Propanoic acid is a carboxylic acid found in some dairy products. Explain why propanoic acid is described as a weak acid, and describe two chemical properties it shares with stronger acids such as hydrochloric acid.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Propanoic acid has the molecular formula CH₃CH₂COOH. It is a weak acid that dissolves in water to form an acidic solution.
Model answer (5 marks)
Propanoic acid is a weak acid because it only partially ionises in water, giving only a small amount of H⁺ and propanoate ions.
1. It reacts with metals such as magnesium or zinc to give a propanoate salt and hydrogen gas:
2 CH₃CH₂COOH + 2 Mg → 2 CH₃CH₂COO⁻Mg²⁺ + H₂↑
2. It reacts with a strong base (e.g. NaOH) to give a propanoate salt and water (neutralisation):
CH₃CH₂COOH + NaOH → CH₃CH₂COONa + H₂O
3. It reacts with a carbonate or metal carbonate to give a propanoate salt, water and carbon dioxide:
2 CH₃CH₂COOH + CaCO₃ → 2 CH₃CH₂COO⁻Ca²⁺ + H₂O + CO₂↑
1. It reacts with metals such as magnesium or zinc to give a propanoate salt and hydrogen gas:
2 CH₃CH₂COOH + 2 Mg → 2 CH₃CH₂COO⁻Mg²⁺ + H₂↑
2. It reacts with a strong base (e.g. NaOH) to give a propanoate salt and water (neutralisation):
CH₃CH₂COOH + NaOH → CH₃CH₂COONa + H₂O
3. It reacts with a carbonate or metal carbonate to give a propanoate salt, water and carbon dioxide:
2 CH₃CH₂COOH + CaCO₃ → 2 CH₃CH₂COO⁻Ca²⁺ + H₂O + CO₂↑
Examiner tips
- Use the term ‘partial ionisation’ for weak acid behaviour. Show the full balanced equations for the metal and base reactions. Mention the salt as a propanoate. Include the carbonate reaction to demonstrate a third property if time allows.
Common mistakes
- Writing the acid as fully dissociated. Using the wrong salt name (e.g. ‘propanoic salt’ instead of ‘propanoate salt’). Omitting the carbonate reaction or the gas evolution.”
Mark scheme (5 marks)
- Propanoic acid is a weak acid because it only partially/incompletely dissociates (ionises) in water
- Propanoic acid reacts with metals (e.g. magnesium/zinc) to produce a salt and hydrogen gas
- Correct description of the salt formed in the metal reaction — named as a propanoate salt
- Propanoic acid reacts with bases/alkalis to produce a salt and water (neutralisation)
- Propanoic acid reacts with carbonates/metal carbonates to produce a salt, water, and carbon dioxide
Key terms in this question
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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