Question

Name the anode used in Hall-Heroults process of electrolysis.

Answer 1

Hall-Heroult Process

• The Hall-Heroult process is widely used in the extraction of Aluminium.
• In Hall-Heroults process, pure${\mathrm{Al}}_2{\mathrm{O}}_3$ is mixed with ${\mathrm{CaF}}_2$ or ${\mathrm{Na}}_3{\mathrm{AlF}}_6$_, to lower the melting point of the mixture and increase it's ability to conduct electricity.
• A steel vessel with the lining of carbon and graphite rods is used.
• The carbon lining acts as cathode and graphite act as an anode.
• Reaction at anode:

$$\begin{gathered} \mathrm{C}\left(\mathrm{s}\right)+{\mathrm{O}}^{2-}\left(\mathrm{g}\right)\to \mathrm{CO}\left(\mathrm{g}\right)+2{\mathrm{e}}^{–} \\ \mathrm{C}\left(\mathrm{s}\right)+2{\mathrm{O}}^{2-}\to {\mathrm{CO}}_2\left(\mathrm{g}\right)+4{\mathrm{e}}^{–} \end{gathered}$$

1. When electricity is passed through the electrolytic cell oxygen is formed at the anode.
2. This oxygen formed reacts with the carbon of the anode to form carbon monoxide and carbon dioxide.

• At the cathode:

${\mathrm{Al}}^{3+}+3{\mathrm{e}}^{–}\to \mathrm{Al}\left(\mathrm{l}\right)$

1. Aluminium ions are produced at the cathode from the aluminium oxide and then sink down because they are heavier than the cryolite solution.

Oxide ions lose electrons and form molecules of oxygen at anode.

Thus, Graphite rod is used as anode in Hall-Heroult's process.