Ethanol belongs to the homologous series called alcohols. Describe four features that define a homologous series and explain why ethanol and propanol both belong to the same homologous series.

AQA A-Level Chemistry (7405) — 3.3.8 Aldehydes and ketones (A-Level only) · Describe · 5 marks · View as Markdown

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

Model answer (5 marks)

A homologous series is a series of compounds that have the same functional group.
1. All members contain the –OH functional group.
2. They show similar chemical properties.
3. Each successive member differs by one CH₂ unit.
4. The general formula is CₙH₂ₙ₊₁OH.
Ethanol (C₂H₅OH) and propanol (C₃H₇OH) both contain the –OH group, so they are both alcohols. Propanol has one more CH₂ group than ethanol, satisfying the CH₂ difference rule, so both belong to the same homologous series.

Examiner tips

  • Use the exact wording ‘same functional group’ and ‘CH₂ unit’ to match the mark scheme.
  • Show the general formula to demonstrate the pattern.
  • Explain the CH₂ difference explicitly between ethanol and propanol.

Common mistakes

  • Writing ‘same carbon chain length’ instead of ‘same functional group’.
  • Omitting the CH₂ difference or the general formula.
  • Using the wrong functional group (e.g., –COOH) for alcohols.

Mark scheme (5 marks)

  1. A homologous series is a series of compounds with the same functional group
  2. Members of a homologous series have similar chemical properties
  3. Each successive member differs by CH₂
  4. Ethanol and propanol both contain the –OH functional group / are both alcohols
  5. Ethanol (C₂H₅OH) and propanol (C₃H₇OH) differ by one CH₂ unit / propanol has one more CH₂ than ethanol

Key terms in this question

homologous series · alcohols

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