Chemistry · Metallurgy

JEE Advanced 2020 — Paper 2 — Question 25

Which among the following statement(s) is(are) true for the extraction of aluminium from bauxite?

Option A:
Hydrated $\mathrm{Al}_{2} \mathrm{O}_{3}$ precipitates, when $\mathrm{CO}_{2}$ is bubbled through a solution of sodium aluminate.
Correct
Option B:
Addition of $\mathrm{Na}_{3} \mathrm{AlF}_{6}$ lowers the melting point of alumina
Correct
Option C:
$\mathrm{CO}_{2}$ is evolved at the anode during electrolysis.
Correct
Option D:
The cathode is a steel vessel with a lining of carbon.
Correct

Answer: A, B, C, D

Step-by-step solution

Sodium meta aluminate is precipitated in the form of $\mathrm{Al}(\mathrm{OH})_{3}$ or $\mathrm{Al}_{2} \mathrm{O}_{3} \cdot \mathrm{XH}_{2} \mathrm{O}$ . i.e hydrated aluminium

hydroxide by passing $\mathrm{CO}_{2}$ gas.

2 \mathrm{NaAlO}_{2}+\mathrm{CO}_{2}(\mathrm{~g})+3 \mathrm{H}_{2} \mathrm{O}(\ell) \longrightarrow \underset{\text { Or Al } \mathrm{O}_{3} \cdot \mathrm{xH}_{2} \mathrm{O}}{2 \mathrm{Al}(\mathrm{OH})_{3} \downarrow+\mathrm{Na}_{2} \mathrm{CO}_{3}}

In electrolytic reduction of $\mathrm{Al}_{2} \mathrm{O}_{3}$ by Hall-Haroult process, cryolite $\left(\mathrm{Na}_{3} \mathrm{AlF}_{6}\right)$ and fluorspar $\left(\mathrm{CaF}_{2}\right)$ are

added in order to reduce the melting temperature of $\mathrm{Al}_{2} \mathrm{O}_{3}$ .

Anode is made up of graphite,

So, $\mathrm{C}(\mathrm{s})+2 \mathrm{O}^{2-} \longrightarrow \mathrm{CO}_{2}(\mathrm{~g})+4 \mathrm{e}^{-}$

Cathode is a steel vessel lined with graphite.

Answer key and solution verified before publishing.

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Exam: JEE Advanced 2020
Paper: Paper 2
Subject: Chemistry
Chapter: Metallurgy
Topic: Individual Extractions of Different Metals + Misc