Properties and Combustion of Hydrocarbons
Master Properties and Combustion of Hydrocarbons for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Shorter hydrocarbons are more volatile, more flammable and less viscous; complete combustion produces carbon dioxide and water.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Hydrocarbons are used as fuels because their combustion releases a great deal of energy. Combustion is an oxidation reaction and it is exothermic.
During complete combustion, the carbon in the fuel is oxidised to carbon dioxide and the hydrogen is oxidised to water. The word equation is hydrocarbon + oxygen → carbon dioxide + water.
Incomplete combustion occurs when there is insufficient oxygen. This produces carbon monoxide, which is a toxic gas that binds to red blood cells and reduces oxygen transport, and also produces solid particulates of carbon which cause soot and can cause respiratory problems.
Revision notes
Complete combustion
Hydrocarbon + oxygen → carbon dioxide + water.
The carbon in the fuel is oxidised to carbon dioxide, and the hydrogen is oxidised to water. Energy is released, so combustion is exothermic.
Incomplete combustion
When there is insufficient oxygen, incomplete combustion occurs.
Carbon monoxide is produced, which is toxic because it binds to red blood cells and reduces the oxygen they can carry. It is colourless and odourless, so it cannot be detected easily. Solid particulates of carbon are also produced.
Other pollutants
Sulfur impurities in fuels burn to produce sulfur dioxide, which causes acid rain.
At the high temperatures inside engines, nitrogen and oxygen from the air react to form oxides of nitrogen, which also cause acid rain and respiratory problems.
Key points
- Combustion is oxidation and is exothermic.
- Complete combustion gives carbon dioxide and water.
- Incomplete combustion gives carbon monoxide.
- Carbon monoxide is toxic and hard to detect.
- Particulates cause soot and respiratory problems.
- Sulfur dioxide causes acid rain.
Worked examples
Example 1
Write the word equation for the complete combustion of a hydrocarbon. [2 marks]
Model answer
Hydrocarbon + oxygen → carbon dioxide + water. Correct reactants (1 mark). Correct products (1 mark).
Example 2
Explain why carbon monoxide is dangerous. [3 marks]
Model answer
Carbon monoxide is toxic (1 mark). It binds to red blood cells, reducing the amount of oxygen they can carry around the body (1 mark). It is also colourless and odourless, so it cannot easily be detected (1 mark).
Example 3
Explain how sulfur dioxide is produced when a fuel is burned. [2 marks]
Model answer
Fuels may contain sulfur impurities (1 mark), and when the fuel is burned this sulfur is oxidised to form sulfur dioxide (1 mark).
Common mistakes
Saying incomplete combustion produces carbon dioxide.
It produces carbon monoxide and particulates because oxygen is insufficient.
Saying carbon monoxide smells.
It is colourless and odourless, which is why it is so dangerous.
Forgetting water as a product.
The hydrogen in the fuel is oxidised to water.
Saying nitrogen oxides come from the fuel.
They form from nitrogen and oxygen in the air at high temperatures.
Exam tips
- Learn both complete and incomplete combustion products.
- Give the full explanation of why carbon monoxide is dangerous.
- Distinguish pollutants from the fuel from those from the air.
- Say oxidised when describing what happens to carbon and hydrogen.
Key terms
- Combustion
- Burning, an exothermic oxidation reaction.
- Complete combustion
- Burning with sufficient oxygen, giving carbon dioxide and water.
- Incomplete combustion
- Burning with insufficient oxygen, giving carbon monoxide.
- Particulate
- A small solid particle of carbon produced by incomplete combustion.
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Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.