Autodissociation, or autoionization is the spontaneous separation of molecules
into ions. A molecule, or a set of molecules will split up to form
a number of positive charged particles (cations) and negative
charged particles (anions). Autodissociation is usually
associated with protic solvents; compounds that can
supply protons (H+ ions) to
stronger bases.
Water (H2O) undergoes autodissociation.
The chemical formula representing this reaction is:
2 H2O = H3O+ + OH-
or simplified (but a less-correct representation of chemical structures):
H2O = H+ + OH-
This is an equilibrium reaction, which means that under ordinary
conditions water consists of both H2O
molecules, and the H3O+ and
OH- ions.
The extent of autodissociation of water is very small. This is
expressed by the equilibrium constant:
K'25 °C = [H+][OH-]/[H2O] = (1.0·10-14)/55.56
The equilibrium constant indicates that at 25 °C, for every 55.6
moles of water, only 1.0·10-14 moles
are dissociated in the corresponding ions (approximately 1 pair of ions per
5.6·1015 molecules of water).
Since the 55.56 mol/L concentration of water molecules is essentially
constant, it is usually omitted. Thus: K25 °C =
[H+][OH-
] = 1.0·10-14
mol2/L2 is used.
However, the autodissociation constant is only constant for a given
temperature, and generally increases for increasing temperature.
Autodissociation is not restricted to water alone: many other
compounds undergo the same type of reaction:
2 HCl = H2Cl+ + Cl- (hydrochloric acid)
2 HF = H2F+ + F- (hydrofluoric acid)
2 H2SO4 = H3SO4+ + HSO4- (sulfuric acid)
2 NH3 = NH4+ + NH2- (Ammonia)
Keep in mind that these simple equations describe only part of the
chemical dissociation reactions that occur in reality. For instance, the
primary ions observed in water can undergo further solvation reaction to ions
such as
H5O2+,
H7O3+,
and
H9O4+.