Hydrolysis of Salts

Dissolution of different salts in water does not always result in neutral solutions. For example, aqueous solution of copper sulphate is acidic whereas aqueous solution of sodium acetate is basic and aqueous solution of sodium chloride forms neutral solution. This is due to the dissociation of the salt in water to form ions. This process of the reaction of anion or cation of the salt with water to produce an acidic or an alkaline solution is called hydrolysis. Thus, hydrolysis is reverse of neutralization.

The equilibrium constant of such a reaction is called hydrolysis constant.

Hydrolysis of anion (A^-)

Hydrolysis of cation (B⁺)

The fraction of the total salt that gets hydrolysed at equilibrium is called degree of hydrolysis. It is written as 'h'.

Relation of Hydrolysis Constant and K_a and K_b

Salt of weak acid and strong base

Here, the anion (A^-) is a stronger base than OH^-, hence it undergoes hydrolysis to give free OH^- ions. Therefore the resulting solution will be basic in character having pH greater than 7. For example

Other examples of this type of salts are CH₃COONa, Na₂CO₃, Na₃PO₄, etc.

The aqueous solution of a salt of weak acid and strong base is alkaline. For the general reaction:

This is called anion hydrolysis.

Hydrolysis constant

The hydrolysis constant may be written as:

Multiplying equation (i) and (ii) and dividing by equation (iii), we get

The hydrolysis constant 'K_h', of the salt is inversely proportional to the dissociation constant, 'K_a' of the weak acid. Therefore, the weaker the acid, the greater is the hydrolysis constant of the salt.

Degree of hydrolysis

If the original concentration of the salt in the solution is 'c' mol/litre and 'h' is the degree of hydrolysis at that concentration, then

Intial concentration	c	0	0
Concentration at eqilibrium	c(1-h)	ch	ch

 

If 'h' is very small as compared to 1, we can assume 1 - h 1.

K_h = ch²

pH of the hydrolysed salt solution [OH^-] = ch

Now, pH = -log [H⁺]

Now, -log K_w = pK_w - log K_a = pK_a

 

 

Knowing the molar concentration 'c' of the solution, K_a and K_w, the pH of the solution can be calculated.