Incomplete Combustion (SL IB Chemistry)

• Complete combustion occurs with a plentiful supply of air / oxygen and produces carbon dioxide and water
  • In a Bunsen burner, complete combustion is characterised by a blue, non-luminous flame
• In comparison, incomplete combustion:
  • Has a limited supply of air / oxygen
  • Still produces water
    • This is the combustion / oxidation product of the hydrogen present in organic compounds 
  • Produces carbon monoxide or carbon
    • These are the combustion / oxidation products of the carbon present in organic compounds as it is not fully oxidised
• Incomplete combustion often takes place inside a car engine and inside faulty boilers due to the limited amount of oxygen present
• In a Bunsen burner, incomplete combustion is characterised by a yellow flame

Carbon monoxide as a product of incomplete combustion

• With a reduced supply of oxygen, carbon monoxide will be produced
• The word equation for incomplete combustion to form carbon monoxide is:

Fuel + oxygen → carbon monoxide + water

• For example, the word and chemical equations for the incomplete combustion of propane to form carbon monoxide are:
  • Incomplete combustion of propane word equation:

Propane + oxygen → carbon monoxide + water

• • Incomplete combustion of propane chemical equation:

C₃H₈ (l) + 3½O₂ (g) → 3CO (g) + 4H₂O (l)

• Carbon monoxide is extremely dangerous as it is colourless and odourless (it doesn’t smell) and will not be noticed
• Carbon monoxide is also a toxic and poisonous gas that binds irreversibly to haemoglobin in the blood
  • This limits the haemoglobin's capacity to bind and transport oxygen
• As no oxygen can be transported around the body, victims will feel dizzy, lose consciousness and if not removed from the carbon monoxide, they can die

Carbon as a product of incomplete combustion

• With a very reduced supply of oxygen, carbon will be produced in the form of soot
  • A sooty, yellow flame is a clear indication that incomplete combustion is taking place
  • Many hydrocarbons derived from benzene burn with a sooty, yellow flame due to their high percentage of carbon content
• The production of soot can be used to distinguish between different organic compounds 
  • Compounds with a higher percentage of carbon content tend to undergo incomplete combustion and produce more soot

• The word equation for incomplete combustion to form carbon is:

fuel + oxygen → carbon + water

• For example, the word and chemical equations for the incomplete combustion of propane to form carbon are:
  • Incomplete combustion of propane word equation:

 propane + oxygen → carbon + water

• • Incomplete combustion of propane chemical equation:

C₃H₈ (l) + 2O₂ (g) → 3C (s) + 4H₂O (l)