Ethene (C₂H₄) is an alkene produced during the cracking of long-chain hydrocarbons. Explain why ethene is described as unsaturated and describe how ethene can undergo addition polymerisation to form poly(ethene).

Edexcel A-Level Chemistry (9CH0) — 6.3 Alkenes · Explain · 5 marks · View as Markdown

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

Ethene is the simplest alkene. It is produced in large quantities by the petrochemical industry and is an important feedstock for making plastics.

Model answer (5 marks)

Ethene is unsaturated because it contains a carbon‑to‑carbon double bond (C=C). The double bond can open or break during a reaction, allowing ethene molecules to join together. In addition polymerisation, many ethene monomers add to each other to form a long chain. No other substance or small molecule is produced – it is an addition reaction. The product is poly(ethene), a long‑chain polymer with repeating –CH₂CH₂– units.

Examiner tips

  • Use the term ‘unsaturated’ and cite the C=C bond; explain that the double bond opens in an addition reaction; mention that no small molecule is released; state the product as poly(ethene) with –CH₂CH₂– repeats

Common mistakes

  • Calling ethene saturated; confusing addition with substitution; forgetting that no small molecule is produced; not specifying the –CH₂CH₂– repeat unit

Mark scheme (5 marks)

  1. Ethene is unsaturated because it contains a carbon-to-carbon double bond (C=C)
  2. The double bond can open / break to allow ethene molecules to join together
  3. Many ethene monomers join together (in a chain)
  4. No other substance / no small molecule is produced / it is an addition reaction
  5. The product is poly(ethene) / a long-chain polymer with repeating –CH₂CH₂– units

Key terms in this question

unsaturated · addition polymerisation · polymer

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