Dissociation or Ionization of Acids And Bases

Acids like HCl, H₂SO₄ ,HNO₃ when dissolved in water dissociates almost completely thus producing a large number of H⁺ ions. These acids are called strong acids.

Acids like CH₃COOH , HF, H₂CO₃, H₃PO₄ dissociates only to a small extent in the aqueous solution giving small amount of H⁺ ions and hence are called weak acids.

Bases like NaOH , KOH dissociate almost completely in the aqueous solution producing a large number of OH‾ ions and are called strong bases.

Bases like NH₄OH , Ca(OH)₂ , Al(OH)₃  dissociate only to a small extent in the aqueous solution and are therefore called weak bases. 

The dissociation of weak acids or weak bases in water can be represented:

CH₃COOH + H₂O  CH₃COO‾ + H₃O⁺

NH₃ + H₂O  NH₄⁺ (aq) + OH‾(aq)

If a weak acid is represented by HA , its dissociation in water can be represented by the equilbrium:

HA + H₂O   A‾ (aq) + H₃O+ (aq)

Applying the law of chemical equilibrium, the expression constant will be:

As water is solvent, its concentration is large and remains almost constant.

HA + (aq)  H⁺ (aq) + A‾ (aq)

 

Knowing the value of the dissociation constant of the acid, Ka and the concentration of the weak acid HA taken, concentration of H₃O+ or H⁺ in the solution can be calculated as:

HA +aq   H⁺ (aq) + A‾ (aq)

Initial conc of HA is C mol L‾¹

Initial conc of H⁺ is 0 M

Initial conc of A‾ is 0 M

Final conc of HA at equilibrium is (C-x) M

Final conc of H⁺ at equilibrium is x M

Final conc of A‾ at equilibrium is x M

Ka = x × x / C-x

Ka = x² / C- x

x is very small as compared to the initial concentration C ,x can be neglected in comparison to C.Hence the equation is simplified to the form:

K_a = x² / C

x= √K_a × C

The dissociation of a weak base in water can be represented by the equilibrium:

BOH + aq  B⁺ (aq) + OH‾ (aq)