Salt Hydrolysis

Salt hydrolysis is defined as the process in which a salt reacts with water to give back the acid and the base.

Salt +water ———-> Acid + Base

BA + H₂O ———> HA + BOH

All salts are strong electrolytes and thus ionize completely in the aqueous solution.

(1) If the acid produced is strong and the base produced is weak.

B⁺ + A‾ + H₂O ——-> H⁺ + A‾ + BOH

or

B⁺ + H₂O ——-> H⁺ + BOH

In this case the cation reacts with water to give an acidic solution. This is called cationic hydrolysis.

(2) If the acid produced is weak and the base produced is strong.

B⁺ + A‾ + H₂O ——-> HA + B⁺ + OH‾

or

A‾ + H₂O ——-> HA + OH‾

In this case the anion reacts with water to give basic solution. This is called acidic hydrolysis.

Salt hydrolysis may be defined as the reaction of the cation or the anion of the salt with water to produce acidic or basic solution.

Depending upon the relative strength of the acid and the base produced, the resulting solution is acidic, basic or neutral.

(1) Salts of strong acid and strong base

NaCl, NaNO₃, Na₂SO₄, KCl, KNO₃ , K₂SO₄

NaCl + H₂O ——-> NaOH + HCl

Na⁺ + Cl‾ + H₂O ——-> Na⁺ + OH‾ + H⁺ + Cl‾

H₂O ——->OH‾ + H⁺

It involves only ionization of water and no hydrolysis.So the solution is neutral.

The salts of strong acids and strong bases do not undergo hydrolysis and the resulting solution is neutral.

 

(2) Salts of weak acid and strong bases

CH₃COONa, Na₂CO₃, K₂CO₃, Na₃PO₄

CH₃COONa + H₂O CH₃COOH + NaOH

CH₃COO‾ + Na⁺ + H₂O CH₃COOH + Na⁺ + OH‾

CH₃COO‾ + H₂O ——->  CH₃COOH + OH‾

As it produces OH‾ ions, the solution of such a salt is alkaline in nature.

(3) Salts of strong acid and weak base

NH₄Cl , CuSO₄, NH₄NO₃ , AlCl₃, CaCl₂

NH₄Cl + H₂O   NH₄OH + HCl

NH₄⁺ + Cl‾ + H₂O   NH₄OH + H⁺ + Cl‾

NH₄⁺ + H₂O ——->   NH₄OH + H⁺

As it produces H⁺ ions, the solution of such a salt is acidic in nature.

(4) Salts of weak acid and weak base

CH₃COONH₄ , AlPO₄ , (NH₄)₂CO₃

CH₃COONH₄  + H₂O  CH₃COOH + NH₄OH

It involves both anionic and cationic hydrolysis

Hydrolysis constant

Equation for the hydrolysis of a salt (BA) may be written as:

Degree of Hydrolysis

The degree of hydrolysis of a salt is defined as the function of the total salt which is hydrolyse i.e.

h = No. of moles of the salt hydrolysed / Total no. of moles the salt taken