Propene and propane are both hydrocarbons with three carbon atoms. Describe how the structures of propene and propane differ, and explain how this structural difference affects their reactions with bromine water.

Edexcel GCSE Chemistry (1CH0) — 10.2 Alkenes (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)

Propene has a carbon–carbon double bond (C=C) whereas propane has only single bonds.

Because of the C=C bond, propene undergoes addition reactions: bromine adds across the double bond, decolourising bromine water from orange to colourless.

Propane lacks a double bond, so it cannot add bromine; bromine water remains orange when propane is present.

Examiner tips

  • Use the term ‘double bond’ and ‘addition reaction’ to show understanding.
  • Show the colour change of bromine water to evidence the reaction.
  • Explain why propane cannot react – lack of a double bond.”]
  • common_mistakes

Mark scheme (4 marks)

  1. Propene contains a carbon–carbon double bond (C=C) whereas propane only contains carbon–carbon single bonds
  2. Propene decolourises / decolourisez bromine water (orange to colourless)
  3. Propane does not decolourise bromine water / bromine water remains orange with propane
  4. The C=C double bond in propene allows addition reactions to occur, so bromine adds across the double bond; propane cannot undergo addition reactions as it has no double bond

Key terms in this question

bromine water

Related

More Alkenes (Chem only) questions

▶ Try answering this question with AI marking (free) →