Ethanol can be oxidised to form ethanoic acid. Ethanoic acid is a member of the carboxylic acid homologous series. Explain why ethanoic acid is classified as a weak acid rather than a strong acid, and describe one chemical reaction that shows ethanoic acid behaves as an acid.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Ethanol is oxidised to ethanoic acid, which is a member of the carboxylic acid series. In water ethanoic acid only partially ionises:
CH₃COOH ⇌ CH₃COO⁻ + H⁺
Only a small fraction of the molecules dissociate, so the concentration of H⁺ in solution is low – this is the definition of a weak acid. In contrast, a strong acid such as HCl or H₂SO₄ completely dissociates in water, giving a high concentration of H⁺.
An example of ethanoic acid behaving as an acid is its reaction with sodium carbonate:
2 CH₃COOH + Na₂CO₃ → 2 CH₃COONa + CO₂ + H₂O
The products are the sodium acetate salt, carbon dioxide gas and water, showing the acidic character of ethanoic acid.
CH₃COOH ⇌ CH₃COO⁻ + H⁺
Only a small fraction of the molecules dissociate, so the concentration of H⁺ in solution is low – this is the definition of a weak acid. In contrast, a strong acid such as HCl or H₂SO₄ completely dissociates in water, giving a high concentration of H⁺.
An example of ethanoic acid behaving as an acid is its reaction with sodium carbonate:
2 CH₃COOH + Na₂CO₃ → 2 CH₃COONa + CO₂ + H₂O
The products are the sodium acetate salt, carbon dioxide gas and water, showing the acidic character of ethanoic acid.
Examiner tips
- Use the term ‘partial ionisation’ to show weak acid behaviour
- Contrast with ‘complete dissociation’ for strong acids
- Give a clear balanced equation with correct products
Common mistakes
- Writing the reaction with a metal instead of a carbonate or base
- Omitting the CO₂ or water product in the carbonate reaction
- Using the wrong stoichiometry for the balanced equation
Mark scheme (4 marks)
- Ethanoic acid does not fully/completely dissociate (ionise) in aqueous solution
- Strong acids fully/completely dissociate in aqueous solution (contrasting with ethanoic acid)
- Named correct reaction showing acidic behaviour — e.g. reacts with a carbonate / metal / alkali / base
- Correct product(s) named for the chosen reaction — e.g. salt (named or 'a salt') AND carbon dioxide (if carbonate used); hydrogen (if metal used); salt and water (if alkali/base used)
Key terms in this question
weak acid · carboxylic acid · ethanoic acid
Related
- All Edexcel GCSE Chemistry (1CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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