Question

Why does dissociation rate of Hydrogen sulphide (H2S) is decreased in presence of HCl in aqueous solution?

Answer 1

The dissociation of H2S is suppressed by the presence of HCl, due to common ion effect. The presence of H+ ion in HCl is a common ion to H2S.
HCl ---> H+ + Cl-
H2S ---> 2H+ + S2-
Due to the suppressed degree of dissociation of H2S, the concentration S2- ion is produced low.

The suppression of the degree of ionisation of a weak electrolyte (weak acid or base) by addition of some strong electrolyte having a common ion is called Common ion effect.