Difference Between Acid and Base Dissociation Constants

The main difference between acid and base dissociation constants (pKa and pKb, respectively) lies in their reaction mechanisms and the ions involved. pKa measures the entropy change of a proton (H+) reading a basic ligand, while pKb measures the enthalpy change of a conjugate base (A-) forming a neutral molecule.

What is the Acid Dissociation Constant (Ka)?

The acid dissociation constant, Ka, is a quantitative measure of the strength of an acid in solution. It represents the extent to which an acid dissociates into its conjugate base and a proton in aqueous solution.

Ka is defined as:

Ka = [H⁺][A^–] / [HA]

Where:

[H⁺] is the concentration of protons produced [A^–] is the concentration of conjugate base produced [HA] is the initial concentration of the acid

A larger Ka indicates a stronger acid that dissociates more completely. A smaller Ka indicates a weaker acid.

What is the Base Dissociation Constant (Kb)?

The base dissociation constant, Kb, is a quantitative measure of the strength of a base in solution. It represents the extent to which a base dissociates into its conjugate acid and a hydroxide ion in an aqueous solution.

Kb is defined as:

Kb = [HB⁺][OH^–] / [B]

Where:

[HB⁺] is the concentration of the conjugate acid [OH^–] is the concentration of hydroxide ions produced [B] is the initial concentration of the base

A larger Kb indicates a stronger base, while a smaller Kb indicates a weaker base.

Differences between Ka and Kb

There are some differences between acid and base dissociation constants.

Acid Dissociation	Base Dissociation
Denoted by Ka	Denoted by Kb
Involves the dissociation of a base	Involves dissociation of a base
Produces H+ and A-	Produces HB+ and OH-
Uses [H+], [A-] and [HA]	Uses [HB+], [OH-] and [B]
Larger Ka = stronger acid	Larger Kb = stronger base
pKa indicates strength	pKb indicates strength