A student is studying two organic compounds: ethanol and ethanoic acid. State the functional group present in each compound and explain why these two compounds belong to different homologous series.

Cambridge International IGCSE Chemistry (0620) — 11.1 Formulae, functional groups and terminology · State and Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Ethanol contains the hydroxyl group –OH.
Ethanoic acid contains the carboxyl group –COOH.
A homologous series is defined by having the same functional group.
Because ethanol and ethanoic acid have different functional groups, they belong to different homologous series.

Examiner tips

  • Use the exact terms – hydroxyl group and carboxyl group. Explain that a homologous series is defined by a common functional group. Show the link between different groups and different series.

Common mistakes

  • Writing ‘hydroxyl’ for ethanol but not specifying –OH. Calling ethanoic acid an ‘acid’ without mentioning the –COOH group. Saying they are in the same series because they both contain oxygen.”

Mark scheme (4 marks)

  1. Functional group of ethanol is the hydroxyl group / –OH
  2. Functional group of ethanoic acid is the carboxyl group / –COOH
  3. Each homologous series has the same functional group / a different functional group gives a different homologous series
  4. Ethanol and ethanoic acid have different functional groups so they cannot belong to the same homologous series

Key terms in this question

functional group · homologous series

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