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3.2.3Organic Chemistry

Combustion of Alkanes — Oxford AQA International A Level Chemistry Revision Notes & Topical Questions

5 min read 5 parts Lesson 9 of 55

📚 Oxford AQA International A Level Chemistry Hub / Topical Revision
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Part 1 of 5Complete and Incomplete Combustion

3.2.3 Combustion of Alkanes
Complete combustion
  • With plenty of oxygen, alkanes produce only CO₂ and H₂O, e.g. CH₄ + 2O₂ → CO₂ + 2H₂O.
Incomplete: CO
  • With limited oxygen, carbon monoxide forms2CH₄ + 3O₂ → 2CO + 4H₂O.
Incomplete: soot
  • With very little oxygen, carbon forms as soot: CH₄ + O₂ → C + 2H₂O.
Octane
  • 2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O for complete combustion of octane in a petrol engine.
Energy
  • Combustion is exothermic, which is why alkanes are used as fuels.

Part 2 of 5Pollutants

Pollutants
Carbon monoxide
  • A toxic gas that binds to haemoglobin in blood, stopping it from carrying oxygen and causing unconsciousness and death.
Nitrogen oxides
  • At the high temperature in engines, N₂ + O₂ → 2NO, and NO then reacts further to give NO₂, which causes acid rain and smog.
Sulfur dioxide
  • Sulfur impurities in fuels burn to SO₂, which dissolves in rain to form acid rain.
Unburnt hydrocarbons
  • These contribute to smog and some are harmful to health.
Carbon dioxide
  • A greenhouse gas which absorbs infrared radiation and contributes to global warming.

Part 3 of 5Reducing Pollution

1
Catalytic converters in car exhausts contain platinum, palladium and rhodium on a ceramic honeycomb with a large surface area.
2
They convert CO and NO into harmless gases: 2CO + 2NO → 2CO₂ + N₂.
3
Unburnt hydrocarbons are oxidised to CO₂ and H₂O in the same device.
4
Sulfur is removed from fuels before burning, and flue gas desulfurisation using calcium oxide or carbonate removes SO₂ from power-station gases.
5
Using less fuel, or fuels from renewable sources, reduces CO₂ emissions.

Part 4 of 5Worked Example

Question

  • Calculate the mass of carbon dioxide produced by the complete combustion of 1.00 g of octane, C₈H₁₈.
  • (Ar: C 12.0, H 1.0, O 16.0)

  1. EquationC₈H₁₈ + 12½O₂ → 8CO₂ + 9H₂O, so 1 mol octane gives 8 mol CO₂.
  2. Mr(C₈H₁₈) = 114; moles = 1.00 ÷ 114 = 0.00877 mol.
  3. Moles of CO₂ = 8 × 0.00877 = 0.0702 mol; Mr(CO₂) = 44.
  4. Mass of CO₂ = 0.0702 × 44 = 3.09 g.

Answer 3.09 g of CO₂.

Part 5 of 5Exam Checklist & Tips

3.2.3 – Key Points for the Exam

  • Complete combustion Burning in plenty of oxygen to form CO₂ and H₂O.
  • Incomplete combustion Burning with limited oxygen, forming CO and/or carbon.
  • Catalytic converter A device that converts exhaust pollutants into less harmful gases.
  • Acid rain Rain made acidic by dissolved SO₂ and nitrogen oxides.

Common Mistakes

  • Writing that NOₓ forms from the fuel; it forms from nitrogen and oxygen in air at high temperature.
  • Saying CO is toxic because it burns the lungs; it binds to haemoglobin.
  • Forgetting to balance O₂ and H₂O in combustion equations.
  • Stating that carbon dioxide is a pollutant that causes acid rain.

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