Water itself dissociates to a small but significant extent
As with ammonia, the constant term, [H2O] = 55.5 moles
per liter, can be incorporated into the equilibrium constant:
in which Kw, is the "ion product" for water.
The numerical value of Kw varies with temperature:
(Can you use these data and Le Chatelier's principle to predict
whether the dissociation of water is endothermic or exothermic?
Check your prediction against Appendix 2.) The value of Kw
that conventionally is used in calculations is
Because this ion product is an equilibrium constant, hydroxide
and hydrogen ion concentrations can be related in aqueous solution.
If we increase the hydrogen ion concentration by adding acid to
a solution, we will repress the dissociation of water. Some of the
added H+ will combine with OH-, and the hydroxide
ion concentration will decrease until the ion product again is 10-14
After equilibrium has been reestablished, the hydrogen and hydroxide
ion concentrations will be different than before, but their product
will be the same. Similarly, if we add hydroxide ions to a solution,
they will combine with some of the H+ originally present,
until the ion product once again is 10-14 .
When an acid and a base are mixed, some of the H+ and
OH- ions combine to form H2O. This is called
neutralization. A solution in which [H+] and [OH-]
are equal is termed a neutral solution.
In pure water or any neutral solution, the hydroxide and hydrogen
ion concentrations are