Molecular 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 H₃O⁺ + OH⁻ – see below

2 NH₃ NH₄⁺ + NH₂⁻

2 H₂SO₄ H₃SO₄⁺ + HSO₄⁻

3 HF H₂F⁺ + HF₂⁻

Non-protic solvents:

2 PF₅ PF₆⁻ + PF₄⁺

2 SO₂ SO₃²⁻ + SO²⁺

2 BrF₃ 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.

See also

• Ionization
• Ion-association