A student tests two hydrocarbon samples, X and Y, with bromine water. Sample X decolourises the bromine water from orange to colourless. Sample Y has no effect on the bromine water. Explain what these results show about the structure and bonding of samples X and Y, and state one use for each type of hydrocarbon.

AQA A-Level Chemistry (7405) — 3.3.10 Aromatic chemistry (A-Level only) · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Sample X contains a carbon–carbon double bond, i.e. it is an alkene. The π‑bond is electron‑rich and reacts with Br₂, giving a dibromide and decolourising the orange bromine water.
Sample Y has only single C–C bonds, i.e. it is an alkane. The σ‑bonds are not reactive towards Br₂, so the bromine water remains orange.
An alkene is used as a monomer for addition polymerisation, e.g. to make polyethylene.
An alkane is used as a fuel, such as petrol or diesel.

Examiner tips

  • Use the word ‘alkene’ and ‘alkane’ explicitly; mention π‑bond vs σ‑bond. Show the reaction of Br₂ with an alkene. Provide one real‑world use for each. Keep answer concise to fit 5 marks.

Common mistakes

  • Confusing alkane with alkene or vice versa. Failing to mention the π‑bond or σ‑bond. Giving a use that is not typical for the hydrocarbon type.

Mark scheme (5 marks)

  1. Sample X is an alkene (unsaturated hydrocarbon)
  2. Sample X has a carbon-carbon double bond, which makes it reactive enough to decolourise bromine water
  3. Sample Y is an alkane (saturated hydrocarbon) / has only single bonds between carbon atoms
  4. A correct use for an alkene — e.g. making polymers / as a monomer for addition polymerisation
  5. A correct use for an alkane — e.g. as a fuel (petrol / diesel / kerosene / LPG)

Key terms in this question

bromine water

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