Question

Reaction between \[{\text{NaCl}}\] and \[{{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}\] gives \[{\text{HCl}}\] and is a:
A. Irreversible
B. Reversible
C. Exothermic
D. Both A and B

Answer 1

Hint: The reaction between sodium chloride and sulphuric acid gives hydrogen chloride. The hydrogen chloride is gas so it escapes from the medium.

Complete answer:
Concentrated sulphuric acid works as a dehydrating agent, so it discards water molecules from the compound and thus dehydrates the compound. These reactions are known as dehydration reactions and the substance which causes dehydration is known as a dehydrating agent.
The reaction of sodium chloride with sulphuric acid is shown as follows:
\[{\text{NaCl}}\,{\text{(s) + }}\,{{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_4}{\text{(s)}}\, \to \,{\text{NaHS}}{{\text{O}}_{\text{4}}}{\text{(s)}}{\text{ + }}\,{\text{HCl(g)}} \uparrow \]
The reaction of sodium chloride and sulphuric acid produces hydrochloric gas as a by-product. The gas hydrogen chloride escapes from the medium so the reaction cannot take place in a backward direction, and thus the reaction is irreversible.
When the product sodium hydrogen carbonate is heated with sodium chloride at $200{\,^ \circ }{\text{C}}$ temperature again the hydrogen chloride gas forms.
The reaction of sodium hydrogen carbonate with sodium chloride is as follows:
\[{\text{NaCl}}\,{\text{(s) + }}\,{\text{NaHS}}{{\text{O}}_{\text{4}}}{\text{(s)}}\, \to \,{\text{N}}{{\text{a}}_{\text{2}}}{\text{S}}{{\text{O}}_4}{\text{(s)}}\,{\text{ + }}\,{\text{HCl(g)}} \uparrow \]

Therefore, option (A) irreversible, is correct.

Note: In case of dehydration reaction water forms as one of the products. Concentrated sulphuric acid is a hygroscopic substance that has a high affinity for water molecules, so the presence of sulphuric acid with the heat on the reactant side in reaction indicates dehydration reaction. The presence of only sulphuric acid, no heat can lead to the oxidation reaction where the sulphuric acid acts as an oxidizing agent hence causing the oxidation of a species.