Ethene (C₂H₄) is an alkene produced during the cracking of long-chain hydrocarbons. Ethene can react with bromine water and can also be used to make poly(ethene). Explain why ethene is described as an unsaturated hydrocarbon, and describe what happens when ethene reacts with bromine water, including the type of reaction that occurs.

Eduqas A-Level Chemistry — 2.4 Organic compounds · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Ethene is a hydrocarbon because it contains only carbon and hydrogen atoms.
Ethene is unsaturated because it has a carbon–carbon double bond (C=C).
When ethene is added to bromine water, the orange/brown colour of the bromine disappears, becoming colourless.
The double bond opens and bromine adds across it, giving 1,2‑dibromopropane.
This is an addition reaction.

Examiner tips

  • Use the term ‘unsaturated hydrocarbon’ and cite the C=C bond.
  • Describe the colour change of bromine water.
  • Explain that the double bond breaks and Br adds across it.
  • State that the reaction is an addition reaction.

Common mistakes

  • Calling the reaction a substitution instead of addition.
  • Forgetting to mention the colour change of bromine water.
  • Using ‘hydrocarbon’ without noting the C=C bond as the reason for unsaturation.

Mark scheme (5 marks)

  1. Ethene is a hydrocarbon because it contains only carbon and hydrogen (atoms/elements)
  2. Ethene is unsaturated because it contains a carbon–carbon double bond (C=C)
  3. Bromine water changes from orange/brown to colourless when ethene is added
  4. The double bond opens/breaks and bromine adds across the double bond
  5. This type of reaction is called an addition reaction

Key terms in this question

unsaturated · hydrocarbon · bromine water

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