Misplaced Pages

Molecular autoionization

Article snapshot taken from Wikipedia with creative commons attribution-sharealike license. Give it a read and then ask your questions in the chat. We can research this topic together.
Chemical reaction between identical molecules to produce ions 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.

In chemistry, molecular autoionization (or self-ionization) is a chemical 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. In most cases the oxidation number on all atoms in such a reaction remains unchanged. Such autoionization can be protic (H
 transfer), or non-protic.

Examples

Protic solvents

Protic solvents often undergo some autoionization (in this case autoprotolysis):

  • 2 H 2 O ↽ − − ⇀ H 3 O + + OH − {\displaystyle {\ce {2 H2O <=> H3O+ + OH-}}}
  • 2 NH 3 ↽ − − ⇀ NH 4 + + NH 2 − {\displaystyle {\ce {2 NH3 <=> NH4+ + NH2-}}}
  • 2 H 2 SO 4 ↽ − − ⇀ H 3 SO 4 + + HSO 4 − {\displaystyle {\ce {2 H2SO4 <=> H3SO4+ + HSO4-}}}
  • 3 HF ↽ − − ⇀ H 2 F + + HF 2 − {\displaystyle {\ce {3 HF <=> H2F+ + HF2-}}}
    • Here proton transfer between two HF combines with homoassociation of F and a third HF to form HF−2


Structure of solid phosphorus pentachloride, illustrating its autoionization into PCl+4 and PCl−6.

Non-protic solvents

  • 2 PF 5 ↽ − − ⇀ PF 6 − + PF 4 + {\displaystyle {\ce {2 PF5 <=> PF6- + PF4+}}}
  • N 2 O 4 ↽ − − ⇀ NO + + NO 3 − {\displaystyle {\ce {N2O4 <=> NO+ + NO3-}}}
  • 2 BrF 3 ↽ − − ⇀ BrF 2 + + BrF 4 − {\displaystyle {\ce {2 BrF3 <=> BrF2+ + BrF4-}}}

These solvents all possess atoms with odd atomic numbers, either nitrogen or a halogen. Such atoms enable the formation of singly charged, nonradical ions (which must have at least one odd-atomic-number atom), which are the most favorable autoionization products. Protic solvents, mentioned previously, use hydrogen for this role. Autoionization would be much less favorable in solvents such as sulfur dioxide or carbon dioxide, which have only even-atomic-number atoms.

Coordination chemistry

Autoionization is not restricted to neat liquids or solids. Solutions of metal complexes exhibit this property. For example, compounds of the type FeX2(terpyridine) (where X = Cl or Br) are unstable with respect to autoionization forming [Fe(terpyridine)2][FeX4].

See also

References

  1. ^ Housecroft C.E.; Sharpe A.G. (2005). Inorganic Chemistry (2nd ed.). Pearson. ISBN 0130-39913-2.
  2. Finch, A.; Fitch, A.N.; Gates, P.N. (1993). "Crystal and Molecular structure of a metastable modification of phosphorus pentachloride". Journal of the Chemical Society, Chemical Communications (11): 957–958. doi:10.1039/C39930000957.
  3. Kamata, K.; Suzuki, A.; Nakai, Y.; Nakazawa, H., "Catalytic Hydrosilylation of Alkenes by Iron Complexes Containing Terpyridine Derivatives as Ancillary Ligands", Organometallics 2012, 31, 3825-3828. doi:10.1021/om300279t


Stub icon

This chemistry-related article is a stub. You can help Misplaced Pages by expanding it.

Categories: