Molecular autoionization

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 (or self-ionization) is a reaction between molecules of the same substance to produce ions. If a pure liquid partially dissociates into ions, it is said to be self-ionizing. 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 H
₂O ⇌ H
₃O⁺
+ OH⁻
The self-ionization of water is particularly well studied, due to its implications for acid-base chemistry of aqueous solutions.

2 NH
₃ ⇌ NH⁺
₄ + NH⁻
₂

2 H
₂SO
₄ ⇌ H
₃SO⁺
₄ + HSO⁻
₄

3 HF ⇌ H
₂F⁺
+ HF⁻
₂ Here proton transfer between two HF combines with homoassociation of F⁻
and a third HF to form HF⁻
₂.

Non-protic solvents:

2 PF
₅ ⇌ PF⁻
₆ + PF⁺
₄

2 N
₂O
₄ ⇌ NO⁺
+ NO⁻
₃

2 BrF
₃ ⇌ BrF⁺
₂ + BrF⁻
₄

The self-ionization reaction 2 SO
₂ ⇌ SO²⁻
₃ + SO²⁺
has also been proposed, but must be considered improbable because it would require the separation of doubly charged ions.

Molecular 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.

See also

• Ionization
• Ion-association