Page 94 - Instant notes

Physical Chemistry 80

the ammonium ion has greater strength as an acid than the chloride ion has as a base and
more hydronium ions will be present in solution.

Polyprotic acids and bases

A polyprotic acid is an acid that can donate more than one proton. An example is
carbonic acid, H 2CO 3. A series of stepwise acid dissociation equilibria are set up in these
systems, each of which involves donation of a single proton. For carbonic acid, these are:

Generally, the stepwise acid dissociation constants are denoted K ax as shown, where the
xth proton is removed from the polyprotic acid in each equilibrium. K ax>K a(x+1) and hence
pK ax<pK a(x+1), which means the acid strength decreases for each successive deprotonation.
This is due to the increased negative charge on the acid after each deprotonation, which
electrostatically attracts the remaining proton(s) and decreases its (their) tendency to be
donated.
At 25°C for carbonic acid, pK a1=6.37 and pK a2=10.25, whilst for phosphoric acid,
pK a1=2.12, pK a2=7.21 and pk a3=12.67. Generally, pK ax values are sufficiently widely
separated (by greater than 2 units) that each proton transfer can be considered to occur
independently and sequentially.

The relationship can therefore be applied to each
stepwise dissociation, with and a HA being the activities of the conjugate base and acid
in the the xth dissociation equilibrium. A consequence is that when the pH of the solution
is at or near pK ax, often it can be assumed that only the xth single proton dissociation
equilibrium is occurring.
The acid dissociation constant for the overall multiproton donation equilibrium, if
required, is simply given by the product of the stepwise acid dissociation constants, for
example:

K a=K a1K a2 or more generally,
The same arguments apply to polyprotic bases, which can be treated in a similar manner.
For example, for: