Question

Consider the reversible reaction $HCN\left(aq\right)\leftrightharpoons H^{+}\left(aq\right)+C{N}^{-}\left(aq\right)$ at equilibrium. The addition of $C{N}^{-}$ (aq) at equilibrium would:

• A. reduce $HCN$ (aq) concentration
• B. decrease $H^{+}$ ion concentration
• C. increase equilibrium constant
• D. decrease equilibrium constant

Answer 1

The correct option is B decrease $H^{+}$ ion concentration

The equilibrium reaction is $HCN\left(aq\right)\leftrightharpoons H^{+}\left(aq\right)+C{N}^{-}\left(aq\right)$.

When cyanide ion is added to the reaction mixture, the concentration of cyanide ions increases. The equilibrium will shift in the backward direction so that more cyanide ions will combine with hydrogen ions to nullify the effect of increased cyanide ion concentration.

This will also decrease the hydrogen ion concentration, as the dissociation of $HCN$ is suppressed.