Question

When caustic soda is manufactured from seawater, one of the important by-products is
(A) Potash
(B) Chlorine
(C) Chromium
(D) Fertilizer

Answer 1

Hint: Caustic soda is manufactured from seawater by the process of evaporation of seawater. The process followed here is called electrolysis. Sodium hydroxide(caustic soda) along with gases is formed in this process.

Complete step by step answer:
Sodium hydroxide primarily known as caustic soda is obtained by the evaporation of seawater. Here the Castner -Kellner method is used in which the sodium hydroxide is obtained by the electrolysis of an aqueous solution of sodium chloride.Caustic soda is produced as the liquid in this process. It is produced by passing an electric current through a brine solution which is common salt dissolved in water.
Electrolysis of concentrated brine solutions produces chlorine gas, hydrogen gas, and aqueous sodium hydroxide that is caustic soda.
The electrolysis process is done as per the following. The reaction at the anode is given by

$2C{l^ - } \to C{l_2} + 2{e^ - }$

The reaction at the cathode is given by

$N{a^ + } + {e^ - } \to Na$

The overall reaction is given as follows

$2Na + 2{H_2}O \to 2N{a^ + } + 2O{H^ - } + {H_2} + 2Hg$

Therefore chlorine is produced in this reaction and is an important by-product of this reaction.

So, the correct answer is Option B .

Note: Here sodium dissolves in the liquid mercury to form an amalgam which is to be removed. Also, aqueous sodium hydroxide and hydrogen gas are collected. Mercury is recycled through the electrolytic cell.

There are other two methods for the preparation of caustic soda named nelson diaphragm cell and membrane cell. In both of the cells, brine solutions are taken and chlorine is formed as a by-product.