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
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

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

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

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

Incomplete combustion as a qualitative measure

Calculate the percentage carbon composition by mass in samples of cyclohexane (C₆H₁₂), nitrobenzene (C₆H₅NO₂) and naphthalene (C₁₀H₈).

cyclohexane
(C₆H₁₂)

nitrobenzene
(C₆H₅NO₂)

naphthalene
(C₁₀H₈)

M / g mol^-1 84 123 128

% of carbon
Using your answer to part (1), explain how the three samples could be distinguished by observing their combustion.

Answer 1:

$\frac{total mass of carbon}{M_{r}} \times 100$

Answer 2:

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

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

The incomplete combustion of organic compounds never produces hydrogen
Hydrogen is always preferentially oxidised by any available oxygen, rather than carbon

Incomplete Combustion (HL IB Chemistry)