Combustion (Edexcel IGCSE Chemistry: Double Science)

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

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