AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
During combustion, the carbon and hydrogen in the fuels are oxidised.
Full specification wording
Properties of hydrocarbons: During combustion, the carbon and hydrogen in the fuels are oxidised.
In combustion, oxygen reacts with the elements in the fuel.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 5 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
In combustion, oxygen reacts with the elements in the fuel. The carbon in the hydrocarbon is oxidised to carbon dioxide, and the hydrogen is oxidised to water. Oxidation here means gain of oxygen, so both elements combine with oxygen. For example, in methane, CH₄, the carbon becomes CO₂ and the hydrogen becomes H₂O. Writing a balanced equation shows that every carbon atom needs one oxygen molecule to form carbon dioxide and every four hydrogen atoms need one oxygen molecule to form two water molecules. This is why complete combustion needs plenty of oxygen and why the products are carbon dioxide and water.
What earns the marks
- Oxidation in this context means the gain of oxygen by an element.
- Carbon in the hydrocarbon is oxidised to carbon dioxide.
- Hydrogen in the hydrocarbon is oxidised to water.
- The products of complete combustion are carbon dioxide and water.
- A balanced symbol equation can show the oxidation, for example CH₄ + 2O₂ → CO₂ + 2H₂O.
Common mistakes and how to put them right
Saying carbon is reduced to carbon dioxide.
Put it right: Carbon gains oxygen, so it is oxidised.
Writing water as the only product and omitting carbon dioxide.
Put it right: Both carbon and hydrogen are oxidised.
Forgetting to balance the equation so oxygen atoms are unequal on each side.
Put it right: Check that every element is balanced.
Exam tips
- Define oxidation as gain of oxygen when explaining what happens to carbon and hydrogen.
- Name both products, carbon dioxide and water, and link each to the element it came from.
- Balance symbol equations by counting atoms of each element on both sides before finalising.
Try the lesson
The start of the Properties of hydrocarbons: During combustion, the carbon and hydrogen in the fuels are oxidised. lesson, made from checked teaching notes for this specification point.
Have a go first
What happens to the carbon and hydrogen atoms in a hydrocarbon fuel when it undergoes complete combustion?
Listen
In combustion, oxygen reacts with the elements in the fuel. The carbon in the hydrocarbon is oxidised to carbon dioxide, and the hydrogen is oxidised to water. Oxidation here means gain of oxygen, so both elements combine with oxygen. For example, in methane, CH₄, the carbon becomes CO₂ and the hydrogen becomes H₂O. Writing a balanced equation shows that every carbon atom needs one oxygen molecule to form carbon dioxide and every four hydrogen atoms need one oxygen molecule to form two water molecules. This is why complete combustion needs plenty of oxygen and why the products are carbon dioxide and water. A strong answer earns marks for these points. Oxidation in this context means the gain of oxygen by an element. Carbon in the hydrocarbon is oxidised to carbon dioxide. Hydrogen in the hydrocarbon is oxidised to water. The products of complete combustion are carbon dioxide and water. Exam tip: Define oxidation as gain of oxygen when explaining what happens to carbon and hydrogen.
See it
Key words
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Oxidation
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Combustion products
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Balancing equations
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Reactions of hydrocarbons
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See every definition
- Oxidation
- In this context it means the gain of oxygen by an element, so carbon and hydrogen combine with oxygen.
- Combustion products
- The products of complete combustion are carbon dioxide and water, with carbon oxidised to carbon dioxide and hydrogen oxidised to water.
- Balancing equations
- A balanced symbol equation can show the oxidation, checking that every element is balanced on each side.
- Reactions of hydrocarbons
- During combustion, the carbon and hydrogen in the fuels are oxidised, which is why complete combustion needs plenty of oxygen.
Label the diagram
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