Ncert Exercises & Solutions

In the metallurgy of aluminium:

1.	Al³⁺is oxidised to Al(s).
2.	The graphite anode is oxidised to carbon monoxide and carbon dioxide.
3.	Oxidation state of oxygen change in the reaction at the anode.
4.	The oxidation state of oxygen changes in the overall reaction involved in the process.

HINT: Graphite anode is oxidised to carbon monoxide and carbon dioxide.

Explanation:

STEP 1:

In the metallurgy of aluminium, electrolysis is performed in a steel vessel with the lining of carbon acting as cathode and graphite act as an anode.

During this process, graphite anode is oxidised to

CO

and

{CO}_{2}

The chemical reaction occurring in this process is as follows:

2 {Al}_{2} O_{3} + 3 C \to 4 Al + 3 {CO}_{2}

STEP 2:

This process is known as the Hall-Heroult process.

The electrolytic reactions are:

At cathode;

{Al}^{3 +} (melt) + 3 e^{-} \to Al (l)

At anode;

C (s) + O^{2 -} (melt) \to CO (g) + 2 e^{-}

C (s) + 2 O^{2 -} (melt) \to {CO}_{2} (g) + 4 e^{-}