Page 92 - Instant notes

Physical Chemistry 78

+
−
HCl (aq)+H 2O (1)→H 3O (aq)+Cl (aq)
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:
−
+
KOH (aq)→K (aq)+OH (aq)
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,
K a, given by (see Topic C1):

The smaller the value of pK a (or as pK a=−log 10K a, the larger the value of K a) the further
the equilibrium position is towards the right-hand side and the more the acid is
dissociated. Thus pK a 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 pK a.
By taking logarithms of both sides, this equation rearranges to:

which is the Henderson-Hasselbach equation (see Topic C4).
From this equation, pK a is the pH at which the activity of the acid, HA, and its
−
conjugate base, A , are equal, when the logarithmic term becomes equal to zero.
Base dissociation constants or basicity constants, K b, can also be used for base
equilibria. The base dissociation constant is the equilibrium constant for the general
reaction: