Question

Electrolytic reduction of alumina to aluminium by Hall-Heroult process is carried out _____.
(a) In presence of of NaCl
(b) in presence of fluorite
(c) in the presence of cryolite which forms a melt with a lower melting temperature
(d) in the presence of cryolite which forms a melt with higher melting temperature

Answer 1

In the extraction of aluminium by the Hall-Heroult process, the pure aluminium is mixed with the cryolite which is a colorless halide mineral with molecular formula $N{a_3}Al{F_6}$.

Complete answer:
The Hall-Heroult process is used for the extraction of pure aluminium from its mixture. In this process, the pure aluminium $A{l_2}{O_3}$ is mixed with calcium fluoride $Ca{F_2}$ or with sodium aluminium hexafluoride $N{a_3}Al{F_6}$ in order to decrease the melting point of the mixture and enhance the capability of conducting electricity.
In the Hall-Heroult process, a vessel made of steel is used in which lining of graphite and carbon rods are applied.
In this set up, the carbon lining behaves as a cathode and the graphite lining behaves as anode. After that electricity is passed through the electrolytic cell containing carbon electrodes, the oxygen is generated at the anode. The oxygen which is formed reacts with the carbon anode to give rise to carbon monoxide and carbon dioxide.
Aluminium ions are thus formed from aluminium oxide at cathode but they sink down as they are heavier in weight than cryolite solution. At the anode, the oxygen from the aluminium oxide is generated and reacts with carbon dioxide with graphite carbon.
Reaction at cathode: $A{l^{3 + }} + 3{e^ - } \to Al(l)$
Reaction at anode: $C(s) + {O^{2 - }} \to CO(g) + 2{e^ - }$
$C(s) + 2{O^{2 - }} \to CO(g) + 4{e^ - }$
Overall reaction: $2A{l_2}{O_3} + 3C \to 4Al + 3C{O_2}$
During the electrolysis process, the aluminium present in ionic form accepts electrons from the cathode to produce molten aluminium and the oxide ions lose its electrons to form oxygen.
Thus, Electrolytic reduction of alumina to aluminium by the Hall-Heroult process is carried out in the presence of cryolite which forms a melt with a lower melting temperature.
Therefore, the correct option is C.

Note: In the Hall-Heroult process of extraction of aluminium, for every 1 Kg of aluminum which is extracted, 0.5kg of carbon anode is burnt.