4.3.2Additional lessons
Incomplete Combustion — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 54 of 98
Section 1 of 6What Happens
Insufficient oxygen changes both the products and the energy released.
Cause
- Incomplete combustion occurs when the supply of oxygen is limited, so not all of the carbon is oxidised to carbon dioxide.
Products
- Carbon monoxide (CO) and carbon (soot, C) can form together with water, and sometimes with unburnt fuel.
Evidence
- A yellow, smoky flame and black soot deposits on cooking vessels indicate that incomplete combustion is taking place.
Energy
- Less energy is released per mole of fuel than for complete combustion, because CO and C still contain stored chemical energy.
Section 2 of 6Equations
1
For carbon monoxide, balance carbon as CO and hydrogen as H₂O, so that methane gives CH₄ + 3/2 O₂ → CO + 2H₂O.
2
For carbon, balance carbon as C(s), so that methane gives CH₄ + O₂ → C + 2H₂O.
3
Multiply by two to remove fractions where needed, for example 2CH₄(g) + 3O₂(g) → 2CO(g) + 4H₂O(l).
4
Compare the oxygen required: complete combustion of CH₄ uses 2 mol O₂, CO formation uses 1.5 mol and carbon formation uses 1 mol.
| Product of carbon | O₂ per mol CH₄ |
|---|---|
| CO₂ | 2 |
| CO | 1.5 |
| C | 1 |
Section 3 of 6Hazards
Hazards
Carbon monoxide
- A colourless, odourless and toxic gas that binds to haemoglobin in preference to oxygen, reducing oxygen transport in the blood.
Soot
- Particles of carbon cause respiratory problems, blacken buildings and contribute to air pollution.
Ventilation
- Properly ventilated appliances and carbon monoxide alarms reduce the risk from faulty heaters and boilers.
Engines
- Vehicle engines produce CO and soot when fuel burns with too little oxygen, and catalytic converters oxidise CO to CO₂.
Section 4 of 6Comparing Complete and Incomplete Combustion
Comparing Complete and Incomplete Combustion
Oxygen supply
- Excess oxygen gives complete combustion, while a limited supply leads to incomplete combustion.
Flame colour
- Complete combustion gives a blue flame, while incomplete combustion often gives a yellow flame.
Energy
- Complete combustion transfers more energy because the carbon is fully oxidised.
Pollutants
- Incomplete combustion adds toxic CO and soot to the exhaust gases.
Section 5 of 6Worked Example
Question Write a balanced equation for the incomplete combustion of octane, C₈H₁₈(l), to carbon monoxide and water.
- 8 carbons gives 8CO, and 18 hydrogens gives 9H₂O.
- Oxygen atoms in the products = 8 + 9 = 17, so 8.5 O₂ are needed.
- Double all coefficients to remove the fraction: 2C₈H₁₈ + 17O₂ → 16CO + 18H₂O.
- Check oxygen34 on the left, and 16 + 18 = 34 on the right.
Answer 2C₈H₁₈(l) + 17O₂(g) → 16CO(g) + 18H₂O(l)
Section 6 of 6Exam Checklist & Tips
4.3.2 – Key Points for the Exam
- Incomplete combustion Burning of a fuel in a limited supply of oxygen, producing CO or C as well as water.
- Carbon monoxide A toxic gas formed by incomplete combustion that reduces the oxygen-carrying capacity of blood.
- Soot Fine particles of carbon formed in incomplete combustion.
- Catalytic converter A device which converts CO and unburnt hydrocarbons in exhaust gases into CO₂ and H₂O.
Common Mistakes
- Thinking carbon monoxide is formed only when there is no oxygen; it forms when oxygen is limited.
- Writing CO₂ in an incomplete combustion equation.
- Not balancing oxygen correctly when the fuel needs a fractional coefficient.
- Saying that carbon monoxide is poisonous because it is acidic, rather than because it binds to haemoglobin.
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