Octane (C₈H₁₈) is an alkane found in petrol. When octane burns in a limited supply of air, incomplete combustion occurs. Explain why incomplete combustion is considered more dangerous than complete combustion, and describe the products formed in each type of combustion.

Edexcel A-Level Chemistry (9CH0) — 6.2 Alkanes · Explain · 5 marks · View as Markdown

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

Petrol engines in cars burn octane, an alkane with the molecular formula C₈H₁₈. The amount of air reaching the engine affects the type of combustion that takes place.

Model answer (5 marks)

1. In complete combustion all the carbon in octane reacts with oxygen to give CO₂ and H₂O only.
2. In incomplete combustion the limited oxygen means some carbon is not fully oxidised, producing CO and/or soot (solid carbon) together with H₂O.
3. CO is toxic because it binds with haemoglobin more strongly than O₂.
4. CO is colourless, odourless and hard to detect, so people may be exposed without noticing.
5. The CO–haemoglobin complex reduces oxygen transport in the blood, leading to hypoxia and potentially death.

Examiner tips

  • Use the exact terms CO₂, H₂O, CO, soot; mention haemoglobin binding; explain detection issue; link to hypoxia

Common mistakes

  • Confusing CO₂ with CO; forgetting to mention soot; claiming CO is visible or odorous

Mark scheme (5 marks)

  1. Complete combustion produces carbon dioxide and water (only)
  2. Incomplete combustion produces carbon monoxide (and/or carbon/soot) and water
  3. Carbon monoxide is toxic / poisonous
  4. Carbon monoxide is colourless and odourless / difficult to detect, making it dangerous
  5. Carbon monoxide prevents the blood from carrying oxygen / reduces oxygen transport in the blood

Key terms in this question

incomplete combustion · complete combustion

Related

More Alkanes questions

▶ Try answering this question with AI marking (free) →