Molecular autoionization

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This article is about molecular autoionization in solution chemistry by reaction of two identical molecules. For autoionization of a single atom or molecule (a different process studied in physics), see autoionization.

Molecular autoionization is a reaction between molecules of the same substance to produce ions and is created by semipolar bonds. The oxidation number on all atoms in such a reaction remains unchanged. Such autoionization can be protic (H+ transfer), or non-protic. Some examples of each are:

Protic solvents:

2 H2O is in equilibrium with H3O+ + OH – see below

3 HF is in equilibrium with H2F+ + HF2

Non-protic solvents:

2 PF5 is in equilibrium with PF6 + PF4+
2 SO2 is in equilibrium with SO32− + SO2+
2 BrF3 is in equilibrium with BrF2+ + BrF4

This type of autoionization can occur in gases or solids, but occurs most readily in liquids. In gases, the molecules are so far apart that ion formation is very unfavorable, and in solids, there is not enough molecular movement. As long as the phase remains constant, autoionization normally increases substantially with increasing pressure.

The self-ionization of water is particularly well studied, due to its implications for acid-base chemistry of aqueous solutions.

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