Water molecules can behave as both acids and bases. One water molecule can donate a Hsup+/sup ion (acid) to another water molecule, which accepts the Hsup+/sup ion (base), forming an OHsup-/sup ion and an Hsub3/subOsup+/sup ion respectively.
However, the OHsup-/sup ion is a very strong base and the Hsub3/subOsup+/sup ion is a very strong acid, therefore they will react together almost immediately to produce water again. At any instant, there is a very small amount of Hsub3/subOsup+/sup and OHsup-/sup ions present. An equilibrium is set up:
(2H2O(l) ⇌ H_3O^+{(aq)} + OH-{(aq)} )
This is usually written in its simplified form:
(H2O(l) ⇌ H^+{(aq)} + OH-{(aq)} )
The equilibrium constant for this slight dissociation of water is known as the ionic product of water, Ksubw/sub.
(Kw = [H^+]OH- )
Like any other equilibrium constant, the value of Ksubw/sub varies with temperature. At room temperature, Ksubw/sub is assumed to be 1.00 x 10sup-14/sup molsup2/sup dmsup-6/sup.
The relationship between Ksubw/sub and pKsubw/sub is the same as that between Ksuba/sub and pKsuba/sub, or H+ and pH.
(pKw = - \log_{10}{K_w} )
At room temperature, pKsubw/sub is assumed to be 14.
bThe variation of Ksubw/sub with temperature/b
The dissociation of water to form Hsup+/sup and OHsup-/sup ions is an endothermic process:
(H2O(l) ⇌ H^+{(aq)} + OH-{(aq)} )
According to Le Chatelier's Principle, if you increase the temperature, the equilibrium will move in the direction that counters the change, i.e. the endothermic direction. So, increasing the temperature will favour the forwards reaction, and produce more Hsup+/sup and OHsup-/sup ions.
Therefore, as temperature increases, the value of Ksubw/sub also increases.