In the electrolytic process for the extraction of aluminium, the electrolyte is:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → A molten mixture of \(Al_2O_3\) and \(Na_3AlF_6\)
Aluminium is extracted by the electrolysis of fused alumina (Hall-Héroult process). Pure \( \ce{Al2O3} \) melts above 2300 K and conducts poorly, so it is mixed with cryolite, Na3AlF6 (or \( \ce{CaF2} \)), which lowers the melting point of the mixture to about 1240 K and increases its conductivity. The molten mixture is electrolysed in a steel cell with carbon electrodes:
\( \ce{2Al2O3} \) + 3C → 4Al + \( \ce{3CO2} \)
Why the other options are incorrect
Option (1) \(Al(OH)_3\) in NaOH solution — Incorrect. This is a step in the leaching (purification) of bauxite, not the electrolyte; an aqueous solution would give hydrogen at the cathode.
Option (2) An aqueous solution of \(Al_2(SO_4)_3\) — Incorrect. Al is highly electropositive; from water, \( \ce{H2} \) is discharged at the cathode instead of aluminium.
Option (4) A molten mixture of \(Al_2O_3\) and \(Al(OH)_3\) — Incorrect. \( \ce{Al(OH)3} \) decomposes to \( \ce{Al2O3} \) and water on heating and does not lower the melting point; cryolite is the additive used.
NCERT (Pre-rationalised edition, 2022-23):
Aluminium – “In the metallurgy of aluminium, purified Al2O3 is mixed with Na3AlF6 or CaF2 which lowers the melting point of the mix and brings conductivity.”