Crude oil is a mixture of hydrocarbons. When a hydrocarbon burns in a limited supply of air, incomplete combustion occurs. Explain why incomplete combustion is considered more dangerous than complete combustion, and state the products formed during each type of combustion of a hydrocarbon.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Crude oil is separated into fractions at an oil refinery. Many of these fractions are used as fuels in homes and vehicles.
Model answer (5 marks)
Incomplete combustion is more dangerous because it produces carbon monoxide, a colourless, odourless gas that is poisonous and cannot be detected by smell or sight, allowing it to accumulate unnoticed.
In complete combustion a hydrocarbon reacts with sufficient oxygen to give:
- carbon dioxide (CO₂)
- water (H₂O)
In incomplete combustion, limited oxygen leads to:
- carbon monoxide (CO)
- water (H₂O)
- elemental carbon (soot) or unburned hydrocarbons
In complete combustion a hydrocarbon reacts with sufficient oxygen to give:
- carbon dioxide (CO₂)
- water (H₂O)
In incomplete combustion, limited oxygen leads to:
- carbon monoxide (CO)
- water (H₂O)
- elemental carbon (soot) or unburned hydrocarbons
Examiner tips
- Use the word ‘produces’ to link reaction to products; list CO₂ and H₂O for complete, CO, H₂O and soot for incomplete; mention CO’s toxicity and lack of smell to justify danger.
- Show the contrast clearly: complete = CO₂+H₂O, incomplete = CO+H₂O+soot; emphasise CO’s properties.
Common mistakes
- Writing that incomplete combustion gives only CO and H₂O, omitting soot; or stating CO is safe instead of toxic; or forgetting to mention CO’s colourless, odourless nature.
Mark scheme (5 marks)
- Complete combustion produces carbon dioxide (and water)
- Incomplete combustion produces carbon monoxide (and water)
- Incomplete combustion also produces carbon (soot) / unburned particles
- Carbon monoxide is toxic / poisonous
- Carbon monoxide is colourless and odourless so cannot be detected by smell / sight, making it more dangerous
Key terms in this question
hydrocarbon · complete combustion · incomplete combustion
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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