JEE Advanced 2022 Paper 1 Q11 Chemistry Inorganic Chemistry Metallurgy Medium

JEE Advanced 2022 Paper 1 · Q11 · Metallurgy

The electrochemical extraction of aluminum from bauxite ore involves

  1. A. the reaction of Al$_2$O$_3$ with coke (C) at a temperature > 2500 $^\circ$C
  2. B. the neutralization of aluminate solution by passing CO$_2$ gas to precipitate hydrated alumina (Al$_2$O$_3 \cdot$3H$_2$O)
  3. C. the dissolution of Al$_2$O$_3$ in hot aqueous NaOH
  4. D. the electrolysis of Al$_2$O$_3$ mixed with Na$_3$AlF$_6$ to give Al and CO$_2$
JEE Advanced multi-correct — pick every correct option, then check.
Reveal answer + step-by-step solution

Correct answer:B, C, D

Solution

Bauxite (Al$_2$O$_3 \cdot$ xH$_2$O) is first concentrated and purified by the Bayer process, then alumina is electrolysed by the Hall–Héroult process.

(A) FALSE. Carbothermal reduction of Al$_2$O$_3$ with coke at very high temperature is not the industrial route for aluminium — Al has too strong an affinity for oxygen and would produce Al$_4$C$_3$ contaminated metal. Al is not extracted by reduction with carbon.

(B) TRUE. In the Bayer process the bauxite is dissolved in hot NaOH to give sodium aluminate, and the aluminate solution is then neutralised by passing CO$_2$, precipitating hydrated alumina Al$_2$O$_3 \cdot$3H$_2$O (gibbsite).

(C) TRUE. Bauxite is leached/dissolved in hot aqueous NaOH (Bayer process) producing soluble NaAlO$_2$ while impurities such as SiO$_2$, Fe$_2$O$_3$, TiO$_2$ separate out.

(D) TRUE. The Hall–Héroult electrolysis is done in molten Al$_2$O$_3$ dissolved in cryolite Na$_3$AlF$_6$ (which lowers the melting point); molten aluminium collects at the cathode, and oxygen released at the carbon anode reacts with the carbon electrode to release CO$_2$.

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