Ethene can react with steam in the presence of a phosphoric acid catalyst to produce ethanol. Explain why ethene is described as an unsaturated hydrocarbon and state the type of reaction that occurs when ethene reacts with steam.

Pearson Edexcel International GCSE Chemistry (4CH1) — 4.4 Alkenes · Explain · 4 marks · View as Markdown

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

Ethene has the molecular formula C₂H₄. It reacts with steam at high temperature and pressure in the presence of a phosphoric acid catalyst to form ethanol, C₂H₅OH.

Model answer (4 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).
The double bond means not all carbon valencies are satisfied by hydrogen, so it can react.
The reaction with steam is an addition reaction.

Examiner tips

  • Use the term ‘unsaturated’ and mention the C=C bond. Explain that the double bond gives extra reactivity. State ‘addition reaction’ clearly. Keep the answer short and to the point.

Common mistakes

  • Calling ethene saturated or missing the double bond. Describing the reaction as a substitution instead of addition. Forgetting to mention that ethene is a hydrocarbon.

Mark scheme (4 marks)

  1. Ethene is a hydrocarbon because it contains hydrogen and carbon atoms only
  2. Ethene is unsaturated because it contains a carbon–carbon double bond (C=C)
  3. The double bond allows ethene to react / the double bond opens / not all carbon valencies satisfied by hydrogen
  4. The reaction with steam is an addition reaction

Key terms in this question

unsaturated · hydrocarbon

Related

More Alkenes questions

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