Methanoic acid is a carboxylic acid found naturally in ant venom. Explain why methanoic acid is described as a weak acid, and describe what you would observe when a small piece of magnesium ribbon is added to a dilute solution of methanoic acid.

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

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

Methanoic acid (HCOOH) is the simplest carboxylic acid. It is a weak acid with a sharp, pungent smell.

Model answer (5 marks)

Methanoic acid is a weak acid because it only partially ionises in water, giving a low concentration of H⁺ ions.

When a small piece of magnesium ribbon is added to a dilute solution of methanoic acid:
1. Effervescence is observed as bubbles of hydrogen gas form.
2. The magnesium ribbon gradually dissolves, becoming smaller.
3. The reaction proceeds more slowly and is less vigorous than with a strong acid of the same concentration.

Examiner tips

  • Use the term ‘partial ionisation’ to explain weakness. Show the reaction equation Mg + 2HCOOH → Mg(CH₃COO)₂ + H₂. Mention the low H⁺ concentration. Describe the gradual dissolution and slower reaction compared to a strong acid.

Common mistakes

  • Claiming methanoic acid is a strong acid. Forgetting to mention the low H⁺ concentration. Describing the reaction as instantaneous or vigorous like with HCl.

Mark scheme (5 marks)

  1. Methanoic acid only partially dissociates (ionises) in water
  2. So the concentration of H⁺ ions in solution is low
  3. Effervescence / bubbles are observed
  4. The magnesium gradually dissolves / gets smaller
  5. The reaction is slower / less vigorous than with a strong acid of the same concentration

Key terms in this question

weak acid · carboxylic acid

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