Question

Explain with reasons:

In the electrolytic reduction of alumina the graphite anode is gradually consumed

Answer 1

• During the Hall - Heroult process that is, electrolytic reduction of alumina, graphite rods (Carbon) are used as anode.
• They are oxidized by the liberated oxygen to form Carbon monoxide $\left(\mathrm{CO}\right)$which further forms Carbon dioxide $\left(C{O}_2\right).$
• Reaction at anode(oxidation) : $$\begin{gathered} \mathrm{C}\left(\mathrm{s}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to {\mathrm{CO}}_2\left(\mathrm{g}\right) \\ \mathrm{Graphite}\mathrm{carbon}\mathrm{dioxide} \end{gathered}$$
• This means that graphite is consumed in reducing oxygen and thus, has to be replaced from time to time.
• Thus the graphite anode is consumed by the liberated oxygen gas during Hall - Heroult process.