Question

Aluminium is obtained by ___________________.
(A) Reduction of $ \text{A}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}} $ with coke
(B) Electrolysis of $ \text{A}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}} $ dissolved in $ \text{N}{{\text{a}}_{3}}\text{Al}{{\text{F}}_{\text{6}}} $
(C) Reduction of $ \text{A}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}} $ with chromium
(D) Heating cryolite with alumina.

Answer 1

Hint: Aluminium is extracted from its ores by two major processes: the Hall-Heroult process and the Bayer’s process. The Hall-Heroult process produces better quality and purity of aluminium, as it is deposited by electrolysis in this process.

Complete Step by step Solution
Aluminium cannot be obtained by reducing its ore, i.e., alumina with coke. This is because alumina is a highly stable oxide and aluminium has high affinity for oxygen. Hence coke cannot reduce alumina to aluminium metal itself getting converted to carbon dioxide.
Aluminium is obtained by the electrolysis of the $ \text{A}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}} $ dissolved in $ \text{N}{{\text{a}}_{3}}\text{Al}{{\text{F}}_{\text{6}}} $ . The pure aluminium oxide has high melting point and it is thus mixed with a certain amount of cryolite to lower the melting point from 2000 $ ^{\text{0}}\text{C} $ to $ \text{60}{{\text{0}}^{\text{0}}}\text{C} $ .
The electrolysis of molten bauxite in presence of cryolite can be represented by the following equations:
 $ \text{A}{{\text{l}}^{\text{+3}}}\text{+3}{{\text{e}}^{\text{-}}}\to \text{Al} $ (At the cathode)
 $ \text{C +2}{{\text{O}}^{\text{2-}}}\to \text{C}{{\text{O}}_{\text{2}}}\text{ + 4}{{\text{e}}^{\text{-}}} $ (At the anode)
During the process of electrolysis, the aluminium ions that are positively charged, move towards the cathode and the oxygen and the fluoride ions being negatively charged move towards the positively charged anode, but oxygen being less electronegative, it is discharged over fluoride as the carbon dioxide gas on reaction with the carbon anode.

Hence, the correct answer is option B.

Notes:
There are three different ores of aluminium from which it can be extracted: Bauxite $ \text{A}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}\text{.2}{{\text{H}}_{\text{2}}}\text{O} $ , Corondom $ \text{A}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}} $ , and Cryolite $ \text{N}{{\text{a}}_{3}}\text{Al}{{\text{F}}_{\text{6}}} $ . The addition of cryolite not only reduces the melting point of aluminium oxide but it also increases the conductivity of the solution for better electrolysis.