Autoionization
Definition and meaning of Autoionization in chemistry.
Autoionization is the spontaneous ionization of a pure liquid into ions without any external catalyst or agent. This occurs due to thermal motion of molecules breaking apart into charged species in a reversible equilibrium.
In more detail
The most common example is water, which undergoes autoionization to produce hydronium and hydroxide ions. In pure water at 25°C, this process reaches an equilibrium where the product of H3O+ and OH- concentrations equals Kw (1.0 × 10^-14), resulting in each ion at approximately 10^-7 molar. This autoionization is fundamental to understanding pH, acid-base chemistry, and why even pure water conducts electricity slightly. Other polar molecules like ammonia and acetic acid also undergo autoionization, though water is the most significant example in chemistry.
Key facts
| Chemical formula | 2H2O ⇌ H3O+ + OH- |
|---|---|
| Field | Physical Chemistry |
| Equilibrium constant (Kw) at 25°C | 1.0 × 10^-14 |
| Ion concentration in pure water | ~1 × 10^-7 M each |
Pure water at 25°C undergoes autoionization according to the equilibrium 2H2O ⇌ H3O+ + OH-, producing equal concentrations of hydronium and hydroxide ions (each 10^-7 M), resulting in a neutral pH of 7.
Frequently asked questions
Why does water autoionize without an external cause?
Molecules possess thermal energy that allows some to spontaneously break their chemical bonds. This is an inherent thermodynamic property of all molecular systems above absolute zero.
Does autoionization occur in substances other than water?
Yes. Polar molecules such as liquid ammonia and acetic acid undergo autoionization, but water is the most significant example in chemistry due to its abundance and central role in aqueous chemistry.