ABSTRACT
Weak acids show much less tendency to form hydronium ions and complete dissociation does not occur. Instead, an acid dissociation equilibrium is established and there is significant undissociated acid in solution. An example is the ammonium ion:
These equilibria have equilibrium constants called acidity constants or acid dissociation constants. It therefore follows that a strong base is a base that completely dissociates into hydroxide ion and its conjugate acid. An example is potassium hydroxide:
A weak base does not. dissociate completely and, as with a weak acid, establishes an equilibrium. An example is ammonia, which is the conjugate base of the weak acid given above:
The acidity constant
The general proton transfer equilibrium between an acid, HA, and water:
has an equilibrium constant called the acidity constant or acid dissociation constant, Ka, given by (see Topic C1):
The smaller the value of pKa (or as pKa=−log10Ka, the larger the value of Ka) the further the equilibrium position is towards the right-hand side and the more the acid is dissociated. Thus pKa is a measure of the acid strength, or the ability of the acid to donate protons and the stronger the acid, the smaller is its value of pKa.
