Ethene (C₂H₄) is a hydrocarbon produced during the cracking of long-chain alkanes. Ethene can react with bromine water. Explain why ethene reacts with bromine water but ethane (C₂H₆) does not, and state what is observed when ethene is added to bromine water.

Eduqas GCSE Chemistry — 2.6 Carbon chemistry / organic chemistry · Explain · 4 marks · View as Markdown

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

Cracking of long-chain alkanes produces smaller, more useful molecules including alkenes such as ethene. Alkenes are unsaturated hydrocarbons because they contain a carbon-to-carbon double bond.

Model answer (4 marks)

Ethene contains a carbon‑to‑carbon double bond, making it unsaturated.
Ethane is saturated, having only single bonds and no double bond.
The double bond in ethene opens during an addition reaction, allowing bromine to add across it.
When ethene is added to bromine water, the orange/brown colour of the bromine is decolourised, giving a colourless solution.

Examiner tips

  • Mention the unsaturation of ethene and saturation of ethane. Explain the addition mechanism. State the colour change of bromine water.
  • common_mistakes
  • :
  • Confusing ethane with ethene. Failing to mention the addition reaction. Not describing the colour change of bromine water.

Mark scheme (4 marks)

  1. Ethene contains a carbon-to-carbon double bond / is unsaturated
  2. Ethane is saturated / contains only single bonds / has no double bond
  3. The double bond in ethene opens/breaks to allow bromine to add across it (addition reaction)
  4. Bromine water is decolourised / changes from orange/brown to colourless (with ethene)

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