A small amount of \(CaF_2\) is added in electrolytic reduction of \(Al_2O_3\) dissolved in fused cryolite:
(A) To decrease the rate of oxidation of carbon at anode
(B) To act as catalyst
(C) To make fused mixture conducting
(D) To lower the fusion temperature of melt
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (C), (D) only
In the Hall–Heroult process, purified alumina is electrolysed in a steel cell lined with carbon (cathode) using graphite anodes. Pure \( \ce{Al2O3} \) melts above 2300 K and is a poor conductor, so it is mixed with Na3AlF6 or CaF2, which lowers the melting point of the mix and brings conductivity.
Cathode: \( \ce{Al^3+(melt) + 3e- -> Al(l)} \)
Anode: \( \ce{C(s) + O^2-(melt) -> CO(g) + 2e-} \); \( \ce{C(s) + 2O^2-(melt) -> CO2(g) + 4e-} \)
(A) To decrease the rate of oxidation of carbon at anode — Incorrect. \( \ce{CaF2} \) has no such effect; the oxygen liberated still burns the graphite anodes, which have to be replaced from time to time.
(B) To act as catalyst — Incorrect. Electrolysis is not a catalysed reaction; \( \ce{CaF2} \) only changes the physical properties of the melt.
(C) To make fused mixture conducting — Correct. The fluoride melt supplies mobile ions and dissolves the alumina, so the mixture conducts well.
(D) To lower the fusion temperature of melt — Correct. The mixture melts at about 1173–1223 K instead of above 2300 K, which saves energy.
NCERT (Pre-rationalised edition, 2022-23):
Aluminium
