Carboxylic acids form a homologous series. State the functional group present in all carboxylic acids, name the first four members of this homologous series, and state one typical chemical property shown by carboxylic acids.

Pearson Edexcel International GCSE Chemistry (4CH1) — 4.6 Carboxylic acids · State · 4 marks · View as Markdown

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

Model answer (4 marks)

Functional group: –COOH

First four members: methanoic acid, ethanoic acid, propanoic acid, butanoic acid

Typical property: reacts with metals to give a salt and hydrogen gas, e.g.
CH₃COOH + Zn → Zn(CH₃COO)₂ + H₂

Examiner tips

  • Use the exact term –COOH for the functional group. List the four acids in order of increasing carbon number. Give a clear example of an acidic reaction, such as with a metal or a base.

Common mistakes

  • Writing the functional group as –COO or –COO⁻ instead of –COOH. Omitting the order of the acids or giving only two members. Stating a non‑acidic property such as a high boiling point.

Mark scheme (4 marks)

  1. Functional group is –COOH
  2. Names two correct members from: methanoic acid, ethanoic acid, propanoic acid, butanoic acid (1 mark for any two correct)
  3. Names the remaining two correct members from: methanoic acid, ethanoic acid, propanoic acid, butanoic acid (1 mark for the other two correct)
  4. States one typical acidic property, e.g. reacts with metals / carbonates / bases / alkalis to form a salt; or has a pH below 7; or turns litmus red / universal indicator red-orange

Key terms in this question

functional group · homologous series · carboxylic acid

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