AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Students should be able to write balanced equations for the complete…
Full specification wording
Properties of hydrocarbons: Students should be able to write balanced equations for the complete combustion of hydrocarbons with a given formula.
To balance a complete combustion equation, start from the general form: hydrocarbon plus oxygen gives carbon dioxide plus water.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 7 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
To balance a complete combustion equation, start from the general form: hydrocarbon plus oxygen gives carbon dioxide plus water. Balance carbon first by placing the correct number of CO₂ molecules, then balance hydrogen using H₂O molecules, and finally balance oxygen by adjusting the number of O₂ molecules. For example, for propane, C₃H₈, the balanced equation is C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. Check that every element has the same number of atoms on each side. If the oxygen count is odd, double the whole equation so that all coefficients are whole numbers, as happens with 2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O. State symbols may be added when required.
What earns the marks
- Write the correct products, carbon dioxide and water, for complete combustion.
- Balance carbon atoms first using the coefficient of CO₂.
- Balance hydrogen atoms next using the coefficient of H₂O.
- Balance oxygen last by adjusting the coefficient of O₂.
- Use the lowest whole-number coefficients, doubling the equation if an odd oxygen count appears.
- Check that the total number of each type of atom is equal on both sides.
Common mistakes and how to put them right
Changing a chemical formula, such as writing C₃H₈ as C₃H₄, instead of changing only the coefficient.
Put it right by keeping every formula fixed and adjusting coefficients only.
Leaving oxygen unbalanced after balancing carbon and hydrogen.
Put it right by recounting oxygen atoms on both sides and adjusting the O₂ coefficient.
Using fractional coefficients such as 6.5O₂.
Put it right by doubling every coefficient to give whole numbers.
Exam tips
- Balance carbon, then hydrogen, then oxygen, and finish with an atom count on each side.
- Show your working or atom tally so a slip in one element is easy to spot and correct.
- Include state symbols only when the question asks for them, and check they match the physical states.
Try the lesson
The start of the Properties of hydrocarbons: Students should be able to write balanced equations for the complete combustion of hydrocarbons with a given formula. lesson, made from checked teaching notes for this specification point.
Have a go first
Which equation correctly represents the complete combustion of methane, CH₄?
Listen
To balance a complete combustion equation, start from the general form: hydrocarbon plus oxygen gives carbon dioxide plus water. Balance carbon first by placing the correct number of CO₂ molecules, then balance hydrogen using H₂O molecules, and finally balance oxygen by adjusting the number of O₂ molecules. For example, for propane, C₃H₈, the balanced equation is C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. Check that every element has the same number of atoms on each side. If the oxygen count is odd, double the whole equation so that all coefficients are whole numbers, as happens with 2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O. State symbols may be added when required. A strong answer earns marks for these points. Write the correct products, carbon dioxide and water, for complete combustion. Balance carbon atoms first using the coefficient of CO₂. Balance hydrogen atoms next using the coefficient of H₂O. Balance oxygen last by adjusting the coefficient of O₂. A common slip: Changing a chemical formula, such as writing C₃H₈ as C₃H₄, instead of changing only the coefficient. Instead, keeping every formula fixed and adjusting coefficients only. Exam tip: Balance carbon, then hydrogen, then oxygen, and finish with an atom count on each side.
Key words
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Conservation of mass in equations
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Systematic balancing strategy
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Representing combustion symbolically
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Whole-number coefficients and formula integrity
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See every definition
- Conservation of mass in equations
- Every element must have the same number of atoms on each side of the equation.
- Systematic balancing strategy
- Balance carbon first using the coefficient of CO₂, then hydrogen using H₂O, and finally oxygen by adjusting the number of O₂ molecules.
- Representing combustion symbolically
- Complete combustion is written as hydrocarbon plus oxygen gives carbon dioxide plus water.
- Whole-number coefficients and formula integrity
- Use the lowest whole-number coefficients, doubling the equation if an odd oxygen count appears, while keeping every formula fixed and adjusting coefficients only.
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