Concept of Hydrolysis

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In this article, we shall study the concept of hydrolysis. Before studying the concept we have to understand different types of salts.

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 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 K_h 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.

Characteristics of 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 the case of salt of a weak acid and a weak base.

Example:

CH₃COONa + HOH  ⇌ CH₃COONa  + NaOH

Applying ionic theory and canceling common ion, we have

CH₃COO^– + HOH ⇌ CH₃COONa  + OH^–

Thus the solution is basic in nature.

Characteristics of 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

The resultant solution is neutral

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.

Hydrolysis of salt of strong acid and weak base.

These salts on hydrolysis produce strong acids and weak bases. The resulting solution is acidic in nature.

Consider hydrolysis of ammonium chloride (NH₄Cl). It gives strong acid HCl and weak base NH₄OH, when treated with water and equilibrium exist as,

NH₄Cl + H₂O   ⇌   NH₄OH    + HCl

(weak base )  (strong acid)

Applying ionic theory

NH₄⁺  + Cl^– + H₂O  ⇌     NH₄OH    + H⁺  + Cl^–

Cancelling common ions of both the sides

NH₄⁺  + H₂O  ⇌    NH₄OH    + H⁺

This solution contains free H+ ion. It is acidic to litmus and pH < 7.

These salts on hydrolysis produce weak acids and strong bases. The resulting solution is basic in nature.

Consider hydrolysis of sodium acetate (CH₃COONa). It gives weak acid CH₃COOH and strong base NaOH, when treated with water and equilibrium exist as,

CH₃COONa  + H 2O ⇌  CH₃COOH + NaOH

(weak acid)  (strong base)

Applying ionic theory

CH₃COO^–  + Na⁺ + H₂O  ⇌    CH₃COOH    + Na⁺  + OH^–

Cancelling common ions of both the sides

CH₃COO^–  +  H₂O   ⇌   CH₃COOH    +  OH^–

This solution contains free OH- ions. It is basic to litmus and pH > 7.

These salts on hydrolysis produce weak acids and weak bases. The nature of resulting solution depends on the relative strengths of the weak acid and the base formed..

Consider hydrolysis of ammonium acetate (CH₃COONH₄). It gives weak acid CH₃COOH and weak base NH₄OH, when treated with water and equilibrium exist as,

CH₃COONH₄ + H₂O  ⇌ CH₃COOH + NH₄OH

(weak acid)  (weak base)

Applying ionic theory

CH₃COO^–  + NH₄⁺ + H₂O  ⇌ CH₃COOH + NH₄OH

As there are no  free OH^– or H⁺ ions. The relative strength of CH₃COOH and NH₄OH are the same. It is neutral to litmus and pH = 7.