Methane undergoes complete combustion according to the equation: CH₄ + 2O₂ → CO₂ + 2H₂O. Explain what is meant by complete combustion and describe the products formed. Then explain why incomplete combustion of methane is considered more dangerous to human health than complete combustion.

Edexcel A-Level Chemistry (9CH0) — 8.1 Enthalpy changes and Hess cycles · Explain · 5 marks · View as Markdown

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

Methane is the main component of natural gas, a fossil fuel used widely for heating and cooking. When methane burns, the conditions of combustion determine which products are formed.

Model answer (5 marks)

Complete combustion means that the fuel reacts with enough oxygen so that all the carbon is oxidised to CO₂ and all the hydrogen to H₂O. The balanced equation for methane is CH₄ + 2O₂ → CO₂ + 2H₂O, so the only products are carbon dioxide and water.

Incomplete combustion occurs when the supply of oxygen is insufficient. In this case not all the carbon is oxidised and carbon monoxide (CO) is produced, together with CO₂ and H₂O.

Carbon monoxide is dangerous because it is colourless, odourless and therefore cannot be detected by smell or sight. It binds to haemoglobin in the blood with a much higher affinity than oxygen, preventing oxygen from being carried to tissues and causing hypoxia.

Examiner tips

  • Use the phrase ‘sufficient oxygen’ to show you understand complete combustion; list CO₂ and H₂O as the only products. Mention CO as the main product of incomplete combustion and explain its toxic effect on haemoglobin. Keep the answer concise – 5 marks can be achieved with 3–4 short sentences.
  • common_mistakes
  • :
  • Writing that incomplete combustion produces only CO and no CO₂; it actually produces both CO and CO₂. Forgetting to explain why CO is harmful – e.g. its colourlessness or its effect on blood. Using vague terms like ‘bad’ instead of ‘toxic’ or ‘dangerous’.

Mark scheme (5 marks)

  1. Complete combustion occurs when there is sufficient oxygen (supply/available)
  2. The only products of complete combustion of methane are carbon dioxide and water
  3. Incomplete combustion occurs when there is insufficient oxygen
  4. Incomplete combustion produces carbon monoxide (CO)
  5. Carbon monoxide is a toxic gas that is dangerous / harmful to humans (because it is colourless and odourless / it cannot be detected / it prevents blood carrying oxygen)

Key terms in this question

complete combustion · incomplete combustion · combustion

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