In order to obtain 1 tonne of aluminium, the following inputs are required: 4 tonnes of bauxite, 150 kg of sodium hydroxide and 600 kg of graphite. The aluminium compound in bauxite is aluminium oxide and the main impurity is iron (III) oxide. Aluminium is obtained by the electrolysis of aluminium oxide dissolved in cryolite.

(a) When bauxite is treated with sodium hydroxide solution, what happens to

(i) the aluminium oxide

(ii) the iron (III) oxide

(b) (i) Name the process used for the purification of bauxite.

(ii) Write the equation for the action of heat on aluminium hydroxide.

(c) (i) Write the formula of cryolite.

(ii) Write down the word which correctly completes the following sentence:

By dissolving aluminium oxide in cryolite a …………… (conducting/non-conducting) solution is produced.

(iii) Why is so much graphite required for the electrolytic process?

(iv) Write the equation for the reaction which takes place at the cathode

(v) What is the cathode made up of?

(a) Aluminium oxide dissolves and forms sodium meta aluminate. Iron (III) oxide is left behind as red mud and is removed by filtration.

Al₂O₃.2H₂O + 2NaOH ⟶ 2NaAlO₂ + 3H₂O

(b) (i) Bayer's process

(ii) The equation for the action of heat on aluminium hydroxide is:

$2\text{Al(OH)}$3 \xrightarrow{1000 \degree\text{C}} \text{Al}2\text{O}3 + 3\text{H}2\text{O}

(c) (i) Na₃AlF₆

(ii) By dissolving aluminium oxide in cryolite, a conducting solution is produced.

iii. Thick Graphite rods attached to copper clamps dipping into fused electrolyte are used as anode. The graphite (anode) is oxidized by oxygen to CO and further forms CO₂, so it is consumed and has to be replaced from time to time. Hence, large amount of graphite is required.
Equation:
2C + O₂ ⟶ 2CO
2CO + O₂ ⟶ 2CO₂

(iv) Reaction at the cathode:
4Al³⁺ + 12e^- ⟶ 4Al

(v) Inner carbon lining of the electrolytic cell