Conjugate Acids & Bases (DP IB Chemistry: HL): Revision Note

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

• Acid–base reactions are often reversible, so in the reverse reaction the products also act as an acid and a base

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

How to deduce the formula of a conjugate acid or base

• To find the conjugate base of an acid, remove one H⁺: take away one H and lower the charge by 1

  • HCl → Cl^-

  • NH₄⁺ → NH₃

  • H₂O → OH^-

  • HCO₃^- → CO₃^2-

• To find the conjugate acid of a base, add one H⁺: add one H and raise the charge by 1

  • NH₃ → NH₄⁺

  • H₂O → H₃O⁺

  • OH^- → H₂O

  • CO₃^2- → HCO₃^-