Combustion (Edexcel IGCSE Science (Double Award))

Combustion of fuels

The combustion of fossil fuels

• A fuel is a substance that, when burned, releases heat energy (exothermic reaction)

• 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 unburned hydrocarbons and carbon particulates

• When the fuel is a hydrocarbon then water and carbon dioxide are the products formed

• Hydrocarbon compounds undergo complete and incomplete combustion

Complete 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

• It occurs in some appliances such as boilers and stoves as well as in internal combustion engines

• The products of these reactions are unburnt fuel (soot), carbon monoxide and water

• Methane for example undergoes incomplete combustion in an oxygen-poor environment:

2CH₄ + 3O₂→ 2CO + 4H₂O

CH₄ + O₂→ C + 2H₂O

Carbon monoxide

Why is carbon monoxide dangerous?

• Carbon monoxide is a toxic and odourless gas which can cause dizziness, loss of consciousness and eventually death

  • The CO binds well to haemoglobin which therefore cannot bind oxygen

  • Oxygen is transported to organs

  Carbon monoxide binding to haemoglobin

The high affinity of CO to haemoglobin prevents it from binding to O₂