Combustion is a reaction of a substance with oxygen that releases energy. For organic compounds the skill is to predict the products, balance the equation and use test results as evidence for what formed. It appears throughout organic reactions and again in energy and environment questions.
What are the products of complete and incomplete combustion?
A hydrocarbon burns completely in plenty of oxygen to give carbon dioxide and water. An alcohol does the same.
With too little oxygen, combustion is incomplete. The products can include carbon monoxide, CO, and carbon, C, as well as water. Each lower-oxygen route needs fewer oxygen molecules, so the balanced equation looks different.
The evidence links each product to a test.
Water condenses as droplets on a cold surface, and a chemical test can confirm it. Carbon dioxide turns limewater milky. A black deposit on a cold surface suggests carbon.
How to write a balanced combustion equation
- Write the fuel and oxygen on the left, and the products on the right.
- Balance carbon first, using the number of CO₂ molecules.
- Balance hydrogen next, using the number of H₂O molecules.
- Balance oxygen last, by counting the total on the right and choosing the O₂ coefficient to match.
- Clear fractions by multiplying every coefficient by 2.
- Count each element on both sides to check.
Worked example
Write the equation for the complete combustion of ethane, C₂H₆.
Step 1: C₂H₆ + O₂ → CO₂ + H₂O
Step 2, carbon: 2 C on the left, so 2CO₂: C₂H₆ + O₂ → 2CO₂ + H₂O
Step 3, hydrogen: 6 H on the left, so 3H₂O: C₂H₆ + O₂ → 2CO₂ + 3H₂O
Step 4, oxygen: the right side has 4 + 3 = 7 oxygen atoms, so 3½ O₂ on the left.
Step 5, clear the fraction: multiply everything by 2.
2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O
Check: left 4 C, 12 H, 14 O. Right 4 C, 12 H, 8 + 6 = 14 O. Balanced.
Amounts follow the same equation. In CH₄ + 2O₂ → CO₂ + 2H₂O, 1 mol of CH₄ (16 g) gives 1 mol of CO₂ (44 g). So 8.0 g of CH₄ is 0.50 mol and gives 0.50 mol of CO₂, which is 22 g.
The mistake to watch for
A common slip is to balance the oxygen first, or to change a subscript to make the numbers work.
Mistaken equation: C₂H₆ + 4O₂ → 2CO₂ + 3H₂O, written after “fixing” a failed count by guessing the oxygen.
Count the oxygen: 8 on the left, 7 on the right. It does not balance.
The correction is to change only coefficients, never subscripts, and to do oxygen last because it appears in two products. Always finish with a full atom count.
Check yourself
1. Balance the complete combustion of butane: C₄H₁₀ + O₂ → CO₂ + H₂O
Show answer
Carbon gives 4CO₂, hydrogen gives 5H₂O, and oxygen on the right is 8 + 5 = 13, so 6½ O₂. Double everything.
2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O
Check: 8 C, 20 H and 26 O on each side.
2. Propane burns in limited oxygen and all the carbon ends up as carbon monoxide. Write the balanced equation.
Show answer
C₃H₈ + O₂ → 3CO + 4H₂O. Oxygen on the right is 3 + 4 = 7, so 3½ O₂. Double.
2C₃H₈ + 7O₂ → 6CO + 8H₂O
Check: 6 C, 16 H and 14 O on each side.
3. A gas from a burning fuel turns limewater milky, and droplets form on a cold tube. What do these results show?
Show answer
They show that carbon dioxide and water formed. So the fuel contained carbon and hydrogen.
Where this leads next
Once combustion is secure, move to addition to an unsaturated molecule. Combustion and its energy change link to energy changes and bonds, and the mole and equation-ratio tutor helps with the amount questions.
Some students can write the equation but lose marks explaining the evidence in words. Our teachers work on exactly that gap in online one-to-one Chemistry tuition.