Write a chemical equation that illustrates the autoionization of water.

The autoionization of water is:

2H₂O ⇄  H₃O⁺  +  OH⁻        Kw

Explanation

2 moles of water can generate hydronium and hydroxide, when they work as an acid or as a base

If we take account that the concentration of protons (hydroniums), at the standard temperature is 1×10⁻⁷ M, it can be considered that the molarity of water is a constant that can be incorporated into a “greater” constant that also includes to Kc and that is known as ionic product of water, Kw. The expression is:

Kw = [H₃O⁺] . [OH⁻] / [H₂O]²

We do not include water → Kw =  [H₃O⁺] . [OH⁻]

Since the water dissociation reaction produces the same concentration of H₃O⁺ as OH⁻, [OH⁻] in pure water will also be 1×10⁻⁷ M

Kw = 1×10⁻⁷ . 1×10⁻⁷ = 1×10⁻¹⁴

pKw = pH + pOH

14 = 7 + 7