Brønsted–Lowry Acid & Base Theory (Edexcel IAL Chemistry)

Brønsted–Lowry Acids & Bases

Spec point — 14.1

• 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 OH-(aq) + H⁺(aq) → H₂O (l)

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

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