Ethene reacts with bromine water. Describe what is observed and explain why ethene reacts in this way but ethane does not.

AQA GCSE Chemistry (8462) — 4.7.2 Reactions of alkenes and alcohols (Chem only) · Describe and Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Bromine water changes from orange to colourless.
Ethene is unsaturated and contains a C=C double bond.
The double bond breaks and bromine adds across it – an addition reaction.
Ethane is saturated, has no double bond, so it does not react with bromine water.

Examiner tips

  • Use the exact phrase ‘orange to colourless’ for the observation. Mention the double bond and addition mechanism. Explain why the absence of a double bond stops the reaction.
  • common_mistakes
  • :
  • Saying the reaction is a substitution instead of addition. Forgetting to mention that ethane is saturated and has no double bond.

Mark scheme (4 marks)

  1. Bromine water turns from orange to colourless
  2. Ethene is unsaturated / has a carbon–carbon double bond
  3. The double bond breaks / opens and bromine adds across the double bond (addition reaction)
  4. Ethane is saturated / has no double bond, so it does not react with bromine water

Key terms in this question

bromine water

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