• A?Br?nsted acid?is a species that can donate a proton
  • For example, hydrogen chloride (HCl) is a Br?nsted acid as it can lose a proton to form a hydrogen (H⁺) and chloride (Cl^-) ion

HCl (aq) → H⁺?(aq) + Cl^-?(aq)

• A?Br?nsted base?is a species that can accept a proton
  • For example, a hydroxide (OH^-) ion is a Br?nsted 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 propanoic acid (CH₃CH₂COOH) partially dissociates in solution to form propanoate (CH₃CH₂COO^-) 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 CH₃CH₂COOH is linked to the base CH₃CH₂COO^-
  • CH₃CH₂COO^-?is, therefore, the?conjugate?base?of CH₃CH₂COOH

CH₃CH₂COOH - CH₃CH₂COO^-?= 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 CH₃CH₂COO^-?is linked to the acid CH₃CH₂COOH
  • CH₃CH₂COOH is therefore?conjugate?acid?of CH₃CH₂COO^-

CH₃CH₂COO^-?- CH₃CH₂COOH = 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^-