Ethene (C₂H₄) is a hydrocarbon. When ethene burns completely in oxygen, carbon dioxide and water are produced. The unbalanced equation for this reaction is: C₂H₄ + O₂ → CO₂ + H₂O. Explain why the total mass of the products is equal to the total mass of the reactants, and state the correctly balanced symbol equation for the complete combustion of ethene.

WJEC A-Level Chemistry (Wales) — 3.2 Hydrocarbons · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

No atoms are lost or made during the reaction, so the total mass of the products equals the total mass of the reactants (conservation of mass). The correctly balanced symbol equation for the complete combustion of ethene is: C₂H₄ + 3O₂ → 2CO₂ + 2H₂O

Examiner tips

  • Show the conservation of mass explicitly; mention that atoms are neither created nor destroyed.
  • Balance the equation step by step: 2C → 2CO₂, 4H → 2H₂O, then balance O atoms.
  • Use correct stoichiometric coefficients (3O₂).

Common mistakes

  • Leaving the equation unbalanced or using the wrong coefficients for O₂.
  • Failing to state the conservation of mass or not explaining why masses are equal.

Mark scheme (5 marks)

  1. No atoms are lost or made during the reaction
  2. The mass of the products equals the mass of the reactants (conservation of mass / law of conservation of mass stated or referenced)
  3. Correct balancing of CO₂: 2CO₂
  4. Correct balancing of H₂O: 2H₂O
  5. Correct balancing of O₂: 3O₂, giving the full balanced equation C₂H₄ + 3O₂ → 2CO₂ + 2H₂O

Key terms in this question

hydrocarbon · complete combustion · balanced equation

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