UNIT 2 CHEMISTRY · PEARSON AAQ · E1.8.1
Complete and Incomplete Combustion
FREE CHEMISTRY REVISION · TOPIC 63
Complete and Incomplete Combustion
Pearson BTEC Level 3 AAQ Applied Science
Unit 2: Principles and Applications of Chemistry · E1.8.1
What you will learn
- Predict complete and incomplete combustion products for hydrocarbons and alcohols.
- Balance equations by conserving carbon, hydrogen and oxygen atoms.
- Explain energy release, fuel uses and the effect of oxygen supply.
1. Combustion and commercial importance
Combustion is an exothermic reaction with oxygen. Fuels such as methane in natural gas, propane in LPG, petrol hydrocarbons and ethanol transfer energy to the surroundings when burned. Useful applications include space heating, cooking, industrial furnaces, electricity generation and transport. Ignition supplies activation energy; the reaction then releases energy overall because forming product bonds releases more energy than breaking reactant bonds requires.
A hydrocarbon contains carbon and hydrogen only. An alcohol also contains oxygen, but still normally needs oxygen from the air for combustion. A fuel being oxygen-containing does not guarantee complete combustion. Good mixing and sufficient oxygen favour complete oxidation; oxygen shortage, poor mixing or cooling can cause incomplete combustion. Real flames may produce a mixture.
2. Products and oxygen supply
| Combustion | Carbon product | Other product and significance |
|---|---|---|
| Complete; sufficient oxygen | CO₂, carbon dioxide | H₂O; maximum oxidation of fuel carbon |
| Incomplete; limited oxygen | CO, carbon monoxide | H₂O; toxic, colourless and odourless CO may be present |
| Incomplete; severe local oxygen shortage | C, carbon/soot | H₂O; soot consists of particles, not a gas |
Carbon dioxide is a greenhouse gas, but it is not the same substance as poisonous carbon monoxide. CO binds strongly to haemoglobin, reducing oxygen transport. Soot particles can damage health; hot soot often makes a flame luminous yellow. Flame colour alone is not a reliable CO detector. Complete combustion still emits CO₂ and is not environmentally impact-free.
3. A reliable balancing method
- Write the fuel and the stated carbon product. Do not guess CO₂ when the question specifies CO.
- Balance carbon, then hydrogen as H₂O.
- Count oxygen atoms on the product side. Subtract any oxygen already in the fuel. Divide the remaining oxygen count by two to find O₂.
- If needed, multiply every coefficient to remove fractions. Recount each element. Never change subscripts to balance an equation.
For complete combustion of an alkane: CₙH₂ₙ₊₂ + (3n+1)/2 O₂ → nCO₂ + (n+1)H₂O. For a saturated monohydric alcohol: CₙH₂ₙ₊₁OH + 3n/2 O₂ → nCO₂ + (n+1)H₂O. These general equations assume the specified complete oxidation products.
Worked and applied examples
Example 1 · Propane, complete
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. Three carbon atoms require 3CO₂; eight H require 4H₂O. Products contain 10 O atoms, so 5O₂. Atom totals on both sides: C=3, H=8, O=10.
Example 2 · Octane, complete
2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O. One octane needs 12.5O₂, so double all coefficients. Totals: C=16, H=36, O=50. This is a model petrol-component equation; petrol is a mixture.
Example 3 · Ethanol, complete
C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O. Ethanol contains 6 H and 1 O. Products need 7 O atoms: 1 from ethanol plus 6 from 3O₂. Totals C=2, H=6, O=7.
Example 4 · Ethanol, incomplete to CO
C₂H₅OH + 2O₂ → 2CO + 3H₂O. Products need 5 O atoms: 1 in ethanol plus 4 in 2O₂. Totals C=2, H=6, O=5. CO formation leaves chemical energy that could be released by further oxidation.
Example 5 · Ethanol, incomplete to soot
C₂H₅OH + O₂ → 2C + 3H₂O. Totals C=2, H=6, O=3. This idealised equation conserves atoms; a real flame can also make CO and CO₂.
Example 6 · Compare propane oxygen demands
2C₃H₈ + 7O₂ → 6CO + 8H₂O has C=6, H=16, O=14. C₃H₈ + 2O₂ → 3C + 4H₂O has C=3, H=8, O=4. Less oxygen is used per mole than in complete combustion; incomplete combustion generally releases less energy per mole of the same fuel.
Common misconceptions and examination tips
- Water is still formed during incomplete combustion of these hydrogen-containing fuels.
- Incomplete combustion can produce CO, carbon, CO₂ and unburned material together; it does not always produce soot only.
- Breaking bonds requires energy. Do not say that breaking fuel bonds releases energy.
- Specify which carbon product is assumed, balance H₂O, and include the oxygen already in an alcohol.
- Link fuel usefulness to energy release and practical storage/transport. Evaluate emissions separately.
Check your understanding
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