Ethene and ethane are both hydrocarbons that can be produced from crude oil. Explain why ethene is described as an unsaturated hydrocarbon and state the general formula of the homologous series to which ethene belongs.

Pearson Edexcel International GCSE Chemistry (4CH1) — 4.4 Alkenes · Explain · 4 marks · View as Markdown

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

Ethene has the molecular formula C₂H₄. Ethane has the molecular formula C₂H₆.

Model answer (4 marks)

Ethene contains a carbon–carbon double bond.
Because of this double bond, ethene does not contain the maximum number of hydrogen atoms – it is not saturated with hydrogen.
The general formula of the homologous series to which ethene belongs is CₙH₂ₙ.
With n = 2 the formula gives C₂H₄, which matches ethene's molecular formula.

Examiner tips

  • Use the word "unsaturated" to link the double bond to lack of hydrogen.
  • Show the general formula CₙH₂ₙ and plug in n=2 to confirm ethene fits.

Common mistakes

  • Writing the saturated formula CₙH₂ₙ₊₂ instead of CₙH₂ₙ.
  • Forgetting to explain that the double bond reduces hydrogen count.

Mark scheme (4 marks)

  1. Ethene contains a carbon–carbon double bond
  2. Because of this double bond, ethene does not contain the maximum number of hydrogen atoms (it is not saturated with hydrogen)
  3. The general formula of alkenes is CₙH₂ₙ
  4. Ethene fits this general formula because with n = 2, the formula gives C₂H₄, which matches ethene's molecular formula

Key terms in this question

unsaturated · general formula · homologous series

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