Question 4
(a) Outline the procedures for the treatment of water for town supply. [4 marks]
(b) (i) State two main impurities present in bauxite.
(ii) Give one reason why bauxite is usually preferred as the ore for the extraction of aluminium.
(iii) Outline the manufacture of aluminium form purified bauxite. [8 marks]
(c) (i) Explain briefly the term fine chemical industry.
(ii) State two differences between a fine chemical and a heavy chemical. [5 marks]
(d) (i) Write a balanced chemical equation for the combustion of coal.
(ii) If 5.4 g of coal is burnt completely, calculate the amount of oxygen measured at s.t.p. that would be required for the combustion.
[C = 12.0, O = 16.0, Molar volume of a gas at s.t.p. = 22.4 dm3] [6 marks]
(e) Name two substances which can be used as electrodes during the electrolysis of acidified water. [2 marks]
Observation
In part (a), majority of the candidates could not outline the procedures for treatment of water for town supply in the correct order.
In part (b), few candidates were familiar with the extraction of aluminum from bauxite.
In part (c), majority of the candidates were familiar with fine chemical and heavy chemical.
In part (d), majority of the candidates were able to write a balanced chemical equation for the combustion of coal.
In part (e), majority of the candidates named two substances which can be used as electrodes during the electrolysis of acidified water.
The expected answers include:
(a)
- addition of alum / addition of coagulating agent
- sedimentation
- filtration
- disinfection / chlorination / addition of chlorine
(b) (i) - iron (III) oxide
- silicon (IV) oxide / sand
- trioxosilicates (IV)
(ii) because aluminium oxide has too high a melting point to eletrolyse on its own / bauxite has low silica content
(iii) - Alumina and molten cryolite
- Melted and electrolysed
- Cell with carbon electrodes
- Molten aluminium produced at the base of cell.
- siphoned off / removed at the base
(c) (i) A fine chemical industry is the one that manufactures chemicals in smaller
quantities / amounts for specific use with high level of purity
(ii)
| Fine chemical | Heavy chemical |
- they are produced in |
- they are produced in larger |
- they are highly pure |
- they are not pure |
- they are final products |
- they are intermediate products |
- they are more costly / |
- less costly / less expensive |
(e) - platinum
- carbon