Explain the method of obtaining pure aluminium from alumina by electrolysis. In this process the carbon rods are replaced from time to time. Why?

Explanation

Aluminiumn is obtained by the electrolysis of alumina dissolved in fused cryolite.
The melting point of alumine is very high. So cryolite is added to it and melted, `Al_2 O_3`, dissolves in molten cryolite, It contain `Al^3+` ions and separate.
Electrolytic reaction
`Al_2 O_3 rarr 2Al^3+ + 3O^2-`
At cathode: `Al^3+ 3e^- rarr Al`
At anode:`2O^2- rarr O_2 + 4e^-`
Since the oxygen liberated at the anode oxidises the carbon rods to from `CO_2`
As a result of this anode is damaged. Hence it is replaced often.