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Chemistry · Ch 3 — Ionic Equilibria

Autoionization of water

3.5

Autoionization of water

Pure water ionizes to a very small extent. The ionization equilibrium of water is represented as,

H2O(l)+H2O(l)⇌H3O+(aq)+OH−(aq)\mathrm{H_2O}(l) + \mathrm{H_2O}(l) \rightleftharpoons \mathrm{H_3O^{+}(aq)} + \mathrm{OH^{-}(aq)}

The equilibrium constant (KK) for the ionization of water is given by

K=[H3O+][OH−][H2O]2...(3.12)K = \frac{[\mathrm{H_3O^{+}}][\mathrm{OH^{-}}]}{[\mathrm{H_2O}]^2} \qquad \text{...(3.12)}

orK[H2O]2=[H3O+][OH−]...(3.13)\text{or} \quad K[\mathrm{H_2O}]^2 = [\mathrm{H_3O^{+}}][\mathrm{OH^{-}}] \qquad \text{...(3.13)}

A majority of H2O\mathrm{H_2O} molecules are undissociated, consequently the concentration of water [H2O][\mathrm{H_2O}] can be treated as constant. Then [H2O]2=K′[\mathrm{H_2O}]^2 = K'. Substituting this in Eq. (3.13) we get,

K×K′=[H3O+][OH−]...(3.14)K \times K' = [\mathrm{H_3O^{+}}][\mathrm{OH^{-}}] \qquad \text{...(3.14)}

Kw=[H3O+][OH−]K_w = [\mathrm{H_3O^{+}}][\mathrm{OH^{-}}]

where Kw=KK′K_w = KK' is called ionic product of water. The product of molar concentrations of hydronium (or hydrogen) ions and hydroxyl ions at equilibrium in pure water at the given temperature is called ionic product of water. …

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