As fuels
2 marks · EasyIncomplete combustion of hydrocarbons can take place when there is insufficient oxygen available. Explain, using your knowledge of combustion, why incomplete combustion of hydrocarbons does not produce as much energy as complete combustion.
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Incomplete combustion produces less energy because hydrocarbons are not completely oxidised; some energy remains in the products such as carbon monoxide or carbon instead of being fully released as heat.
Also accept: Less energy is released because the hydrocarbon does not fully react with oxygen; some energy stays in the carbon monoxide or soot.
Common mistakes: Reversing complete/incomplete explanation; Giving only the products without energy reasoning
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Marking points
M1 · 1 mark · Mentions incomplete combustion gives products that are not fully oxidised (e.g. carbon monoxide or carbon).
A1 · 1 mark · Explains that less energy is released because not all energy is transferred to heat as in complete combustion.
Worked solution
Incomplete combustion does not fully oxidise the hydrocarbon, so products like carbon monoxide or carbon (soot) are made.
Some energy remains stored in these products instead of being released as heat, so less energy comes out compared to complete combustion.