Question

Why dissociation of HCN is suppressed by the addition of HCL?

Answer 1

HCN and HCL both dissociate to produce H+ ions which are common to both.

HCN is a weak electrolyte. It dissociates to a little extent.
HCN_(aq) ⇌ H⁺_(aq) + CN^- 

HCL is a strong ·electrolyte. It undergoes complete dissociation.
HCl_(aq) → H⁺_(aq) + Cl^-_(aq) 
both HCN and HCL provide H⁺ ions.

The concentration of H⁺ ions in the solution increases due to the complete dissociation of HCL

According to Le-Chatelier's principle, the effect of the stress (the addition of H⁺ ions from HCL) applied to the ionisation equilibrium of HCN is reduced by shifting the equilibrium in the backward direction.

H⁺ ions combine with CN^- ions to produce unionised HCN. Thus. the dissociation of HCN is suppressed by the addition of HCL.