Self Dissociation of Water

Water plays an important role in acid base chemistry. It helps in the dissociation of acids and bases resulting in the formation of H+(aq) and OH (aq) ions respectively. Water itself undergoes dissociation process which is called self-dissociation of water.

Water dissociates into H+ (aq) and OH (aq) ions as:

H2O (l) ⇌ H+ (aq) + OH (aq)

The dissociation of water is extremely small and only about two out of every billion (109) water molecules are dissociated at 25°C. As a result, the concentrations of H+ (aq) and OH (aq) ions formed is also extremely low. At 25°C (298 K),

[H+] = [OH] = 1.0 × 10–7 mol L–1

Here, square brackets denote the molar concentration of the species enclosed within. Thus, [H+] denotes the concentration of H+ (aq) ions in moles per litre and [OH] the concentration of OH (aq) ions in moles per litre. In pure water and in all aqueous neutral solutions,

[H+] = [OH]

Also, in pure water as well as in all aqueous solutions at a given temperature, product of concentrations of H+ (aq) and OH (aq) always remains constant. This product is called ionic product of water and is given the symbol Kw. It is also called ionic product constant of water. Thus,

Kw = [H+] [OH]

At 25°C (298 K), in pure water, Kw can be calculated as:

Kw = (1.0 × 10–7) × (1.0 × 10–7)

= 1.0 × 10–14