The following questions relate to the extraction of aluminium by electrolysis :
(i) Name the other aluminium containing compound added to alumina and state it’s significance.
(ii) Give the equation for the reaction that takes place at the cathode.
(iii) Explain why is it necessary to renew the anode periodically.

(i) The compound is cryolite [Na₃AlF₆]. It —

1. lowers the fusion point of the mixture i.e., mixture fuses around 950°C instead of 2050°C.
2. enhances the mobility of the fused mixture by acting as a solvent for the electrolytic mixture.
3. enhances the conductivity of the mixture since, pure alumina is almost a non-conductor of electricity.

(ii) 2Al³⁺ + 6e^– ⟶ 2Al

(iii) The anodes are continuously replaced during the electrolysis because :

1. The oxygen evolved at the anode escapes as a gas or reacts with the carbon anode.
2. The carbon anode is thus oxidized to carbon monoxide which either burns giving carbon dioxide or escapes out through an outlet. 2C + O₂ ⟶ 2CO [2CO + O₂ ⟶ 2CO₂]
3. The carbon anode is hence consumed and renewed periodically after a certain period of usage.