Ethanoic acid is a carboxylic acid. Describe what happens when ethanoic acid reacts with sodium carbonate solution, and explain why ethanoic acid is described as a weak acid.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Ethanoic acid is the carboxylic acid found in vinegar. Its molecular formula is CH₃COOH.
Model answer (5 marks)
Ethanoic acid reacts with sodium carbonate according to the equation
CH₃COOH + Na₂CO₃ → 2 NaCH₃COO + CO₂↑ + H₂O.
The reaction gives the salt sodium ethanoate, releases carbon dioxide gas (effervescence) and water.
Ethanoic acid is a weak acid because it only partially dissociates in aqueous solution:
CH₃COOH ⇌ CH₃COO⁻ + H⁺.
Unlike a strong acid, the equilibrium lies far to the left, so the concentration of H⁺ ions is lower than that of a strong acid of the same concentration.
CH₃COOH + Na₂CO₃ → 2 NaCH₃COO + CO₂↑ + H₂O.
The reaction gives the salt sodium ethanoate, releases carbon dioxide gas (effervescence) and water.
Ethanoic acid is a weak acid because it only partially dissociates in aqueous solution:
CH₃COOH ⇌ CH₃COO⁻ + H⁺.
Unlike a strong acid, the equilibrium lies far to the left, so the concentration of H⁺ ions is lower than that of a strong acid of the same concentration.
Examiner tips
- Write the balanced equation with CO₂ and H₂O; include the salt name.
- Explain partial dissociation and compare to a strong acid; mention lower H⁺ concentration.
Common mistakes
- Omitting CO₂ or H₂O from the equation
- Claiming the acid is strong or saying it fully dissociates
- Not linking weak acid to incomplete ionisation
Mark scheme (5 marks)
- Ethanoic acid reacts with sodium carbonate to produce a salt (sodium ethanoate)
- Carbon dioxide gas is produced / effervescence / bubbles observed
- Water is also produced in the reaction
- A weak acid only partially / incompletely dissociates / ionises in aqueous solution
- Unlike a strong acid, ethanoic acid does not completely dissociate, so the concentration of H⁺ ions in solution is lower than for a strong acid of the same concentration
Key terms in this question
carboxylic acid · weak acid · sodium carbonate
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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