Question

In Castner-Kellner cell for production of sodium hydroxide
(A) Brine is electrolyzed using graphite electrodes
(B) Molten sodium chloride is electrolyzed
(C) Sodium amalgam is formed at mercury cathode
(D) Brine is electrolyzed with platinum electrodes.

Answer 1

Hint : The Castner-Kellner process is a highly efficient method of producing sodium hydroxide in a pure form at an industrial scale. The process involves the use of electrolytic methods and salt solutions like sodium chloride (brine solution).

Complete Step By Step Answer:
The Castner-Kellner process is used for the manufacturing of caustic soda or sodium hydroxide which is not just a strong alkali but an important starting material for the production of a variety of special and complex chemicals.
The first step in the process is the electrolysis of brine which is a highly concentrated solution of sodium chloride in the presence of mercury electrodes. As a result of electrolysis, the positively charged sodium ions get deposited at the mercury electrodes forming an amalgam and the chlorine that has evolved at anodes is removed from the chamber.
The reactions taking place can be written as follows:
At Cathode:
$2N{a^ + } + 2{e^ - } \to 2Na(s)$
$Na(s) + Hg \to Na - Hg(ama\lg um)$
At Anode:
$2C{l^ - } \to C{l_2} + 2{e^ - }$
Since, sodium chloride is used as a solution and not in its molten form and mercury electrodes are used instead of graphite or platinum electrodes.
The only correct option is that Sodium amalgam is formed at Mercury cathode which is option (c).

Note :
Sodium hydroxide is a bulk chemical that is used in a lot of industries and is therefore required in large amounts. The large scale industrial production of sodium hydroxide can be carried out by Gibbs method and Nelson’s method apart from the Castner-Kellner method.