Alkanes belong to a homologous series. State the general formula of alkanes and describe three features that are shared by all members of a homologous series.

Pearson Edexcel International GCSE Chemistry (4CH1) — 4.3 Alkanes · State and Describe · 4 marks · View as Markdown

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

Model answer (4 marks)

General formula of alkanes is CₙH₂ₙ₊₂.
All members of a homologous series:
1. Have the same functional group – a single C–C single bond (alkane).
2. Show similar chemical properties – e.g. similar flammability and low reactivity.
3. Differ by a constant unit of CH₂ – each successive alkane has one more C and two more H atoms.

Examiner tips

  • Use the exact formula CₙH₂ₙ₊₂. List the three features in separate points. Mention the CH₂ unit explicitly.
  • Ensure you state that the functional group is the same (alkane).

Common mistakes

  • Writing the formula as CₙH₂ₙ instead of CₙH₂ₙ₊₂. Omitting the CH₂ unit as the difference between members. Confusing the functional group with the entire molecule (e.g. saying ‘alkane’ instead of ‘C–C single bond’).

Mark scheme (4 marks)

  1. General formula of alkanes is CₙH₂ₙ₊₂
  2. All members of a homologous series have the same functional group
  3. All members have similar chemical properties
  4. Each successive member differs by CH₂ (one carbon and two hydrogen atoms) / members differ by a constant unit of CH₂

Key terms in this question

homologous series · general formula

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