Autoprotolysis

Autoprotolysis

Autoprotolysis, also known as self-ionization or self-dissociation, is a chemical process in which a pure substance undergoes ionization to form ions without the addition of any external substances. This phenomenon is most commonly observed in water, but it can occur in other solvents as well. Autoprotolysis is a fundamental concept in acid-base chemistry and is crucial for understanding the behavior of amphoteric substances.

Mechanism

In autoprotolysis, a molecule of the substance acts as both an acid and a base. This dual role allows the molecule to donate a proton (H⁺) to another molecule of the same substance, resulting in the formation of ions. The general reaction for autoprotolysis can be represented as:

A + A ⇌ A⁺ + A⁻

In the case of water, the autoprotolysis reaction is:

2 H₂O ⇌ H₃O⁺ + OH⁻

Here, one water molecule donates a proton to another, forming a hydronium ion (H₃O⁺) and a hydroxide ion (OH⁻). This reaction is in equilibrium, and the equilibrium constant for this reaction is known as the ion product of water, denoted as K_w.

Ion Product of Water

The ion product of water (K_w) is a crucial parameter in aqueous chemistry. It is defined as the product of the concentrations of the hydronium and hydroxide ions in pure water:

K_w = [H₃O⁺][OH⁻]

At 25°C, the value of K_w is approximately 1.0 × 10⁻¹⁴. This value is temperature-dependent and increases with rising temperature.

Importance in Chemistry

Autoprotolysis is important for several reasons:

• pH and pOH Calculations: The concept of autoprotolysis is essential for calculating the pH and pOH of solutions. In pure water, the concentrations of H₃O⁺ and OH⁻ are equal, leading to a neutral pH of 7 at 25°C.

• Buffer Solutions: Understanding autoprotolysis helps in the design of buffer solutions, which resist changes in pH upon the addition of small amounts of acids or bases.

• Acid-Base Equilibria: Autoprotolysis is a key concept in understanding the behavior of weak acids and bases in solution.

Examples in Other Solvents

While water is the most common example, autoprotolysis can occur in other solvents such as ammonia and acetic acid. For instance, in liquid ammonia, the autoprotolysis reaction is:

2 NH₃ ⇌ NH₄⁺ + NH₂⁻

Also see

• Acid-base reaction
• pH
• Amphoteric
• Buffer solution
• Ionization