Show by means of an equilibrium equation the self-ionisation of water that occurs even in 'deionised' water. - Leaving Cert Chemistry - Question 1 - 2021

In pure water, the self-ionization can be represented by the following equilibrium equation:

$H_2O + H_2O \rightleftharpoons H_3O^+ + OH^-$

This equation indicates that two water molecules interact, resulting in the generation of hydronium ions (H₃O⁺) and hydroxide ions (OH⁻).

Explanation

Even in deionized water, which has had most of its ions removed, the self-ionization still occurs due to the inherent properties of water molecules. The equilibrium constant for this reaction at 25 °C is represented by the ion product constant of water, $K_w$, calculated as:

$K_w = [H_3O^+][OH^-] = 1.0 \times 10^{-14}$

This illustrates that even in the absence of added ions, water molecules can still dissociate and establish a dynamic equilibrium, albeit at a very low concentration of ions.