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Definition of Self-ionization of water

Self-ionization of water Definition from Encyclopedia Dictionaries & Glossaries
Wikipedia English - The Free Encyclopedia
The self-ionization of water (also autoionization of water, and autodissociation of water) is an ionization reaction in pure water or an aqueous solution, in which a water molecule, H2O, loses the nucleus of one of its hydrogen atoms to become a hydroxide ion, OH-. The hydrogen nucleus, H+]], immediately protonates another water molecule to form hydronium, H3O+. It is an example of autoprotolysis, and exemplifies the amphoteric nature of water.

Equilibrium constant
Chemically pure water has an electrical conductivity of 0.055 µS·cm-1. According to the theories of Svante Arrhenius, this must be due to the presence of ions. The ions are produced by the self-ionization reaction
H2O + H2O H3O+ + OH-
This equilibrium applies to pure water and any aqueous solution.

Approximating activities by concentrations, the chemical equilibrium constant, Keq, for this reaction is given by:

If the concentration of dissolved solutes is low, the concentration [H2O] can be taken as being constant at c. 55.5M.

Expressed with activities , instead of concentrations, the thermodynamic equilibrium constant for the water ionization reaction is:


which is numerically equal to the more traditional thermodynamic equilibrium constant written as:

under the assumption that the sum of the chemical potentials of H+ and H3O+ is formally equal to twice the chemical potential of H2O at the same temperature and pressure.
, immediately protonates another water molecule to form hydronium, H3O+. It is an example of autoprotolysis, and exemplifies the amphoteric nature of water.

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