Ionic product of water, Kw
INSERT [IW_CI_03_ima]
Water can behave as an acid or a base. The remarkably tiny amount of water molecules that self-ionise to form ions, re-form water molecules as quickly as these ions have formed gives the dynamic equilibrium:
H2O(l) + H2O(l) H3O+(aq) + OH-(aq)
Similar to other examples of equilibria that students may have met already, an expression can be written according to the equilibrium law:
Kw = [H3O+ ] [OH-] where Kw is the equilibrium constant for the ionisation of water.
Why does [H2O] not appear in this expression?
It is not necessary to write [H2O] in the expression as so little of it is ionised and the concentration would be almost constant.
What sort of number of water molecules are ionised at any moment in neutral water?
There are about one in 10 million water molecules that each give a proton to another water molecule, forming ions. That is 1.0 x 10-7 at 25 °C. Remember that the position of an equilibrium is temperature dependent.
What can be noted about the relative number of OH– ions compared with the number of H3O+ ions?
There are always the same numbers of OH– ions and H3O+ ions present in pure water. This 1:1 ratio can be seen clearly in the equilibrium. [H3O+] = [OH–] in pure water and neutral solutions, and this is what is meant by neutral.
How does the value of pH 7 and neutral relate to this?
The value of Kw = [H+] [OH-] = 10-14 at 25 ºC.
[H+] = [OH-] = 10-7 mol dm-3, and since pH = –log10 [H+] then pH = 7 (at 25 °C).