Explain why hydrochloric acid is described as a strong acid, and why ethanoic acid is described as a weak acid. In your answer, refer to the degree of ionisation of each acid in water.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Hydrochloric acid is a strong acid because it completely ionises in water. All or nearly all HCl molecules dissociate to give H⁺ and Cl⁻ ions, so the concentration of H⁺ in solution is very high.
Ethanoic acid is a weak acid because it only partially ionises in water. Only a small proportion of CH₃COOH molecules release H⁺ ions; an equilibrium is established with most molecules remaining un‑ionised.
Consequently, at the same concentration, HCl gives a higher [H⁺] and therefore a lower pH than ethanoic acid.
Ethanoic acid is a weak acid because it only partially ionises in water. Only a small proportion of CH₃COOH molecules release H⁺ ions; an equilibrium is established with most molecules remaining un‑ionised.
Consequently, at the same concentration, HCl gives a higher [H⁺] and therefore a lower pH than ethanoic acid.
Examiner tips
- Use the term ‘completely ionises’ for HCl and ‘partially ionises’ for CH₃COOH.
- Mention the equilibrium for ethanoic acid and the high [H⁺] for HCl.
- Show the link to pH – higher [H⁺] gives lower pH.
Common mistakes
- Saying HCl is only ‘strong’ without explaining ionisation.
- Confusing the degree of ionisation with the strength of the acid.
- Omitting the equilibrium statement for ethanoic acid.
Mark scheme (5 marks)
- Hydrochloric acid completely / fully ionises in water
- All / nearly all HCl molecules produce H⁺ ions (and Cl⁻ ions) in solution
- Ethanoic acid only partially / incompletely ionises in water
- Only a small proportion of ethanoic acid molecules release H⁺ ions in solution / an equilibrium is established with mostly un-ionised molecules present
- Therefore hydrochloric acid has a higher concentration of H⁺ ions than ethanoic acid at the same concentration, so has a lower pH
Key terms in this question
strong acid · weak acid · ionisation
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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