Acids and Bases in Water

Due to the fact that the majority of acid-base reaction take place in aqueous solution, acids and bases are most commonly defined by how they react with water.

Auto-ionisation of water

Water is able to act as an acid and a base. Equilibrium is produced by a small number of the water molecules in an aqueous solution undergoing acid-base reactions with one another, a process known as auto-ionisation of water:

2H₂O(l) ? H₃O+(aq) + OH^–(aq)

The equilibrium expression for this reaction is a follows

K_c = ([H₃O⁺][OH^–]) / [H₂O]²

As the fraction of water which dissociates into ions is so small, the concentration of water in these conditions is not significantly altered. Therefore, it is possible to assume that the concentration of water is constant and so can be incorporated into K_c:

K_c [H₂O] = K_w = [H₃O⁺][OH^–]

The final expression is called the ionic product of water. At the temperature of 25