspond to A,B,C.Justify your answer

5. Match the element in column A to the corresponding atomic radius in column B.Justify your answer

8.2. Reaction of aluminium

8.2.1. Reaction of aluminium with oxygen

Activity 8.2(a)

In groups learners make research, discuss and explain the following statement:

(a)aluminium is a metal therefore it forms compounds with the covalent character.(

b) aluminium forms amphoteric oxide.

Activity 8.3 (b)

Experiment: burning an aluminium metal in air/oxygen and amphoteric properties of Al2O3

Apparatuses: deflagrating spoon, Bunsen burner, glass beaker

.Chemicals: aluminum sheet, hydrochloric acid, sodium hydroxide Other requirements: match box and petroleum gas

Procedure:

1. Cut a small piece of aluminium

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

3. When combustion is complete, divide the solid residue into two portions and place each portion in two different glass test tubes

4. Add 10ml solution of HCl To the first test tube and 10 ml of NaOH to the second test tube. Observe

Study questions

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

b) Name the product that was formed.

c) Write the equations of reaction between the product in (b) with HCl and with NaOH

Aluminium burns in oxygen to form aluminium oxide

•In its compounds, Aluminum occurs exclusively in the +3 oxidation state. It rapidly reacts with oxygen in air to give water in soluble coating of Al2O3. This oxide layer protects the metal beneath from further corrosion.

•All other Group 13 elements also produce compounds of the formula M2O3:

) where M represents Al, Ga, In, or Tl

Aluminum, gallium, and indium have +3 as the most stable oxidation state, whereas thallium has +1 oxidation state as the most stable.

The most common oxide form of boron, B2O3 or boron trioxide, it is produced by heating boric acid:

8.2.2. Reactions with acids

Activity 8.2 (c)

Reaction of aluminium with acids

Experiment

Learners perform experiments to investigate the reaction of aluminium with moderately concentrated HCl on warming and concentrated H2SO4

Apparatuses: Bunsen burner, gas jar, thistle, water bath, delivery tube, wooden splint

Chemicals: aluminum sheet, concentrated hydrochloric acid concentrated H2SO4, solu-tion of KMnO4. Other requirements: match box and petroleum gas

Procedure:

Put aluminium powder into two different flat bottomed flasks and connect on each a thistle, delivery tube as shown on the figure A and B.

a. Pour in the thistle of the apparatus A moderately concentrated HCl open the tap of the thistle and let HCl flow on Aluminium powder in the flat bottomedflaskCollect the gas evolved and test the gas using a burning wooden splint.

b. Pour in the thistle of the apparatus B concentrated H2SO4, open the tap of the thistle and let H2SO4 flow on Aluminium powder in the flat bottomed flaskCollect the gas evolved and test the gas using a violet solution of KMnO4.

Study Questions

a. write the equations of reaction when alumimium reacts with:

(i) moderately concentrated HCl

(ii) concentrated H2SO4

b. From your observations, explain the chemical test described for the gas evolved in each case, illustrate by chemical equations if any.

•Reaction with HCl '

Aluminium reacts when warmed with moderately concentrated hydrochloric acid forming aluminium chloride and hydrogen gas:

Hydrogen is tested using a burning splint.Apop sound is heard during the test.

•Reaction with H2SO4

Aluminium does not react with dilute sulphuric acid but reacts with concen-trated sulphuric acid forming aluminium sulphate, sulphur dioxide and water:

Sulphur dioxide gas is tested using a violet solution of potassium manganate(VII)that decolorizes to give colorless (aq) ion

Note: Aluminium does not react with HNO3 because of the insoluble layer of Al2O3 formed which prevents further reaction.

8.2.3. Reaction with alkalis

Activity 8.2 (d):

Reaction of aluminium with concentrated NaOH solution

Experiment

Learners perform experiments to investigate the reaction of aluminium with NaOH solution

Apparatuses: thermometer, pyrex beaker,stirrer

Chemicals: aluminium powder,40% sodium hydroxide solution

Procedure:

•Prepare 40% of sodium hydroxide by mixing 60cm3of water with 40g of sodium hydroxide

•Take 0.5 g of aluminium powder into a pyrex beaker

•Pour the solution of sodium hydroxide in the pyrex beaker containing aluminium powder and allow the reaction to proceed for about 5 minutes.

•Use thermometer to record the temperature during the process

Aluminium reacts vigorously with sodium hydroxide solution forming sodium aluminate and hydrogen gas.

8.2.4. Reaction with halogens

Activity 8.2.4

a. In terms of s,p,d,f orbitals give the electronic structure of aluminium (Z=13)

b. How many valency electrons does aluminium possess? Explain how you reach this conclusion

c. Give the formula of the compound formed between aluminium and chlorine(Z=17)

d. Explain why the compound AlCl is not formed

•Aluminium fluoride is made by direct combination of the metal with fluorine.

• Aluminium chloride is made by passing chlorine gas over heated aluminium metal

Aluminium can be reacted with moderately concentrated HCl

Al, Ga, In and Tl react with halogens to give binary halides of formula EX3. In this case, all 3 valence electrons are used in the reaction. All trihalides of group 13 elements are known except Tl(III) iodide ,because iodine is a weak oxidizing agent.

E:is a group 13 element, TlI3 doesn’t exist.Fluorides are ionic and have high melting points due to the small size of fluoride ion.

For the same halide ion bonded to group 13 element the ionic character increases down the group as the size of cation of group element increases. For example AlCl3is more covalent than TlCl3.

For the same element of group 13 the covalent character the covalent character increases with increasing size of the anion. For exampleGaCl3 is ionic while GaI3 is covalent

The chemistry of gallium is similar to that of aluminium.

The chemistry of indium is similar to that of aluminium and gallium except that compounds containing the 1+ ion are known, such as InCl and In2O, in addition to those with more common 3+ ions.

The chemistry of thallium is completely metallic. For example, Tl2O3 is a basic oxide. Both the +1 and +3 oxidation states are quite common for thallium; Tl2O3 and TlCl3 and TlCl are all well-known compounds.

The tendency for heavier members of group 13 to exhibit the +1 as well as the expected +3 oxidation states results from an effect called “inert pair effect”.

In this effect, since s electrons are closer and more attracted by the nucleus down the group, they are less and less available to participate in bond formation of compounds; and only p electron will participate, hence the oxidation state +1.

For lighter members such B and Al the s and p valency electrons, having almost the same energy, are always available and used at the same time to form compounds where they are in oxidation state +3.

Checking up 8.2

1. Using chemical reactions differentiate and explain the action of HCl, H2SO4 and HNO3 on aluminium metal.

2. In the groups learners discuss and explain the following statement: aluminium utensils are not washed in strong alkaline solutions.

3. Write and balance the equation of reaction between

a) Aluminium and bromine

b) Gallium and chlorine

c) Gallium and hydrochloric acid

d) If b) and c) above give different products, explain why.

e) Indium and iodine

8.3. Oxides and hydroxides of group 13 elements

Activity 8.3

Write chemical reactions to illustrate the amphoteric character of aluminium hydroxide.

Aluminium oxide is amphoteric:

•it dissolves in mineral acids to form aluminium salts: in this case it acts as a base.

•it dissolves in caustic alkali to form aluminate: in this case it acts as an acid.

Aluminium hydroxide also shows amphoteric properties.

Checking up 8.3

a) Aluminium oxide is said to be amphoteric, whereas calcium oxide is said to be basic, yet the two oxides are metal oxides? Explain the origin of that difference

b) Explain the concept of amphoterism using aluminium hydroxide

8.4. Anomalous properties of boron

Activity 8.4

In groups learners make research on internet and in library, discuss and explain the following statement:

a) Boron is a bad conductor of electricity

b) Boron has higher boiling and melting points than other member of the group

c) Boron oxide is an acidic oxide

Boron – the first member of group 13 shows anomalous behavior due to the small size and high nuclear charge/size ratio, high electronegativity. This makes boron typically non-metal whereas the other members of the group are metals. Hereafter are some other anomalies.

The melting point and boiling points of boron are much higher than those of other elements of group 13.

Boron forms only covalent compounds whereas aluminum and other elements of group 13 form even some ionic compounds. The oxide of boron is acidic in nature whereas those of others are amphoteric and basic.

The trihalides of boron (BX3) exist as monomers. The reason being that due to its small size, it cannot accommodate 4 large sized halogens atoms around it.

The hydrides of boron i.e. boranes are quite stable and are formed by many molecules such as diborane (B2H6) ,triboranes(B3H8)...while those of aluminum are unstable.

Dilute acids have no action on boron; other group 13 elements are dissolved in acids to liberate H2. Borates are more stable than aluminates.

Boron does not decompose steam water while other members do so.Boron combines with metals to give borides e.g. Mg3B2. Other members form simply alloys.

Concentrated nitric acid oxidizes boron to boric acid but no such action is noticed with other group members.

Checking up 8.4

a) What is the cause of abnormal behavior of boron

b) State any anomalous properties of boron

8.5. Identification of Al3+ ion in solution

Activity 8.5

In group of 6 learners perform experiment to test for the presence of Al3+ in solution:

Apparatuses: beaker, test tubes, droppers, test tube rack

Chemicals: aluminum salt solution, sodium hydroxide, ammonia solution

Procedure:

•Pour about 2cm3 of a solution of aluminium salt into 2 different test tubes

•To 2cm3 of solution of aluminium salt in the first test tube add drop by drop a solution of sodium hydroxide until excess. Then observe what happens.

•In a second test tube, to a 2cm3 of aluminium salt add drop by drop a solution of ammonia until excess and observe

Study question

Make a report on your findings and explain what happened

Aqueous solution of aluminium salt when reacted with a solution of sodium hydroxide, a white precipitate of Al(OH)3 is formed. Al(OH)3 precipitate dissolves in excess of sodium hydroxide as soluble complex ion:

Transparent and colorless solution

When reacted with a solution of ammonia, aluminium ion Al3+ produces a white precipitate of Al(OH)3 insoluble in excess ammonia solution

Checking up 8.5

State the reagent that should be used to distinguish between a solution containing aluminium chloride salt and that containing calcium chloride salt. Illustrate your answer by chemical equations and expected observations.