Atmospheric pollutants from fuels (AQA GCSE Combined Science: Trilogy): Revision Note

Combustion of fossil fuels

• The combustion of fossil fuels is a major source of atmospheric pollution

• Fossil fuels include coal, oil, natural gas, oil shales, and tar sands

• These fuels are mainly hydrocarbons, meaning they contain carbon and hydrogen, and often small amounts of sulfur

• When these fuels are burned, they release energy, but also a variety of pollutants that can harm the environment and human health

Combustion products

• Burning fossil fuels releases several gases into the atmosphere:

  • Carbon dioxide (CO₂) – produced during complete combustion

  • Water vapour (H₂O) – also from complete combustion

  • Carbon monoxide (CO) – produced during incomplete combustion when oxygen is limited

  • Oxides of nitrogen (NO and NO₂) – formed from nitrogen and oxygen in the air at high temperatures in engines

  • Sulfur dioxide (SO₂) – produced from sulfur impurities in the fuel.

  • Solid particles and unburned hydrocarbons can also be released, forming soot (carbon particulates) and contributing to particulate pollution

Complete vs incomplete combustion

• Complete combustion happens when there is plenty of oxygen

  • hydrocarbon + oxygen → carbon dioxide + water

  • Example: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

• Incomplete combustion happens when there is not enough oxygen

  • hydrocarbon + oxygen → carbon monoxide + water 

  • Example: 2CH₄ + 3O₂ → 2CO + 4H₂O

• Incomplete combustion is common in engines and poorly ventilated stoves

• It is more likely with fuels like diesel or in environments with limited airflow

Formation of key pollutants

Carbon monoxide (CO)

• Carbon monoxide is produced when fuels burn with limited oxygen 

• The carbon in the fuel is only partially oxidised, forming carbon monoxide instead of carbon dioxide

• For example:

2CH₄ + 3O₂ → 2CO + 4H₂O

Soot (carbon particles)

• Soot forms when there is very little oxygen, causing hydrocarbons to break down without fully combusting

• This results in tiny solid carbon particles, often seen as black smoke

• Soot is common in diesel engines and open flames

• For example: 

C₈H₁₈ + 8½O₂ → 8C (soot) + 9H₂O

Sulfur dioxide (SO₂)

• Some fossil fuels contain sulfur as an impurity

• During combustion, this sulfur reacts with oxygen in the air to form sulfur dioxide, a gas that can cause acid rain

• For example:

S + O₂ → SO₂

Oxides of nitrogen (NO and NO₂)

• Oxides of nitrogen form when nitrogen and oxygen from the air react at high temperatures, such as in car engines and power stations. 

• These reactions do not happen during normal combustion but occur due to the extreme heat

• For example:

N₂ + O₂ → 2NO

2NO + O₂ → 2NO₂

• These gases can irritate the lungs and contribute to acid rain and smog