Incomplete combustion equations

An incomplete combustion equation shows a fuel burning without enough oxygen, so its carbon ends up as carbon monoxide (CO) or carbon (soot) instead of carbon dioxide. The hydrogen still forms water. For methane, incomplete combustion making carbon monoxide is 2CH₄ + 3O₂ → 2CO + 4H₂O, and making soot it is CH₄ + O₂ → C + 2H₂O. Compare complete combustion, CH₄ + 2O₂ → CO₂ + 2H₂O, which needs more oxygen.

Complete and incomplete combustion

AQA's GCSE Chemistry specification says the complete combustion of a hydrocarbon produces carbon dioxide and water, and that alkenes tend to burn in air with smoky flames because of incomplete combustion. It also asks students to predict the products of burning a fuel from its composition and the conditions it is used in.

Word equations

• Complete: hydrocarbon + oxygen → carbon dioxide + water
• Incomplete, making carbon monoxide: hydrocarbon + oxygen → carbon monoxide + water
• Incomplete, making soot: hydrocarbon + oxygen → carbon + water

In practice, a poorly burning flame often makes a mixture of carbon dioxide, carbon monoxide and soot all at once.

How to write an incomplete combustion equation

1. Write the hydrocarbon and oxygen on the left.
2. Write the products on the right: CO (or C) and H₂O.
3. Balance the carbon: one CO, or one C, for each carbon atom in the fuel.
4. Balance the hydrogen: one H₂O for every two hydrogen atoms.
5. Balance the oxygen last, by counting the oxygen atoms on the right and putting half that number of O₂ on the left.
6. If you end up with a half, double everything.

Incomplete combustion equation examples

Example 1: methane making carbon monoxide

CH₄ + O₂ → CO + H₂O. Carbon: 1 CO. Hydrogen: 4 H, so 2H₂O. Oxygen on the right: 1 + 2 = 3 atoms, which is 1½O₂.

Double to clear the half:

Example 2: propane making carbon monoxide

C₃H₈ + O₂ → CO + H₂O. Carbon: 3CO. Hydrogen: 8 H, so 4H₂O. Oxygen on the right: 3 + 4 = 7 atoms, which is 3½O₂. Doubling:

Example 3: propane making soot

With even less oxygen, the carbon is left as soot. Carbon: 3C. Hydrogen: 4H₂O. Oxygen on the right: 4 atoms, so 2O₂:

Example 4: ethene, an alkene

Ethene burning to carbon monoxide: 2CO, 2H₂O, and 2 + 2 = 4 oxygen atoms on the right, so 2O₂:

Example 5: a mixture of products

A propane flame with a little too little oxygen makes both carbon dioxide and carbon monoxide. If two-thirds of the carbon becomes CO₂, then from two propane molecules (6 C), 4 C become CO₂ and 2 C become CO:

Check the oxygen: 4 × 2 + 2 + 8 = 18 atoms on the right, and 9 × 2 = 18 on the left.

Example 6: pentane making soot

Pentane is C₅H₁₂. Carbon: 5C. Hydrogen: 12 H, so 6H₂O. Oxygen on the right: 6 atoms, all in the water, so 3O₂ on the left:

No halving or doubling is needed here, because the oxygen count on the right is already even. Check: 5 carbon, 12 hydrogen and 6 oxygen atoms on each side.

The oxygen pattern

For the same fuel, less oxygen means less fully oxidised carbon:

Now balance a combustion equation in the course, then press Mark to check it there.

The Equation Balancer balances any combustion equation you type, so you can check your coefficients.

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Why incomplete combustion matters

• Carbon monoxide is dangerous. AQA's specification describes it as a toxic gas that is colourless and odourless, so it is not easily detected. You cannot see or smell it building up.
• Soot causes problems. AQA says particulates cause global dimming and health problems for humans. Soot also blackens appliances and chimneys.
• It wastes fuel. Less of the fuel's carbon is fully oxidised, so less energy is released from the same amount of fuel.

Common mistakes

• Writing CO₂ for incomplete combustion. Incomplete combustion makes CO or C. It may make some CO₂ as well, but the question will say if it wants a mixture.
• Forgetting the water. The hydrogen in the fuel still becomes water.
• Balancing oxygen first. Do carbon, then hydrogen, then oxygen.
• Leaving a half. 1½O₂ is fine as a working step, but double everything for the final answer unless the question allows fractions.
• Writing Co for carbon monoxide. Co is cobalt. Carbon monoxide is CO, with a capital O.

Practise chemistry in a short timed paper

GCSE Chemistry Higher and GCSE Chemistry Foundation each give you a short timed paper every day, with 8 questions worth 25 to 35 marks across the course, so fuels and combustion turn up regularly but not in every paper. Our balancing equations guide has more practice.

Read next

Chemistry · EnergeticsEnthalpy of combustion: definition, values and calculationsStandard enthalpy of combustion: the definition, a table of values, measuring it with a calorimeter, why results come out low, and using it in Hess's law.