Conjugate Acids & Bases (Cambridge International A-Level Chemistry)

Conjugate Acids & Bases

• A Brønsted-Lowry acid is a species that can donate a proton
• A Brønsted-Lowry base is a species that can accept a proton
• In an equilibrium reaction, 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

  • A conjugate acid-base pair is two species that are different from each other by a H⁺ ion

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

Worked Example Identifying conjugate acid-base pairs

Identify the conjugate acid-base pairs in the following equilibrium reaction:

NH₃(g) + H₂O (l)$\rightleftharpoons$NH₄⁺(aq) + OH^–(aq)

Answer:

• In the forward reaction:

  • NH₄⁺ is the conjugate acid of the base NH₃
  • OH^- is the conjugate base of the acid H₂O

• In the reverse reaction

  • NH₃ is the conjugate base of the acid NH₄⁺
  • H₂O is the conjugate acid of the base OH^–

Worked Example In the equilibrium reaction shown below, which species are a conjugate acid-base pair?

CH₃CH₂CH₂COOH (aq) + H₂O (l) ⇌ CH₃CH₂CH₂COO^- (aq) + H₃O⁺(aq)

A. CH₃CH₂CH₂COOH and H₂O

B. H₂O and H₃O⁺

C. H₂O and CH₃CH₂CH₂COO^-

D. CH₃CH₂CH₂COO^- and H₃O⁺

Answer:

• The correct option in B

  • A conjugate acid-base pair differ only by an H⁺ ion