AQA A Level Chemistry

Revision Notes

5.5.1 Brønsted–Lowry Acid & Bases

Brønsted–Lowry Acid & Bases

  • 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)

                                    acid                   base            conjugate base      conjugate acid 

  • The reactant CH3COOH is linked to the product CH3COOby 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

Worked Example

Identify the acid-base conjugate pairs in the following reactions:

  1. HCO3(aq) + H2O (l) ⇌CO32- (aq) + H3O+ (aq)
  2. HCO3(aq) + H3O+(aq) ⇌ CO2 (g) + H2O (l) + H2O (l)
  3. H2SO4 (aq) + HNO3 (aq) ⇌ HSO4(aq) + NO2+ (aq) + H2O (l)
  4. HSO4(aq) + OH(aq) ⇌ SO42- (aq) + H2O (l)


The pairs in the order acid/base are:

    1. HCO3and CO32- ; H3O+ and H2O
    2. H3O+ and H2O ;  (CO2 + H2O) and HCO3
    3. H2SO4 and HSO4–  ; (NO2+ + H2O) and HNO
    4. HSO4and SO42- ; H2O and OH– 

Exam Tip

You can see from No. 2 and 3, that conjugate acid-base pairs can consist of more than one species


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