Base Dissociation Constants (Kb)
- Kb, the base dissociation constant or base ionisation constant, is an equilibrium constant that refers to the dissociation, or ionisation, of a base.
- For the reaction in which the Arrhenius base, BOH, dissociates to form the ions OH- and B+:
BOH OH- + B+
Kb = [OH-][B+] [BOH]
For a Brönsted-Lowry base:
B + H2O BH+ + OH-
Kb = [OH-][BH+] [B]
The concentration of water is absorbed into the value of Kb
- Kb provides a measure of the equilibrium position
(i) if Kb is large, the products of the dissociation reaction are favoured
(ii) if Kb is small, undissociated base is favoured.
- Kb provides a measure of the strength of a base
(i) if Kb is large, the base is largely dissociated so the base is strong
(ii) if Kb is small, very little of the base is dissociated so the base is weak.
- The degree to which a base dissociates can be represented as a percentage:
% dissociation (ionization) = [OH- at equilibrium] ÷ [base initial] × 100
(i) If %dissociation ≈ 100%, the base is a strong base
(ii) If %dissociation is small, the base is a weak base