JEE Main · 2015 · 04 Apr 2015 · Offline · General Principles and Processes of Isolation of Metals

In the context of the Hall-Heroult process for the extraction of Al, which of the following statements is false?

(A) $Al_{2} O_{3}$ is mixed with $CaF_{2}$ which lowers the melting point of the mixture and brings conductivity
(B) $Al^{3 +}$ is reduced at the cathode to form Al
(C) $Na_{3} AlF_{6}$ serves as the electrolyte✓ Correct
(D)CO and $CO_{2}$ are produced in this process

Answer: (C)

Solution

The question hunts for the untrue claim about the Hall-Heroult cell. It is genuinely true that

A l^{3 +}

is reduced to aluminium at the cathode, that the carbon anode is eaten by the liberated oxygen to give

CO

and

C O_{2}

, and that fluoride additives lower the melting point and raise the conductivity. The false statement is that

N a_{3} Al F_{6}

(cryolite) is the electrolyte: cryolite is really the molten solvent that dissolves the alumina, and it is the dissolved

A l_{2} O_{3}

, furnishing

A l^{3 +}

and

O^{2 -}

, that is actually electrolysed. Calling cryolite itself the electrolyte is the very misconception being tested.

JEE Main · 2015 · 04 Apr 2015 · Offline · Single correct option