Question

A.) What ions are formed when KCN dissolves in water? Identify the ion in KCN which will hydrolyze.

B.) Write the chemical equation showing the hydrolysis of this ion. Write the equilibrium constant expression for the hydrolysis of this ion.

Answer 1

A.

KCN is a salt of strong base KOH and weak acid HCN, Hence, the ions that forms when KCN dissolves in water are ions as seen in the following equation.

.

Now, among the two ion formed, K+ can react with water to form KOH which is fully soluble in water and hence gives back K+ ions and OH- ions. Hence, K+ does not hydrolyse in water.

Now, CN^- ion reacts with water to form HCN which only dissociates partially as it is a weak acid. Hence, will hydrolyse in water.

B.

The chemical reaction for the hydrolysis of CN- ion is

Basically, CN- being the conjugate base of HCN abstracts a proton from water which reacts as an acid in the reaction.

The equilibrium constant of the above hydrolysis can be expressed as

Note that H2O does not appear in the equilibrium constant expression as it is the solvent and its concentration remains virtually unchanged during the reaction.