7.11.9 Hydrolysis of Salts and the pH of their Solutions

Salts formed by the reactions between acids and bases in definite proportions, undergo ionization in water.

The cations/anions formed on ionization of salts either exist as hydrated ions in aqueous solutions or interact with water to reform corresponding acids/bases depending upon the nature of salts. The later process of interaction between water and cations/anions or both of salts is called hydrolysis.

The pH of the solution gets affected by this interaction. The cations (e.g., Na+ , K+ , Ca2+ , Ba2+ , etc.) of strong bases and anions (e.g., Cl , Br , NO3 , ClO4 etc.) of strong acids simply get hydrated but do not hydrolyse, and therefore the solutions of salts formed from strong acids and bases are neutral i.e., their pH is 7. However, the other category of salts do undergo hydrolysis.

 

We now consider the hydrolysis of the salts of the following types:

(i) salts of weak acid and strong base e.g., CH3COONa.

(ii) salts of strong acid and weak base e.g., NH4Cl, and

(iii) salts of weak acid and weak base, e.g., CH3COONH4

In the first case, CH3COONa being a salt of weak acid, CH3COOH and strong base, NaOH gets completely ionised in aqueous solution

CH3COONa → CH3COO-(aq) + Na+(aq)

 

Acetate ion thus formed undergoes hydrolysis in water to give acetic acid and OH ions

CH3COO-(aq) + H2O ⇔  CH3COOH(aq) + OH(aq)

 

Acetic acid being a weak acid (Ka = 1.8 × 10–5) remains mainly unionised in solution. This results in increase of OH ion concentration in solution making it alkaline. The pH of such a solution is more than 7.

Similarly, NH4Cl formed from weak base, NH4OH and strong acid, HCl, in water dissociates completely.

NH4Cl(aq) → NH4+(aq) + Cl-(aq)

Ammonium ions undergo hydrolysis with water to form NH4OH and H+ ions.

NH4+(aq) + H2O(l) ⇔ NH4OH(aq) + H+(aq)

Ammonium hydroxide is a weak base (Kb = 1.77 × 10–5) and therefore remains almost unionised in solution. This results in increased of H+ ion concentration in solution making the solution acidic. Thus, the pH of NH4Cl solution in water is less than 7.

Consider the hydrolysis of CH3COONH4 salt formed from weak acid and weak base. The ions formed undergo hydrolysis as follow:

CH3COO- + NH4+ + H2O ⇔  CH3COOH(aq) + NH4OH(aq)

CH3COOH and NH4OH, also remain into partially dissociated form:

CH3COOH ⇔  CH3COO- + H+

NH4OH ⇔  NH4+ + OH

H2O ⇔  H+ + OH

 

Without going into detailed calculation, it can be said that degree of hydrolysis is independent of concentration of solution, and pH of such solutions is determined by their pK values:

pH = 7+ ½(pKa - pKb)

The pH of solution can be greater than 7, if the difference is positive and it will be less than 7, if the difference is negative.

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