Propene and butene are both members of the alkene homologous series. Explain why propene is classified as an unsaturated hydrocarbon, and state the empirical formula and general formula of the alkene homologous series.

Pearson Edexcel International GCSE Chemistry (4CH1) — 4.1 Introduction · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Propene contains a carbon‑to‑carbon double bond, which means it does not have the maximum number of hydrogen atoms that a saturated hydrocarbon would have. Therefore propene is classified as an unsaturated hydrocarbon.

The empirical formula of alkenes is CH₂.

The general formula of the alkene homologous series is CₙH₂ₙ.

Examiner tips

  • Use the double‑bond explanation to justify unsaturation; mention hydrogen deficiency. Include both empirical and general formulas exactly as CH₂ and CₙH₂ₙ. Keep the answer concise – one sentence each point.
  • Use UK spelling (e.g., ‘hydrocarbon’).

Common mistakes

  • Writing the empirical formula as CH instead of CH₂. Confusing the general formula with CₙH₂ₙ₊₂ (saturated alkanes). Adding unnecessary detail such as the molecular formula of propene (C₃H₆).

Mark scheme (4 marks)

  1. Propene contains a carbon-to-carbon double bond
  2. The presence of the double bond means propene is unsaturated (not all bonds are single bonds / not the maximum number of hydrogen atoms)
  3. Empirical formula of alkenes is CH₂
  4. General formula of alkenes is CₙH₂ₙ

Key terms in this question

unsaturated · empirical formula · general formula · alkene · homologous series

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