Concept of Hydrolysis

Types of Salts:

  • Salts are classified on the basis of the nature of the acids and bases from which they are derived. There are four types of salts.

Salts of strong acids and strong bases:

Formula Strong Acid Strong  Base
NaCl HCl NaOH
KCl HCl KOH
NaNO3 HNO3 NaOH
KNO3 HNO3 KOH
Na2SO4 H2SO4 NaOH
K2SO4 H2SO4 KOH

Salts of strong acids and weak bases:

Formula Strong Acid Weak  Base
NH4Cl HCl NH4OH
FeCl3 HCl Fe(OH)3
PbNO3 HNO3 PbOH
CuSO4 H2SO4 Cu(OH)2

Salts of weak acids and strong bases:

Formula Weak Acid Strong  Base
CH3COONa CH3COOH NaOH
CH3COOK CH3COOH KOH
HCOONa HCOOH NaOH
HCOOK HCOOH KOH
Na2CO3 H2CO3 NaOH
K2CO3 H2CO3 KOH

Salts of weak acids and weak bases:



Formula Weak Acid Weak  Base
CH3COONH4 CH3COOH NH4OH
HCOONH4 HCOOH NH4OH
(NH4)2CO3 H2CO3 NH4OH

 

Hydrolysis of a Salt:

  • The process in which cation or anion or both the ions of a salt react with water to produce acidity or basicity to the solution is called as hydrolysis.
  • Explanation :
  • Hydrolysis is reverse of neutralization.  When salt is added to the water, then cation, anion or both the ions of salt react with water and if the solution becomes either acidic or basic then it is hydrolysis process.
  • When a cation of the salt reacts with water, weak base and acid solution is formed.

BA  +   H2O  ⇌    BOH(aq)   +     H+(aq)

When anion of the salt reacts with water, weak acid and basic solution are formed.

BA  +   H2O  ⇌ HA (aq)  +   OH(aq)



  • When both ions of the salt react with water, weak base and weak base both are formed. The nature of solution depends on the relative strengths of acid and base.

BA(aq)  +   H2O ⇌  BOH(aq) +  HA(aq)

Degree of Hydrolysis (h):

  • The fraction of the total number of moles of a salt that hydrolyzed at equilibrium is called as the degree of hydrolysis. It is denoted by ‘h’

hydrolysis

Hydrolysis Constant:

  • Consider following hydrolysis reaction.

Salt + Water ⇌ Acid + Base

Applying the law of mass action we have



  • In hydrolysis water is a reactant as well as medium for the reaction, hence water is in large excess. Therefore [Water] = constant.

Where Kh is a constant called hydrolysis constant.

  • Hydrolysis constant is the modified equilibrium constant, obtained by applying the law of mass action to the hydrolysis reaction at equilibrium.

Distinguish Between Hydrolysis and Neutralization:

Hydrolysis:

  • It is defined as a reaction in which cation or anion or both the ions of a salt react with water to produce acidity or alkalinity to the solution,
  • Salt + Water Acid + Base
  • It takes place to a very small extent.
  • It is a reversible reaction.
  • It is not possible for a salt of a strong acid and strong base.
  • Dilution increases the degree of hydrolysis except in case of a salt of weak acid and a weak base.
  • Example:

CH3COONa + HOH  ⇌ CH3COONa  + NaOH



Applying ionic theory and canceling common ion, we have

CH3COO + HOH ⇌ CH3COONa  + OH

Neutralisation:

  • It is a reaction between H+ ions of the acid and OH- ions of the base to form practically undissociated water.
  • Acid + Base →  Salt + Water
  • It is almost complete.
  • It is an irreversible reaction.
  • It is possible with any type of acid and base.
  • Dilution has no effect on neutralization reaction.
  • Example:

HCl + NaOH →  NaCl  + H2O

Applying ionic theory and cancelling common ion, we have



H+ +    OH–  →   H2O

Salt of a strong acid and strong base do not undergo hydrolysis.

  • salts of a strong acid and strong base produce strong acid and strong base when treated with water but the solution is neither acidic nor basic, hence these salts do not undergo hydrolysis.
  • Explanation:
  • Consider NaCl salt. It gives strong acid (HCl) and strong base (NaOH). In water solution, equilibrium exists as,

NaCl + H2O  ⇌ NaOH  + HCl

NaOH is a strong base and HCl is a strong acid

By ionic theory,



Na+ + Cl +  H2O  ⇌ Na+ + OH + H+ + Cl

On cancelling common ions of both the sides

H2O   ⇌  H+ +   OH

  • In this case [H+] = [ OH] . The solution is neither acidic nor basic i.e. it is neutral to litmus. Neither cation nor anion reacts with water. Hence salts of strong acids and strong bases do not undergo hydrolysis.

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