Autoprotolysis
Definition and meaning of Autoprotolysis in chemistry.
Autoprotolysis is a spontaneous chemical reaction that happens inside completely pure liquids. The liquid molecules act as both an acid and a base at the same time. They transfer protons between themselves to create a balanced pair of charged ions.
In more detail
During this unique process, one solvent molecule gives up a tiny hydrogen proton. Another molecule of that exact same solvent quickly accepts the wandering hydrogen proton. This single transfer creates a negative conjugate base and a positive conjugate acid.
This self splitting behavior is an inherent natural property of all protic solvents. Protic solvents are simply liquids that have hydrogen atoms attached to oxygen or nitrogen. This ongoing process establishes a specific autoionization constant for every unique liquid solvent.
We use the special math symbol Kw to represent this constant for pure water. This physical constant is extremely important because it dictates the basic chemical rules. It determines the natural pH range and the normal ion concentration in the pure liquid.
Autoprotolysis explains why even ultra pure water can still conduct a tiny amount of electricity. It also explains why pure liquid ammonia shows acidic or basic properties all on its own. The total amount of splitting is very small, but it controls how other reactions happen.
Key facts
| Field | Physical Chemistry |
|---|---|
| Reaction Type | Spontaneous proton transfer between identical molecules |
| Requirement | The liquid must be a protic solvent with available hydrogen atoms |
| Water Equilibrium | 2 H2O ⇌ H3O⁺ + OH⁻ |
| Ionic Product of Water (Kw) | 1.0 × 10⁻¹⁴ at 25 degrees Celsius |
| pH of Pure Water | 7.0 (neutral despite having charged ions) |
Liquid water undergoes this autoprotolysis reaction constantly every single day at normal room temperature. Two neutral water molecules crash together to form one hydronium ion and one hydroxide ion. We write the chemical equation for this proton transfer as 2 H2O ⇌ H3O⁺ + OH⁻. The equilibrium constant for this specific water reaction is 1.0 × 10⁻¹⁴ at 25 degrees Celsius. Because of this constant, pure water always contains exactly 10⁻⁷ moles per liter of both ions. This perfectly balanced concentration is exactly why pure water has a neutral pH of 7.0.
Frequently asked questions
Why is autoprotolysis important if pure water has a neutral pH?
This process establishes the Kw constant that defines the relationship between acids and bases. This hidden constant controls all chemical equilibria in the water regardless of what you dissolve in it.
Does this self splitting reaction occur only in pure liquid water?
No, any protic solvent like liquid ammonia or pure acetic acid will undergo this process. However, these other liquids have different equilibrium constants and very different degrees of natural ionization.
How does autoprotolysis affect the electrical conductivity of a pure liquid?
The moving protons create a very small number of charged ions in the pure liquid. These free floating ions allow the liquid to conduct a tiny but measurable amount of electricity.