THE AUTOIONIZATION OF H 2 O
This might surprise you, but a glass of water is not entirely composed of
molecules of H 2 O. Small amounts of H+(aq) and OH−(aq) are also present; these
are formed during the spontaneous dissociation of water, a process called
autoionization.
H 2 O(aq) H+(aq) + OH−(aq)Autoionization is reversible, and an equilibrium exists in which [H+], [OH−],
and [H 2 O] are stable.
This equilibrium can be upset by the addition of compounds that alter [H+] or
[OH−], as predicted by Le Chatelier’s principle. In fact, you’ll see that aqueous
acid-base chemistry is nothing new; it is simply the study of how other
compounds introduced into solution can disturb the autoionization equilibrium
of water.
Dissociation Constant for Water, K (^) w
The equilibrium expression for the autoionization of water is
Kw = [H+][OH−]
(Remember that H 2 O doesn’t appear in the equilibrium expression because it is
the solvent.)
For pure water at 25°C, both [H+] and [OH−] are 10−7 M. Therefore, in pure
water
Kw = [H+][OH−]