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+.

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