Ethene (C₂H₄) is a hydrocarbon. A student states: 'When ethene burns completely in oxygen, the total mass of the products must equal the total mass of the reactants.' Explain why the student is correct, and state the names of the two products formed during the complete combustion of ethene.

Eduqas A-Level Chemistry — 3.2 Hydrocarbons · Explain · 5 marks · View as Markdown

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

Ethene is a hydrocarbon gas used in the production of plastics. Like all hydrocarbons, it can undergo combustion reactions.

Model answer (5 marks)

The student is correct because of the law of conservation of mass. No atoms are lost or made during a chemical reaction, so the mass of the products equals the mass of the reactants. One product is carbon dioxide (CO₂) and the other product is water (H₂O).

Examiner tips

  • Use the phrase ‘law of conservation of mass’ to justify the equality of masses.
  • Mention that no atoms are lost or created during the reaction.
  • Name the two products: CO₂ and H₂O.
  • Keep the answer concise and directly linked to the marks given.

Common mistakes

  • Writing the reaction incorrectly (e.g. missing O₂ or H₂O).
  • Forgetting to state the law of conservation of mass.
  • Using vague terms like ‘mass balance’ instead of the specific law.

Mark scheme (5 marks)

  1. The student is correct because of the law of conservation of mass
  2. No atoms are lost or made during a chemical reaction
  3. Therefore the mass of the products equals the mass of the reactants
  4. One product is carbon dioxide
  5. The other product is water

Key terms in this question

complete combustion · hydrocarbon · products · reactants

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