Ethanoic acid is a carboxylic acid. Explain why ethanoic acid is described as a weak acid and describe what is observed when ethanoic acid reacts with sodium carbonate.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Ethanoic acid (CH₃COOH) is the carboxylic acid found in vinegar. It can react with several substances, including sodium carbonate (Na₂CO₃).
Model answer (5 marks)
Ethanoic acid is a weak acid because it is only partially dissociated in water; only a small fraction of the CH₃COOH molecules release H⁺ (or H₃O⁺) ions, so the concentration of H⁺ is low compared with a strong acid.
When ethanoic acid reacts with sodium carbonate, the acid protonates the carbonate ion to form carbonic acid, which immediately decomposes:
CH₃COOH + Na₂CO₃ → 2 NaCH₃COO + CO₂↑ + H₂O
Effervescence (bubbling) is observed as CO₂ gas is released, and a sodium ethanoate salt and water are also produced.
When ethanoic acid reacts with sodium carbonate, the acid protonates the carbonate ion to form carbonic acid, which immediately decomposes:
CH₃COOH + Na₂CO₃ → 2 NaCH₃COO + CO₂↑ + H₂O
Effervescence (bubbling) is observed as CO₂ gas is released, and a sodium ethanoate salt and water are also produced.
Examiner tips
- Use the term ‘partially dissociated’ to show understanding of weak acid behaviour. Show the balanced equation and note the CO₂ effervescence. Mention the salt and water as products to hit all marks.
Common mistakes
- Writing the acid as fully dissociated or calling it a strong acid. Forgetting to include the CO₂ gas or the salt in the reaction description.
Mark scheme (5 marks)
- Ethanoic acid is partially/incompletely dissociated (ionised) in aqueous solution
- This means only a low concentration of H⁺ ions (or H₃O⁺) is produced compared to a strong acid
- Effervescence / bubbling is observed when ethanoic acid reacts with sodium carbonate
- Carbon dioxide gas is produced in the reaction
- A salt (sodium ethanoate) and water are also formed as products
Key terms in this question
weak acid · carboxylic acid · sodium carbonate
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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