Propanoic acid is a carboxylic acid used as a food preservative. Describe what is observed and explain what happens when propanoic acid reacts with sodium carbonate powder.

Eduqas A-Level Chemistry — 3.4 Carbonyls and acids · Describe and Explain · 5 marks · View as Markdown

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

Propanoic acid is a carboxylic acid with the formula CH₃CH₂COOH. It is a weak acid used to prevent mould growth in bread and other baked goods.

Model answer (5 marks)

Effervescence (bubbles/fizzing) is observed when propanoic acid reacts with sodium carbonate powder.

The reaction produces carbon dioxide gas, water and sodium propanoate:
CH₃CH₂COOH + Na₂CO₃ → 2 NaCH₃CH₂COO + CO₂ + H₂O.

Propanoic acid, being a carboxylic acid, donates a proton (H⁺) to the carbonate ion. The carbonate ion accepts the proton, forming bicarbonate and then carbon dioxide, which escapes as a gas. The remaining sodium ion combines with the propanoate anion to give the salt sodium propanoate.

Examiner tips

  • Mention effervescence first to show observation. Include the balanced equation to demonstrate understanding of products. Explain the acid–base mechanism: donation of H⁺ and formation of CO₂.
  • common_mistakes
  • :
  • Omitting the gas (CO₂) or the salt (sodium propanoate). Failing to explain that the acid donates H⁺ to the carbonate. Not balancing the equation or using incorrect stoichiometry.

Mark scheme (5 marks)

  1. Effervescence / bubbles / fizzing is observed
  2. Carbon dioxide gas is produced / released
  3. A salt (sodium propanoate) is formed
  4. Water is also produced
  5. Carboxylic acids react with carbonates because they are acids / they donate H⁺ ions / the acid neutralises the carbonate

Key terms in this question

carboxylic acid · carbonate

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