Atmospheric pollutants from fuels: describe how carbon monoxide, soot (carbon particles), sulfur dioxide and oxides of nitrogen are produced by burning fuels

The idea

Burning fuels can release four named atmospheric pollutants. Carbon monoxide and soot form when a fuel burns in a limited supply of oxygen, a process called incomplete combustion. For example, methane burning in too little air can produce carbon (soot) and carbon monoxide instead of just carbon dioxide and water. Sulfur dioxide forms when sulfur impurities in fuels such as coal or petrol oxidise during burning. Oxides of nitrogen form when nitrogen and oxygen from the air react at the high temperatures found in car engines and furnaces. You should be able to describe each production route clearly, linking the pollutant to its source and the conditions needed, and use formulae such as CO, SO₂ and NOₓ where helpful.

What earns the marks

• Carbon monoxide is produced by incomplete combustion of a carbon-containing fuel when oxygen supply is limited.
• Soot consists of carbon particles and is also produced by incomplete combustion of fuels.
• Sulfur dioxide is produced when sulfur impurities in fuels burn and oxidise, forming SO₂.
• Oxides of nitrogen are produced when nitrogen and oxygen from the air react at high temperatures, for example in vehicle engines.
• Complete combustion in plenty of air produces carbon dioxide and water, so the four pollutants require either limited oxygen, fuel impurities or high temperature.
• Each pollutant should be linked to a named fuel or context, such as coal, petrol, diesel or natural gas, to show understanding of production.

Common mistakes and how to put them right

• Saying carbon monoxide is produced by complete combustion.

  Put it right: Carbon monoxide is a product of incomplete combustion when oxygen is limited.

• Confusing soot with carbon dioxide.

  Put it right: Soot is solid carbon particles, while carbon dioxide is a colourless gas.

• Stating that sulfur dioxide comes from nitrogen in the air.

  Put it right: Sulfur dioxide comes from sulfur impurities in the fuel, whereas oxides of nitrogen come from nitrogen and oxygen in the air at high temperature.

Exam tips

• Structure each description as fuel plus condition gives pollutant, for example petrol burning in limited air produces carbon monoxide and soot.
• Use the correct formula for each pollutant where you can: CO, SO₂ and NOₓ, and write carbon for soot.
• Check that you have covered all four pollutants, because exam questions often award credit for each separate production route.