AQA A Level Chemistry复习笔记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 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


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. HCO3- and CO32- ; H3O+ and H2O
    2. H3O+ and H2O ;  (CO2 + H2O) and HCO3-
    3. H2SO4 and HSO4-  ; (NO2+ + H2O) and HNO3
    4. HSO4- and 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