Question

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

Answer 1

Aluminium is manufactured by the reduction of alumina using electricty as reducing agent since the reativity of aluminium is very high.
Electrolysis of Alumina:

The alumina $\left({\mathrm{Al}}_2{O}_3\right)$ obtained by concentration of bauxite is mixed with molten cryolite $\left({\mathrm{Na}}_3{\mathrm{AlF}}_6\right)$
and subjected to electrolysis. The melting point of alumina is very high. Cryolite is added to alumina to reduce its melting point and increase its electrical conductivity.

On passing electric current, molten aluminium is deposited on cathode and dioxygen gas is produced at the anode. The molten aluminium collected at the bottom of the cell is taken out.
${\mathrm{Al}}_2{O}_3\to 2{\mathrm{Al}}^{3+}+3O^{2-}$
At Cathode: ${\mathrm{Al}}^{3+}+3e^{-}\to \mathrm{Al}$
At Anode: $2O^{2-}\to O_2+4e^{-}$
And this process, carbon rods are replaced from time to time because the dioxygen gas produced at the anode reacts with the rod of carbon and forms carbon dioxide.
$C+O_2\to {\mathrm{CO}}_2$
Thus anode gets used up. So anode is replaced frequently.