JEE Advanced 2022 — Paper 1 — CHEM-II solution

JEE Advanced2022Paper 1CHEM-II

The electrochemical extraction of aluminium from bauxite ore involves

A
the reaction of Al $_{2}$ O $_{3}$ with coke (C) at a temperature $> 250 0^{\circ}$ C.
B
the neutralization of aluminate solution by passing CO $_{2}$ gas to precipitate hydrated alumina (Al $_{2}$ O $_{3}$ ·3H $_{2}$ O).
C
the dissolution of Al $_{2}$ O $_{3}$ in hot aqueous NaOH.
D
the electrolysis of Al $_{2}$ O $_{3}$ mixed with Na $_{3}$ AlF $_{6}$ to give Al and CO $_{2}$ .

Solution

The industrial route comprises the Bayer process (purification of bauxite) followed by the Hall-Héroult process (electrolysis).

(A) Direct carbothermic reduction of Al $_{2}$ O $_{3}$ is not used because aluminium forms carbides (Al $_{4}$ C $_{3}$ ) and the process is thermodynamically and practically unsuitable. Wrong.

(B) After dissolving bauxite in hot NaOH, the sodium aluminate solution is treated by bubbling CO $_{2}$ which neutralizes the alkali and precipitates hydrated alumina. Correct.

(C) Bauxite is leached with hot, concentrated aqueous NaOH (around 473 to 523 K, 35 to 36 bar) to selectively dissolve Al $_{2}$ O $_{3}$ as sodium aluminate. Correct.

(D) Purified Al $_{2}$ O $_{3}$ is dissolved in molten cryolite (Na $_{3}$ AlF $_{6}$ ) to lower the melting point, and electrolysed using carbon electrodes. Aluminium is deposited at the cathode and CO $_{2}$ is evolved at the anode as the carbon anode is consumed. Correct.

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