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Hydrolysis of Salts




Hydrolysis of Salts

  • Salts undergo ionization in water
  • The cations/anions formed on ionization of salts exists as hydrated ions in aqueous solutions in case of strong Base Cations and strong Acids Anions. therefore the solutions of salts formed from strong acids and bases are neutral i.e., their pH is 7.
  • In some cases ions in aqueous solutions or interact with water to reform corresponding acids/bases depending upon the nature of salts
  • This is known as as salt hydrolysis. This is reverse of neutalisation reaction
  • This happened in case of Salt of weak acid and strong base, weak base and strong acid , weak acid and weak base
  • The pH of these solutions is not neutral

Example of Hydrolysis of Salts

Lets take an example of $NH_4Cl$
It made with
$NH_4OH + HCl -> NH_4Cl + H_2O$
i.e Strong Acid and Weak Base
Now their hydrolysis:
$NH_4Cl + H_2O -> NH_4OH + H^+ + Cl^-$
The hydrolysis of $NH_4Cl$ gives the solution with pH < 7 i.e. acidic

Hydrolysis of Salt of weak Base and strong Acid

The hydrolysis of weak Base and strong Acid. is also known as catinic hydrolysic because only cations of this salt take part in hydrolysis.
Example
$N_H4Cl + H_2O -> NH_4OH + H^+ + Cl^-$
$NH_4NO_3 + H_2O ->NH_4OH + H^+ + NO3^-$
Ammonium hydroxide is a weak base,therefore remains almost unionised in solution. This results in increased of H+ ion concentration in solution making the solution acidic.
Therefore pH of NH4Cl solution in water is less than 7

Hydrolysis of Salt of weak Acid and strong Base

The hydrolysis of weak Acid and strong Base. is also known as Anionic hydrolysis. hydrolysic because only Anions of this salt take part in hydrolysis.
Example
$CH_3COONa + H_2O -> CH_3 COOH + Na^+ + OH^-$
Acetic acdd is a weak Acid,therefore remains almost unionised in solution. This results in increased of OH- ion concentration in solution making the solution Basic.
Therefore pH of $CH_3COONa$ solution in water is more than 7
Other Examples will be NaCO3, Na3O4

Hydrolysis of Salt of weak Acid and Weak Base

Example ares $CH_3 COONH_4$ (Ammonium acetate), $(NH_4)_2CO_3$
Hydrolysis of Ammonium acetate
$CH_3 COONH_4 + H_2O -> CH_3 COOH + NH_4 OH$
Now both are weak acid and weak , so they dont disociate completely
$CH_3 COOH -> CH_3 COO^- + H^+$
$NH_4 OH -> NH_4^+ + OH^-$
pH of solution depends upon relative strength of acid and base and it isindependent of concentration of solution
It is given
$pH = 7 + \frac {1}{2}(pK_a – pK_b)$

Example

1.The pKa of acetic acid and pKb of ammonium hydroxide are 4.76 and 4.75 respectively. Calculate the pH of ammonium acetate solution.
Solution
$pH = 7 + \frac {1}{2}(pK_a – pK_b)$
$ = 7 + \frac {1}{2} [4.76 – 4.75]$
$ = 7 + \frac {1}{2} [0.01] = 7 + 0.005 = 7.005$

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