AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Properties of hydrocarbons: The combustion of hydrocarbon fuels releases energy. During combustion, the carbon and hydrogen in the fuels are oxidised. The complete combustion of a hydrocarbon produces carbon dioxide and water.
Hydrocarbon fuels store chemical energy in their bonds.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Hydrocarbon fuels store chemical energy in their bonds. When they burn, they react with oxygen from the air and release energy, usually as heat and light. In the reaction, each carbon atom is oxidised to form carbon dioxide and each hydrogen atom is oxidised to form water. Complete combustion happens when there is plenty of oxygen, so the only products are carbon dioxide and water.
For example: Burning methane: CH₄ + 2O₂ → CO₂ + 2H₂O. The carbon in CH₄ is oxidised to CO₂ and the hydrogen is oxidised to H₂O. If oxygen is limited, combustion is incomplete and carbon monoxide or carbon particles may form instead.
What earns the marks
- Combustion of hydrocarbon fuels is an exothermic reaction that releases energy.
- Carbon in the fuel is oxidised to carbon dioxide during complete combustion.
- Hydrogen in the fuel is oxidised to water during complete combustion.
- Complete combustion requires a good supply of oxygen and produces carbon dioxide and water only.
- A balanced symbol equation can represent the reaction, such as CH₄ + 2O₂ → CO₂ + 2H₂O.
- Oxidation in this context means reaction with oxygen, so the carbon and hydrogen atoms gain oxygen.
Common mistakes and how to put them right
Writing water as the only product.
Put it right by including carbon dioxide as well.
Saying energy is destroyed or used up.
Put it right by stating that energy is transferred to the surroundings during combustion.
Confusing complete and incomplete combustion.
Put it right by stating that complete combustion produces carbon dioxide and water, while incomplete combustion can produce carbon monoxide or carbon.
Exam tips
- Name both products of complete combustion: carbon dioxide and water.
- Use a balanced equation to show the reactants and products clearly.
- State that combustion is exothermic and that energy is released to the surroundings.
Try the lesson
The start of the Properties of hydrocarbons: The combustion of hydrocarbon fuels releases energy. During combustion, the carbon and hydrogen in the fuels are oxidised. The complete combustion of a hydrocarbon produces carbon dioxide and water. lesson, made from checked teaching notes for this specification point.
Have a go first
Which two products are formed when a hydrocarbon such as methane undergoes complete combustion?
Listen
Hydrocarbon fuels store chemical energy in their bonds. When they burn, they react with oxygen from the air and release energy, usually as heat and light. In the reaction, each carbon atom is oxidised to form carbon dioxide and each hydrogen atom is oxidised to form water. Complete combustion happens when there is plenty of oxygen, so the only products are carbon dioxide and water. For example, burning methane: CH₄ + 2O₂ → CO₂ + 2H₂O. The carbon in CH₄ is oxidised to CO₂ and the hydrogen is oxidised to H₂O. If oxygen is limited, combustion is incomplete and carbon monoxide or carbon particles may form instead. A strong answer earns marks for these points. Combustion of hydrocarbon fuels is an exothermic reaction that releases energy. Carbon in the fuel is oxidised to carbon dioxide during complete combustion. Hydrogen in the fuel is oxidised to water during complete combustion. Complete combustion requires a good supply of oxygen and produces carbon dioxide and water only. A common slip: Writing water as the only product. Instead, including carbon dioxide as well. Exam tip: Name both products of complete combustion: carbon dioxide and water.
Key words
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Oxidation
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Energy transfer
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Combustion products
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Chemical reactions
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See every definition
- Oxidation
- During combustion, the carbon and hydrogen in the fuels are oxidised, meaning they react with oxygen.
- Energy transfer
- Combustion is exothermic and energy is transferred to the surroundings during combustion.
- Combustion products
- The complete combustion of a hydrocarbon produces carbon dioxide and water.
- Chemical reactions
- When hydrocarbon fuels burn, they react with oxygen from the air and release energy, usually as heat and light.
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