Ethene is produced industrially by the cracking of long-chain hydrocarbons. Explain why ethene is described as an unsaturated hydrocarbon and describe one addition reaction that ethene can undergo to form a saturated product.

Cambridge International IGCSE Chemistry (0620) — 11.5 Alkenes · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Ethene is an unsaturated hydrocarbon because it contains a carbon–carbon double bond (C=C). Unsaturated means the molecule does not contain the maximum number of hydrogen atoms; the double bond means there are fewer C–H bonds than in a saturated alkane of the same carbon number.

One addition reaction that ethene can undergo is hydrogenation:

C₂H₄ + H₂ → C₂H₆

In this reaction the double bond is broken and two hydrogen atoms add across it, giving ethane, which is saturated because all C–C bonds are now single bonds.

Examiner tips

  • Use the term "unsaturated" and link it to the presence of a C=C bond; mention lack of maximum H atoms. State a valid addition reaction and show the balanced equation. Explain that the product is saturated because the double bond has been converted to single bonds.
  • common_mistakes
  • :
  • Calling ethene saturated or confusing it with alkanes. Giving a non‑addition reaction or omitting the balanced equation. Failing to state that the product is saturated.

Mark scheme (4 marks)

  1. Ethene is unsaturated because it contains a carbon–carbon double bond (C=C)
  2. Unsaturated means the molecule does not contain the maximum number of hydrogen atoms (i.e. not all single bonds between carbons)
  3. Correct identification of a valid addition reaction, e.g. ethene reacts with hydrogen (hydrogenation) to form ethane, OR ethene reacts with water (steam) to form ethanol, OR ethene reacts with bromine to form dibromoethane
  4. The product is saturated because the double bond has broken and the two new atoms / groups are added across it, so all carbon–carbon bonds are now single bonds

Key terms in this question

unsaturated · addition reaction · saturated · hydrocarbon

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