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:
- HCO3- (aq) + H2O (l) ⇌CO32- (aq) + H3O+ (aq)
- HCO3- (aq) + H3O+(aq) ⇌ CO2 (g) + H2O (l) + H2O (l)
- H2SO4 (aq) + HNO3 (aq) ⇌ HSO4- (aq) + NO2+ (aq) + H2O (l)
- HSO4- (aq) + OH- (aq) ⇌ SO42- (aq) + H2O (l)
Answers
The pairs in the order acid/base are:
- HCO3- and CO32- ; H3O+ and H2O
- H3O+ and H2O ; (CO2 + H2O) and HCO3-
- H2SO4 and HSO4- ; (NO2+ + H2O) and HNO3
- HSO4- and SO42- ; H2O and OH-
Examiner Tip
You can see from No. 2 and 3, that conjugate acid-base pairs can consist of more than one species