What is the net reaction in Hall-Heroult process?

2 A l_{2} O_{3} + 3 C \to 4 A l + 3 C O_{2}

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4 A l_{2} O_{3} + 3 C \to 4 A l + 6 C O_{2}

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F e_{2} O_{3} + A l_{2} O_{3} \to A l + F e_{2} O_{3}

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N a_{3} A l F_{6} \to A l F_{3} + 3 N a F

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Solution

The correct option is A $2 A l_{2} O_{3} + 3 C \to 4 A l + 3 C O_{2}$
In the metallurgy of aluminium, purified $A l_{2} O_{3}$ is mixed with $N a_{3} A l F_{6}$ or $C a F_{2}$ which lowers the melting point of the mix and brings conductivity. The fused matrix is electrolysed. Steel vessel with lining of carbon acts as cathode and graphite anode is used.

Cathode reation:
$A l^{+ 3} + 3 e^{-} \to A l$

Anode reaction:
$C (s) + O^{2 -} m e l t \to C O (g) + 2 e^{-}$

$C (s) + 2 O^{2 -} m e l t \to C O_{2} (g) + 4 e^{-}$

The overall reaction may be written as

$2 A l_{2} O_{3} + 3 C \to 4 A l + 3 C O_{2} .$

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