Acids & Bases (CIE 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)

• 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

• 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

• Conjugate acid-base pairs are a pair of reactants and products that are linked to each other by the transfer of a proton
• For example, in the equilibrium reaction below, the ethanoic acid (CH₃COOH) partially dissociates in solution to form ethanoate (CH₃COO^-) and hydrogen (H⁺) ions
• 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

• In the forward reaction:
  • The acid CH₃COOH is linked to the base CH₃COO^-
  • CH₃COO^- is, therefore, the conjugate base of CH₃COOH

CH₃COOH - CH₃COO^- = conjugate acid-base pair

• • The base H₂O is linked to the acid H₃O⁺
  • H₃O⁺ is, therefore, the conjugate acid of H₂O

H₂O - H₃O⁺ = conjugate acid-base pair

• In the reverse reaction
  • The base CH₃COO^- is linked to the acid CH₃COOH
  • CH₃COOH is therefore conjugate acid of CH₃COO^-

CH₃COO^- - CH₃COOH = conjugate acid-base pair

• • The acid H₃O⁺ is linked to the base H₂O
  • H₂O is, therefore, the conjugate base of H₃O⁺

H₃O⁺ - H₂O = conjugate acid-base pair

Worked Example: Identifying conjugate acid-base pairs

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^-