autoionization of water
Summary
Autoionization of water is the reaction in which two water molecules transfer a proton to form H₃O⁺ and OH⁻: 2H₂O ⇌ H₃O⁺ + OH⁻. At 25 °C, Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴, so pure water has pH 7 and the product stays fixed in any aqueous solution.
Detail
Meaning: Water is amphoteric. One molecule acts as a Brønsted-Lowry acid and donates a proton to another that acts as a base. The equilibrium lies far to the left, so only a tiny fraction of molecules are ionized. In pure water, [H₃O⁺] = [OH⁻] = 1.0 × 10⁻⁷ M at 25 °C. H⁺ and H₃O⁺ are used interchangeably.
Equation: Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25 °C, where concentrations are in mol/L (M) and Kw is unitless by convention. Taking −log of both sides gives pH + pOH = 14 = pKw.
On the MCAT: You are given [H⁺] or [OH⁻] for a strong acid, strong base, or salt solution and asked for pH, pOH, or the other ion's concentration. Kw is also the link between a conjugate pair: Ka × Kb = Kw. Passages may give a temperature other than 25 °C. Autoionization is endothermic, so heating water raises Kw, lowers pKw, and lowers the pH of pure water below 7. The water is still neutral because [H⁺] = [OH⁻].
Common confusion: Students think pH 7 means neutral at every temperature. Neutral means [H⁺] = [OH⁻]. pH 7 is neutral only when Kw = 1.0 × 10⁻¹⁴, which is true at 25 °C. Also, adding acid or base does not change Kw. It only shifts the balance of H⁺ and OH⁻ so their product stays equal to Kw.
Sources
- Chemistry: The Central Science
- Chemistry (Zumdahl)
- General Chemistry: Principles and Modern Applications
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.