Brønsted–Lowry Acid & Base Theory (Edexcel International A Level Chemistry): Revision Note

Brønsted–Lowry Acids & Bases

• The Brønsted-Lowry Theory defines acids and bases in terms of proton transfer between chemical compounds

• A Brønsted acid is a species that can donate a proton

  • For example, hydrogen chloride (HCl) is a Brønsted acid as it can lose a proton to form a hydrogen (H⁺) and chloride (Cl^-) ion

            HCl (aq) → H⁺ (aq) + Cl^- (aq)

• A Brønsted base is a species that can accept a proton

  • For example, a hydroxide (OH^-) ion is a Brønsted base as it can accept a proton to form water

  OH^- (aq) + H⁺ (aq) → H₂O (l)

Conjugate acid-base pairs

• Conjugate acid-base pairs are a pair of reactants and products that are linked to each other by the transfer of a proton

• In a reaction equilibrium, the products are formed at the same rate as the reactants are used

CH₃COOH (aq) + H₂O (l)         ⇌    CH₃COO^- (aq)          +         H₃O⁺ (aq)

 acid                 base                    conjugate base             conjugate acid

• The reactant CH₃COOH is linked to the product CH₃COO^- by the transfer of a proton from the acid to the base

• Similarly, the H₂O molecule is linked to H₃O⁺ ion by the transfer of a proton

• These pairs are therefore called conjugate acid-base pairs

  • Conjugate here means related

  • In other words, the acid and base are related to each other by one proton difference