Propanoic acid reacts with sodium carbonate solution. Describe what you would observe during this reaction and explain why carboxylic acids are described as weak acids.

Eduqas A-Level Chemistry — 4.5 Carboxylic acids and their derivatives · Describe and Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Propanoic acid (CH₃CH₂COOH) is a carboxylic acid found in some dairy products. Like all carboxylic acids, it can react with carbonates and only partially dissociates in water.

Model answer (5 marks)

During the reaction the solution fizzes, producing bubbles of gas.
The gas is CO₂.
A carboxylic acid is a weak acid because it only partially dissociates in water, so only a small proportion of its molecules release H⁺.
A strong acid, by contrast, fully dissociates in water.

Examiner tips

  • Use the word ‘fizzing’ or ‘effervescence’ for the observation. State the gas as CO₂. Explain that weak acids only partially dissociate, not fully.
  • common_mistakes
  • :
  • Saying the acid is a ‘strong’ acid. Forgetting to mention that only a small proportion of molecules release H⁺. Using vague terms like ‘gas’ without specifying CO₂.

Mark scheme (5 marks)

  1. Observation: effervescence / bubbles / fizzing is seen
  2. The gas produced is carbon dioxide (CO₂)
  3. A weak acid only partially dissociates / ionises in water
  4. A strong acid fully / completely dissociates in water
  5. Carboxylic acids are weak because only a small proportion of their molecules release hydrogen ions (H⁺) in solution

Key terms in this question

carboxylic acid · weak acid

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