Question

In the manufacture of caustic soda by Castner Kellner process, the by-products formed are:
(A) ${{H}_{2}},C{{l}_{2}}$
(B) $C{{l}_{2}},{{O}_{2}}$
(C) ${{H}_{2}},{{O}_{2}}$
(D) ${{N}_{2}},C{{l}_{2}}$

Answer 1

Caustic soda is another name for sodium hydroxide($NaOH$) and in the Castner Kellner process the aqueous sodium chloride ($NaCl$) is electrolyzed. By writing the different steps in this electrolysis process, we would be able to identify the by-products formed in the Castner Kellner process.

Complete step by step answer:
-The Castner Kellner process is a method of producing an alkali hydroxide through the electrolysis of an alkali chloride solution in aqueous medium. Since we are asked about the manufacture of sodium hydroxide (caustic soda), the electrolysis of brine or aqueous solution of sodium chloride is used.
- The apparatus used in the Castner Kellner process has two types of cells. In the first cell, sodium chloride solution is used as the electrolyte along with a mercury cathode and a graphite anode. This second cell is surrounded by the first cell from both sides.
- In the second cell, an iron cathode and a mercury anode are used with the sodium hydroxide as electrolyte. Both the cells are separated by a wall and mercury is allowed to flow under them and this is the central cell.
- Let's look at the reactions happening in different cells. At the cathode, the half cell reaction includes the formation of chlorine gas as a byproduct. This reaction is given below
$C{{l}^{-}}\to \dfrac{1}{2}C{{l}_{2}}+{{e}^{-}}$
- At the mercury cathode, the sodium ion reacts with the electron formed from the above reaction giving sodium which on reaction with mercury produces sodium amalgam. The reaction I given below
$N{{a}^{+}}+{{e}^{-}}\to Na(amalgam)$
- The anode reaction takes place in the central cell at the interface of sodium hydroxide solution and mercury
$2Na(amalgam)\to 2N{{a}^{+}}+{{e}^{-}}$
- At the iron cathode of the central cell, the following reaction happens
$2{{H}_{2}}O+2{{e}^{-}}\to 2O{{H}^{^{-}}}+{{H}_{2}}$
As we can see the major byproducts formed are chlorine gas and hydrogen.
Therefore, the answer is option (A) ${{H}_{2}},C{{l}_{2}}$

Note: As the reaction proceeds the concentration of $NaCl$ in the first cell is decreased and the concentration of $NaOH$ at the central cell increases. For completing the production of sodium hydroxide or caustic soda, the concentrated solution of sodium hydroxide is taken from the inner compartment and it is evaporated to dryness for obtaining the solid sodium hydroxide.