Brønsted–Lowry Acid & Bases (AQA A Level Chemistry)

• 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)

Weak acids dissociating

• In an equilibrium reaction, the products are formed at the same rate as the reactants are used
• This means that at equilibrium, both reactants and products are present in the solution
• For example, ethanoic acid (CH₃COOH) is a weak acid that partially dissociates in solution
• When equilibrium is established there are CH₃COOH, H₂O, CH₃COO^- and H₃O⁺ ions present in the solution
• The species that can donate a proton are acids and the species that can accept a proton are bases

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

• The reactant CH₃COOH is linked to the product CH₃COO^- by the transfer of a proton from the acid (CH₃COOH) to the base (CH₃COO^-)
• Similarly, the H₂O molecule is linked to H₃O⁺ ion by the transfer of a proton
• These pairs are therefore called conjugate acid-base pairs
• A conjugate acid-base pair is two species that are different from each other by an H⁺ ion
  • Conjugate here means related
  • In other words, the acid and base are related to each other by one proton difference

Answers

The pairs in the order acid/base are:

1. 1. HCO₃^- and CO₃^2- ; H₃O⁺ and H₂O
   2. H₃O⁺ and H₂O ;  (CO₂ + H₂O) and HCO₃^-
   3. H₂SO₄ and HSO₄^-  ; (NO₂⁺ + H₂O) and HNO₃ 
   4. HSO₄^- and SO₄^2- ; H₂O and OH^-