Propanoic acid is a carboxylic acid found in some fermented foods. Describe the chemical properties of propanoic acid, including its reactions with metals, carbonates and alkalis. Explain how these reactions show that propanoic acid is a weak acid rather than a strong acid.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Propanoic acid (CH₃CH₂COOH) is a carboxylic acid that belongs to the same homologous series as methanoic acid, ethanoic acid and butanoic acid. Unlike hydrochloric acid, propanoic acid only partially dissociates in water.
Model answer (5 marks)
Propanoic acid reacts with metals such as magnesium or zinc to give a propanoate salt and hydrogen gas:
CH₃CH₂COOH + Mg → CH₃CH₂COO⁻Mg²⁺ + H₂↑
With carbonates it gives a propanoate salt, water and CO₂:
2 CH₃CH₂COOH + Na₂CO₃ → 2 CH₃CH₂COONa + H₂O + CO₂↑
With alkalis it undergoes neutralisation to give a propanoate salt and water:
CH₃CH₂COOH + NaOH → CH₃CH₂COONa + H₂O
In aqueous solution propanoic acid only partially dissociates:
CH₃CH₂COOH ⇌ CH₃CH₂COO⁻ + H⁺
The equilibrium lies far to the left, so the [H⁺] is much lower than that of a strong acid of the same concentration.
Because the acid is weak it has a higher pH than a strong acid of the same molarity and reacts more slowly with metals or carbonates, which is evidence that it is not a strong acid.
CH₃CH₂COOH + Mg → CH₃CH₂COO⁻Mg²⁺ + H₂↑
With carbonates it gives a propanoate salt, water and CO₂:
2 CH₃CH₂COOH + Na₂CO₃ → 2 CH₃CH₂COONa + H₂O + CO₂↑
With alkalis it undergoes neutralisation to give a propanoate salt and water:
CH₃CH₂COOH + NaOH → CH₃CH₂COONa + H₂O
In aqueous solution propanoic acid only partially dissociates:
CH₃CH₂COOH ⇌ CH₃CH₂COO⁻ + H⁺
The equilibrium lies far to the left, so the [H⁺] is much lower than that of a strong acid of the same concentration.
Because the acid is weak it has a higher pH than a strong acid of the same molarity and reacts more slowly with metals or carbonates, which is evidence that it is not a strong acid.
Examiner tips
- Write the balanced equations for each reaction; include the salt formed.
- Show the equilibrium for dissociation and explain the low [H⁺].
- Mention the higher pH and slower reaction rates as evidence of weakness.
Common mistakes
- Forgetting to include the salt in the metal reaction; writing only H₂ gas.
- Using the wrong stoichiometry for the carbonate reaction; omitting CO₂.
- Confusing the dissociation equilibrium with complete ionisation; claiming full dissociation.
Mark scheme (5 marks)
- Propanoic acid reacts with metals (e.g. magnesium/zinc) to produce a salt and hydrogen gas
- Propanoic acid reacts with carbonates to produce a salt, water and carbon dioxide
- Propanoic acid reacts with alkalis (e.g. sodium hydroxide) to produce a salt and water only (neutralisation)
- Propanoic acid only partially dissociates / ionises in water, so the concentration of H⁺ ions is lower than for a strong acid of the same concentration
- Evidence of being a weak acid: higher pH than a strong acid of the same concentration / reacts more slowly with metals or carbonates than a strong acid
Key terms in this question
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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