Incomplete Combustion (DP IB Chemistry): Revision Note

Incomplete combustion

• 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

• Complete combustion happens with a plentiful supply of oxygen, producing carbon dioxide and water:

• In a Bunsen burner, this gives a blue, non-luminous flame

• Incomplete combustion occurs when oxygen is limited:

  • Still produces water, from hydrogen in the fuel

  • Produces carbon monoxide (CO) or carbon (C), as carbon is not fully oxidised

• It is common in:

  • Car engines

  • Faulty boilers, where oxygen supply is restricted

• In a Bunsen burner, incomplete combustion gives a yellow, luminous flame.

Carbon monoxide as a product of incomplete combustion

• When oxygen supply is limited, carbon monoxide (CO) forms instead of carbon dioxide:

fuel + oxygen → carbon monoxide + water

• For example, Incomplete combustion of propane:

propane + oxygen → carbon monoxide + water

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

• Carbon monoxide is extremely dangerous because it is:

  • Colourless and odourless (can’t be seen or smelled)

  • Hard to detect without a sensor

• It is a toxic and poisonous gas that binds irreversibly to haemoglobin in the blood.

  • This prevents haemoglobin from carrying oxygen.

• Lack of oxygen transport leads to:

  • Dizziness

  • Loss of consciousness

  • Potentially death if not treated

Carbon as a product of incomplete combustion

• With a very limited oxygen supply, carbon is produced as soot

• A yellow, sooty flame is a clear sign of incomplete combustion

• Hydrocarbons with high carbon content, such as those derived from benzene, often burn with a sooty yellow flame

• Soot production can help distinguish between different organic compounds:

  • Compounds with a higher carbon percentage are more likely to produce soot during incomplete combustion

• 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)