Question

For the effective extraction of aluminium, the carbon anodes are to replaced from time to time. Justify.

Answer 1

In extraction of aluminium following reaction takes place:

$2{Al}_2{O}_3⟶4Al+3O_2$

Oxygen is given off at the positive carbon anode. Carbon dioxide is also given off at the carbon anode because hot oxygen reacts with the carbon anode to form carbon dioxide gas.

$C+O_2⟶{CO}_2$

The carbon anodes slowly disappear because each molecule of carbon dioxide which is given off takes a little piece of carbon away with it. The carbon anodes need to be replaced when they become too small.