Salt Hydrolysis

It is defined as the reaction in which the cation and anion of a salt reacts with H+ and OH ions of the water to produce acidic and basic solution.

Even in simpler terms, it is defined as the process in which a salt reacts with water to give back the acid and the base. Hydrolysis is the reverse of neutralization.

When water get feebly ionized, then it dissociates as follows:

$$\ce{H2O<=>H+ + OH-}$$

Now,  the salt obtained from the base (say NaOH) or acid (say HCl) dissociates as:

$$\ce{NaCl<=>Na+ + Cl-}$$

Now,  Na+ from salt combines with OH from water to give NaOH and Cl from acid combine with H+ from water to give HCl establishing the equilibrium.

$$\ce{Na+ + OH- <=>NaOH}$$

$$\ce{H+ + Cl-<=>HCl}$$

Thus the nature of cation or anion determines the solution of salt will be acidic or basic or neutral. There are four types of salts based upon the phenomenon of hydrolysis:

  1. Salt of strong acids and strong bases
  2. Salt of  strong acids and weak bases
  3. Salt of  weak acids and strong bases
  4. Salt of weak acids and weak bases

Cationic hydrolysis and anionic hydrolysis:

Cationic hydrolysis: The reaction in which cation react with water to give to acidic solution is called cationic hydrolysis.

Example: Potassium cyanide ionizes as:

$$\ce{KCN ->K+ + CN-}$$

Then,

$$\ce{ CN+ H2O<=> HCN + OH-}$$

Cationic hydrolysis: The reaction in which anion react with water to give base is called anionic hydrolysis.

For example, NH4Cl ionizes as:

$$\ce{NH4Cl ->NH4+ + Cl-}$$

Then,     

 $$\ce{NH4+ + H2O<=>NH3 + H3O+}$$

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