Complete Combustion

Specification Point 8.7:
  • Describe the complete combustion of hydrocarbon fuels as a reaction in which:
    a) carbon dioxide and water are produced
    b) energy is given out
  • A fuel is a substance which releases energy in an exothermic reaction called combustion (burning).
  • When the fuel is a hydrocarbon then water and carbon dioxide are the products formed.
  • Hydrocarbon compounds undergo complete and incomplete combustion.
  • Complete combustion occurs when there is excess oxygen.
  • Propane for example undergoes combustion according to the following equation:

C3H8 + 5O2 → 3CO2 + 4H2O           ΔH = -2219 kJ/mol

Incomplete Combustion

Specification Point 8.8:
  • Explain why the incomplete combustion of hydrocarbons can produce carbon and carbon monoxide
  • Incomplete combustion occurs when there is insufficient oxygen to burn.
  • The products of these reactions are unburnt fuel (soot), carbon monoxide and water.
  • Methane for example undergoes incomplete combustion in an oxygen poor environment:

2CH4+ 3O2→ 2CO + 4H2O

CH4+ O2→ C + 2H2O

Toxicity of Carbon Monoxide

Specification Point 8.9:
  • Explain how carbon monoxide behaves as a toxic gas
  • Carbon monoxide is an extremely poisonous gas as it combines with hemoglobin in blood and prevents it from carrying oxygen.
  • It is particularly malevolent as it is colourless, odourless and tasteless, making it difficult to detect.
  • A lack of oxygen supply to the brain can lead to fainting, coma or in worst case scenarios, even death.

The Problem of Incomplete Combustion

Specification Point 8.10:
  • Describe the problems caused by incomplete combustion producing carbon monoxide and soot in appliances that use carbon compounds as fuels
  • The main products of incomplete combustion are carbon monoxide and soot although some H2O and CO2 are inevitably produced as well.
  • Incomplete combustion occurs in some appliances such as boilers and stoves as well as in internal combustion engines, where space is cramped.
  • Incomplete combustion of gasoline in car engines is a major source of CO and C:

C8H18 + 8.5O2 → 8CO + 9H2O

C8H18 + 4.5O2 → 8C + 9H2O

  • The carbon particles released clump together to form soot which gradually fall back to the ground.
  • Soot causes respiratory problems and covers buildings and statues, making them look unclean.

Soot Statue, Edexcel GCSE Chemistry

Statues and monuments in densely populated areas become blackened over time from soot

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Edexcel GCSE Chemistry Notes

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.