Decane (C₁₀H₂₂) is a hydrocarbon found in crude oil. When decane burns incompletely, carbon monoxide and water are produced instead of carbon dioxide and water. Explain why incomplete combustion occurs and describe two differences between the products of complete combustion and incomplete combustion of decane.

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).

Hydrocarbons are compounds that contain hydrogen and carbon only. They are found in crude oil and are widely used as fuels. The combustion of hydrocarbons can be complete or incomplete depending on the conditions.

Model answer (5 marks)

Incomplete combustion occurs when there is insufficient oxygen available for the reaction.

During complete combustion of decane the reaction is:
C10H22 + 15.5O2 → 10CO2 + 11H2O

During incomplete combustion the reaction is:
C10H22 + 7.5O2 → 10CO + 11H2O

Thus, in incomplete combustion carbon is oxidised only to CO instead of CO2.

Two differences between the products:
1. Complete combustion gives CO2, whereas incomplete combustion gives CO.
2. CO is toxic and poisonous, whereas CO2 is not toxic.

Incomplete combustion also releases less energy and may produce soot (solid carbon particles) which is not produced in complete combustion.

Examiner tips

  • Explain the cause of incomplete combustion first; then list the two differences clearly.
  • Use the exact chemical formulas (CO2 vs CO) and mention toxicity and energy release.
  • Keep the answer concise – 5 marks can be earned with 3–4 short points.

Common mistakes

  • Writing the wrong chemical equations (e.g. CO2 for incomplete combustion).
  • Failing to mention the lack of oxygen as the cause of incomplete combustion.
  • Using vague terms like "less energy" without specifying the difference in products.

Mark scheme (5 marks)

  1. Incomplete combustion occurs when there is insufficient/limited oxygen supply
  2. Complete combustion produces carbon dioxide; incomplete combustion produces carbon monoxide (instead)
  3. Carbon monoxide is toxic / poisonous whereas carbon dioxide is not (toxic)
  4. Incomplete combustion releases less energy than complete combustion
  5. Carbon (soot/particles) may also be produced during incomplete combustion but not during complete combustion

Key terms in this question

hydrocarbon · complete combustion · incomplete combustion · carbon monoxide · carbon dioxide

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