Question

Consider the reaction $HCN(aq.)\rightleftharpoons {{H}^{+}}(aq.)+C{{N}^{-}}(aq.)$. At equilibrium, the addition of $C{{N}^{-}}(aq.)$would [NCERT$1979$]
A. Reduce $HCN(aq.)$concentration
B. Decrease the ${{H}^{+}}(aq.)$ion concentration
C. Increase the equilibrium constant
D. Decrease the equilibrium constant

Answer 1

Hint: When cyanide ion is added to the given reaction mixture at equilibrium, the concentration of cyanide ions increases in the system. According to Le-Chatelier’s principle, the equilibrium will shift in the backward direction to form hydrogen cyanide$(HCN)$.

Complete answer:If any system is already in equilibrium, then if an extra amount of the reactants or products, the system will try to restore equilibrium conditions. According to Le-Chatelier’s principle, two possibilities will occur. If an extra amount of one of the reactants is added then the system will adjust by allowing the forward reaction to happen. Whenever an extra amount of one of the products is added then the system will adjust by allowing the backward reaction to occur to some extent. This is because the system uses these added reactants or products to restore equilibrium.
Here is the given reaction $HCN(aq.)\rightleftharpoons {{H}^{+}}(aq.)+C{{N}^{-}}(aq.)$
If at equilibrium, the addition of $C{{N}^{-}}$to the reaction system, the amount of cyanide ion $(C{{N}^{-}})$increases. Now according to Le-Chatelier’s principle, the reaction system will restore a new equilibrium in such a way as to reduce the concentration. The equilibrium moves toward the backward direction for the formation of hydrogen cyanide$(HCN)$.
As a result, the number of ${{H}^{+}}$ions decreases due to the reaction with $C{{N}^{-}}$ion to form hydrogen cyanide.
Therefore for the given equilibrium condition, the addition of $C{{N}^{-}}$ions would decrease the ${{H}^{+}}(aq.)$ion concentration according to Le-Chatelier’s principle.

Thus, option (B) is correct.

Note: When inert gas is added to an equilibrium system at constant volume, the total pressure will increase but the concentration of products and reactants will be unchanged. When inert gas is added to equilibrium at constant pressure, the total volume will increase but the concentration of reactants and products will decrease.