Combustion (Edexcel IGCSE Science (Double Award)): Revision Note

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 (CO) is a highly poisonous gas produced during the incomplete combustion of carbon-containing fuels

• It is particularly dangerous because it is:

  • Odourless

  • Colorless

  • Tasteless

• This means it can be inhaled without a person realising it is present

How carbon monoxide affects the body

The basic explanation

• When inhaled, carbon monoxide is absorbed from the lungs into the red blood cells

• It binds tightly to the molecules that normally transport oxygen

• This means the blood's capacity to carry oxygen around the body is significantly reduced

• This lack of oxygen supply to tissues and organs leads to symptoms such as:

  • Headaches and dizziness

  • Nausea and vomiting

  • Shortness of breath

  • Loss of consciousness and, in high concentrations, death

The detailed explanation

• Inside red blood cells, haemoglobin is responsible for binding to oxygen in the lungs and transporting it to the body's tissues

• Carbon monoxide can also bind to haemoglobin

• However, haemoglobin has a much higher affinity for carbon monoxide than for oxygen

  • It binds around 200-250 times more strongly

• This means CO binds preferentially and irreversibly to haemoglobin

  • This forms a stable compound called carboxyhaemoglobin

• This process prevents haemoglobin molecules from binding to oxygen and transporting it around the body