Question

The following questions relate to the extraction of aluminium by electrolysis.
(a) Give the equation for the reaction which takes place at the cathode.
(b) Explain, why it is necessary to renew the anode periodically.

Answer 1

(a) The extraction of aluminium from pure ore by electrolysis is called the Hall-Heroult's process. When electricity is passed through the electrodes, the following reactions occur:
Dissociation of alumina:
$2{\mathrm{Al}}_2{\mathrm{O}}_3\to 4{\mathrm{Al}}^{3+}+3{\mathrm{O}}_{}^{2-}$
The equation for the reaction that takes place at the cathode is as follows:
${\mathrm{Al}}^{3+}+3{\mathrm{e}}^{-}\stackrel{}{\to }\mathrm{Al}$
Aluminium, which is formed in a molten state at the cathode, settles at the bottom of the cell. It is collected as and when it is formed.The aluminium obtained through this process is 99% pure.

(b) The reaction that takes place during the extraction of aluminium at the anode is:
$$\begin{gathered} {\mathrm{O}}^{2-}\stackrel{}{\to }\mathrm{O}+2{\mathrm{e}}^{-} \\ \mathrm{O}+\mathrm{O}\stackrel{}{\to }{\mathrm{O}}_2 \end{gathered}$$
Oxygen so produced reacts with the carbon anode and forms CO and CO₂.
$\mathrm{C}+{\mathrm{O}}_2\stackrel{}{\to }{\mathrm{CO}}_2$
Due to oxidation at the carbon anode, it gets used up. Hence, it is necessary to renew carbon rods periodically.