Combustion of Fuels (Oxford AQA IGCSE Combined Science Double Award) : Revision Note

Atmospheric Pollutants

• Fuels are substances that react with oxygen to release energy 

• This reaction is called combustion 

  • Due to the release of energy, combustion reactions are exothermic 

• Most fuels contain carbon, hydrogen and sometimes traces of sulfur 

• When the fuel burns, these elements react with oxygen to produce gases that are released into the atmosphere including:

  • Carbon dioxide 

  • Particulates

  • Sulfur dioxide 

  • Oxides of nitrogen 

Carbon Dioxide 

• Carbon dioxide is produced when a fuel containing carbon burns in plenty of oxygen

  • This is called complete combustion

methane  +  oxygen  →  carbon dioxide  +   water

• Increasing levels of carbon dioxide in the atmosphere contribute to climate change 

Particulates 

• Particulates (solid carbon particles and unburnt hydocarbons) are produced when a fuel containing carbon burns in limited oxygen 

  • This is called incomplete combustion

• Carbon monoxide can also be produced during incomplete combustion but you are not required to know about this for this specification

methane  +  oxygen  →  carbon monoxide +  carbon  +   water

• Particulates can clump together to form soot which gradually falls back to the ground 

• Particulates also cause global dimming 

  • This happens as they reflect sunlight back into space reducing the amount of light reaching the earth 

Sulfur Dioxide

• Some fuels contain sulfur impurities 

• When a fuel burns, the sulfur will oxidise to form sulfur dioxide 

• The sulfur dioxide released mixes with clouds and readily dissolves in rainwater

• SO₂ is a non-metal oxide so it forms an acidic solution in water, and therefore forms acid rain

• Acid rain causes corrosion to metal structures, buildings and statues made of carbonate rocks

• It causes damage to aquatic organisms, pollutes crops and water supplies, and irritates lungs, throats and eyes

Statues and monuments in very polluted areas become blackened over time from soot

Oxides of Nitrogen

• Oxides of nitrogen come largely from vehicle exhausts and also contribute to acid rain

• They are produced when nitrogen and oxygen react at high temperatures which are reached in a vehicle engine

Diagram to show how acid rain is caused

Oxides of sulfur and nitrogen contribute to the formation of acid rain