Atmospheric pollutants from fuels (AQA GCSE Chemistry: Combined Science)

• The combustion of fossil fuels is the major source of atmospheric pollution
• Fossil fuels include: coal, oil, natural gas, oil shales and tar sands
• Non-renewable fossil fuels are obtained from crude oil by fractional distillation
• Petrol is used as a fuel in cars, kerosene is used to fuel aircraft and diesel oil is used as a fuel in some cars, trucks and heavy vehicles such as tanks and trains
• Coal is used in power stations and also steel production
• Natural gas consists mainly of methane, CH₄
• There are finite amounts of fossil fuels and they all contribute to pollution and global warming
• All these fuels contain carbon, hydrogen and small quantities of sulfur

Combustion Products

• The burning of fossil fuels releases the gases carbon dioxide, carbon monoxide, oxides of nitrogen and oxides of sulfur
• In addition, incomplete combustion of the fuels gives rise to unburnt hydrocarbons and carbon particulates

Complete versus Incomplete Combustion

• A fuel is a substance which releases energy in an exothermic reaction
• When the fuel is a hydrocarbon then water and carbon dioxide are the products formed
• Hydrocarbon compounds undergo complete and incomplete combustion
• Complete combustion occurs when there is excess oxygen
• For example, the combustion equation for propane is:

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

Incomplete Combustion

• Incomplete combustion occurs when there is insufficient oxygen to burn
• Some of the fuel does not undergo combustion and unburned hydrocarbons are released into the atmosphere
• Fuels such as diesel contain larger hydrocarbon molecules which do not always burn completely in the engine
• It occurs in some appliances such as boilers and stoves as well as in internal combustion engines
• In addition to water, carbon particles (soot) and carbon monoxide may be produced during incomplete combustion 
• Methane for example undergoes incomplete combustion in an oxygen-poor environment:

2CH₄ + 3O₂→ 2CO + 4H₂O

CH₄ + O₂→ C + 2H₂O

How are oxides of nitrogen produced in a car engine?

• Although nitrogen is not present in fossil fuels, nitrogen oxides are a product of the combustion of fuels in car engines
• The oxides are mostly a mixture of nitrogen monoxide and nitrogen dioxide
• Nitrogen is normally an unreactive gas, but high temperatures inside combustion engines can make nitrogen in the air react with oxygen to produce oxides of nitrogen: 

N₂ (g) + O₂ (g) → 2NO (g)

2NO (g) + O₂ (g) → 2NO₂ (g)

How is sulfur dioxide produced in a car engine?

• Fossil fuels are often contaminated with small amounts of sulfur impurities
• When these contaminated fossil fuels are combusted, the sulfur in the fuels get oxidised to sulfur dioxide

S (s) + O₂ (g) → SO₂ (g)

• Sulfur dioxide is a major atmospheric pollutant