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Q.
In Hall-Heroult process, how much carbon anode is burnt away to produce each $1 \,kg$ of aluminium?
General Principles and Processes of Isolation of Elements
Solution:
Reaction at cathode:
$Al ^{3+}+3 e ^{-} \rightarrow Al$ .... (i)
Reaction at anode:
$ C + O^{2-} \rightarrow CO + 2e^-$ .....(ii)
$C + 2O^{2-} \rightarrow CO_2 + 4e^- $ ..... (iii)
By performing $2 \times( i )+$ (ii) $+$ (iii):
$2 Al ^{3+}+ \underset{(2 \times 12 \,kg )}{2 C +3 O ^{2-} } \rightarrow \underset{(2 \times 27 \,kg )}{2 Al + CO} + CO _2$
$2 \times 27 \,kg$ of Al is required $=2 \times 12 \,kg$ of carbon
$1 \,kg$ of Al is required $=\frac{2 \times 12}{2 \times 27}\, kg$ of carbon $=0.5\, kg$.