Propane and butane are alkanes used as fuels in camping gas canisters. When propane burns completely in excess oxygen, the products are carbon dioxide and water. However, when the oxygen supply is limited, different products can form. Explain what happens when propane undergoes complete combustion and incomplete combustion, and describe why incomplete combustion can be dangerous.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Camping gas canisters contain propane (C₃H₈) or butane (C₄H₁₀). These alkanes are used as fuels because they release energy when they burn.
Model answer (5 marks)
Propane (C₃H₈) can be oxidised in two ways.
1. Complete combustion – when there is excess or sufficient O₂, the reaction is
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. The products are carbon dioxide and water.
2. Incomplete combustion – when the O₂ supply is limited, the reaction cannot proceed to CO₂ and H₂O. Instead, carbon monoxide (CO) and sometimes elemental carbon (soot) are produced, e.g. C₃H₈ + 2O₂ → 3CO + 4H₂O or C₃H₈ + 2O₂ → 3CO + 2H₂O + C.
Incomplete combustion is dangerous because CO is a colourless, odourless, poisonous gas that binds haemoglobin more strongly than oxygen, leading to hypoxia and potentially death.
1. Complete combustion – when there is excess or sufficient O₂, the reaction is
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. The products are carbon dioxide and water.
2. Incomplete combustion – when the O₂ supply is limited, the reaction cannot proceed to CO₂ and H₂O. Instead, carbon monoxide (CO) and sometimes elemental carbon (soot) are produced, e.g. C₃H₈ + 2O₂ → 3CO + 4H₂O or C₃H₈ + 2O₂ → 3CO + 2H₂O + C.
Incomplete combustion is dangerous because CO is a colourless, odourless, poisonous gas that binds haemoglobin more strongly than oxygen, leading to hypoxia and potentially death.
Examiner tips
- Show the two balanced equations, label the products clearly
- Explain the role of oxygen supply in each case
- Mention the toxic nature of CO and its lack of smell or colour
Common mistakes
- Writing the wrong stoichiometry for complete combustion
- Forgetting to mention that CO is colourless and odourless
- Confusing CO with CO₂ as the dangerous product
Mark scheme (5 marks)
- Complete combustion requires excess/sufficient oxygen
- Complete combustion produces carbon dioxide and water
- Incomplete combustion occurs when there is a limited/insufficient supply of oxygen
- Incomplete combustion produces carbon monoxide (and/or carbon/soot)
- Carbon monoxide is dangerous because it is a toxic/poisonous gas that can cause death/is colourless and odourless so cannot be detected
Key terms in this question
complete combustion · incomplete combustion · alkane · combustion
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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