In the electrolysis of alumina to obtain Aluminium metal, the cryolite is added mainly to
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → lower the melting point of alumina.
Aluminium is extracted by the electrolysis of alumina (the Hall–Héroult process). Pure alumina melts at about 2323 K, which is far too high a temperature to run an electrolytic cell. Purified \( \ce{Al2O3} \) is therefore mixed with cryolite, which brings the working temperature down to about 1173 K. The fused mixture is electrolysed with a steel cathode and graphite anodes:
\( \displaystyle \text{Cathode: }\ce{Al^3+(melt) + 3e^- -> Al(l)} \)
\( \displaystyle \text{Anode: }\ce{C(s) + O^2-(melt) -> CO(g) + 2e^-} \)
\( \displaystyle \text{Anode: }\ce{C(s) + 2O^2-(melt) -> CO2(g) + 4e^-} \)
\( \displaystyle \text{Cell reaction: }\ce{2Al2O3 + 3C -> 4Al + 3CO2} \)
The main reason for adding cryolite is to lower the melting point, so that the cell can work at a much lower temperature.
Why the other options are incorrect
Option (2) dissolve the alumina in the molten cryolite. — Incorrect. Alumina does dissolve in the molten cryolite, but this is the way the lower melting mixture is formed, not the purpose for which cryolite is added.
Option (3) remove the impurities of alumina. — Incorrect. The alumina is purified before electrolysis, by leaching the bauxite with NaOH; cryolite plays no part in that.
Option (4) increase the electrical conductivity. — Incorrect. The better conductivity of the melt is a useful side effect, but the main purpose of cryolite is to bring the working temperature down.
NCERT (Pre-rationalised edition, 2022-23):
Extraction of Aluminium – …
