Question

Consider the reversible reaction,

$HCN\left(aq\right)\rightleftharpoons H^{+}\left(aq\right)+C{N}^{-}\left(aq\right)$

At equilibrium, the addition of $C{N}^{-}\left(aq\right)$ would :

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

Answer 1

The correct option is B decrease the $H^{+}\left(aq\right)$ ion concentration

At equilibrium, the addition of $C{N}^{-}\left(aq\right)$ would decrease the $H^{+}\left(aq\right)$ ion concentration.

$K_a=\frac{\left[H^{+}\right]\left[C{N}^{-}\right]}{\left[HCN\right]}$

An increase in $\left[C{N}^{-}\right]$ will decrease $\left[H^{+}\right]$ to maintain $K_a$ constant.

In a general way: Le Chatelier's principle states that if the system is changed in a way that increases the concentration of one of the reacting species, it must favour the reaction in which that species is consumed. In other words, if there is an increase in products, the reaction quotient is increased, making it greater than the equilibrium constant.