Question

How is sodium hydroxide manufactured in industries? Name the process. In this process a gas X is formed as a by-product. This gas reacts with lime water to give a compound Y, which is used as a bleaching agent in the chemical industry. Identify X and Y and write the chemical equation of the reactions involved.

Answer 1

Hint: To answer this question, you need to recall the various methods for the preparation of sodium hydroxide. Sodium hydroxide can be prepared industrially by a number of methods but those methods are preferred more if the by-products can be used further. The gas X is a halogen.

Complete step by step answer:
One of the most common methods for industrial preparation of sodium hydroxide in which useful side products are also obtained is the electrolysis of brine solution. Brine is a saturated solution of common salt (sodium chloride) in water. This process of preparation of sodium hydroxide is known as chlor- alkali process.

Let us write the reactions occurring in the process to solve the question. Brine is the aqueous solution of sodium chloride and it decomposes on electrolysis and we obtain:
$2NaCl(aq) + 2{H_2}O(l) \to 2NaOH(aq) + C{l_2}(g) + {H_2}(g)$
From the reaction we can clearly see that the side product obtained in the preparation of sodium hydroxide is chlorine gas. As a result, it is evident as to why the reaction is named as chlor- alkali process. If we consider $C{l_2}$ gas to be X, it must react with lime water to give a bleaching agent.

The chemical formula for lime water is known to be $Ca{\left( {OH} \right)_2}$ and its reaction with chlorine gas can be given by the equation:
$Ca{\left( {OH} \right)_2} + C{l_2} \to CaOC{l_2} + {H_2}O$
We know that $CaOC{l_2}$ has bleaching properties and is thus known as bleaching powder.

Thus, we can conclude that the gas X is $C{l_2}$ and the product Y is $CaOC{l_2}$.

Note:
A common confusion possibly arising could be why do we not consider hydrogen gas as X since it is a by- product of the reaction as well. In such questions, we need to fix the unknown elements on the basis of the reactions given in the question. ${H_2}$ gas does not have any such reaction with lime water and thus we can neglect it and proceed using $C{l_2}$ as X.