Autoprotolysis
Definition and meaning of Autoprotolysis in chemistry.
Autoprotolysis is the spontaneous ionization of a pure liquid where molecules act as both acid and base, donating and accepting protons to form ion pairs. It represents the self-dissociation equilibrium of a solvent.
In more detail
In autoprotolysis, a solvent molecule transfers a proton to another molecule of the same solvent, creating both a conjugate acid and conjugate base. This is an inherent property of all protic solvents and establishes the solvent's autoionization constant (Kw for water). The process is crucial because it determines the pH range and ion concentration in pure solvents, affecting chemical reactions and equilibria in those systems. Autoprotolysis explains why pure water is slightly conductive and why ammonia and other polar solvents show acidic or basic properties even when pure.
Key facts
| Water equilibrium reaction | 2 H2O ⇌ H3O⁺ + OH⁻ |
|---|---|
| Ionic product of water (Kw) | 1.0 × 10⁻¹⁴ at 25°C |
| Field | Physical Chemistry |
| pH of pure water | 7.0 (neutral despite autoionization) |
Water undergoes autoprotolysis via the equilibrium 2 H2O ⇌ H3O⁺ + OH⁻, with Kw = 1.0 × 10⁻¹⁴ at 25°C. This means pure water contains 10⁻⁷ M of both H3O⁺ and OH⁻ ions.
Frequently asked questions
Why is autoprotolysis important if pure water has a neutral pH?
Even though pH is neutral, autoprotolysis establishes Kw, the ionic product, which defines the pH-pOH relationship and controls all aqueous equilibria regardless of solution composition.
Does autoprotolysis occur only in water?
No, any protic solvent, ammonia, methanol, or acetic acid, undergoes autoprotolysis, though with different equilibrium constants and degrees of ionization.