Combustion of FuelsOxford AQA IGCSE Chemistry: Revision notes
Section 1
What is released when fuels burn?
Most fuels contain carbon and/or hydrogen, and may also contain sulfur as an impurity. When a fuel burns, a range of products can be released depending on its composition and how completely it burns:
- Carbon dioxide (CO2)
- Water (vapour)
- Carbon monoxide (CO)
- Sulfur dioxide (SO2)
- Oxides of nitrogen
- Solid particles (particulates, including soot)
Section 2
What environmental problems do these products cause?
Each combustion product causes a different environmental problem:
| Product | Environmental effect |
|---|---|
| Sulfur dioxide, oxides of nitrogen | Cause acid rain |
| Carbon dioxide | Increased levels may cause climate change |
| Solid particles (particulates) | Cause global dimming (particles in the atmosphere reflect and absorb sunlight) |
Match each product to its correct environmental problem precisely — examiners penalise mixing up acid rain (SO2/NOx) with climate change (CO2) with global dimming (particulates).
Section 3
What is the difference between complete and incomplete combustion?
- Complete combustion happens when there is plenty of oxygen. The fuel is fully oxidised, producing only carbon dioxide and water (plus sulfur dioxide if sulfur is present)
- Incomplete combustion happens when there is insufficient oxygen. This produces carbon monoxide and/or solid carbon (soot/particulates) as well as, or instead of, carbon dioxide
Carbon monoxide is toxic because it is colourless and odourless but stops blood carrying oxygen effectively — this is a common extended-answer point examiners look for.
Section 4
How do the products relate to what is in the fuel?
The products of combustion depend directly on the elements present in the fuel:
- A fuel containing only carbon and hydrogen (a hydrocarbon) produces carbon dioxide and water (complete combustion), or carbon monoxide/soot and water (incomplete combustion)
- A fuel containing sulfur as an impurity additionally produces sulfur dioxide
- Nitrogen oxides usually form from nitrogen and oxygen in the air reacting at the high temperatures reached during combustion, rather than from nitrogen in the fuel itself
Must Know
- Burning fuels can release CO2, water vapour, CO, SO2, oxides of nitrogen, and particulates
- SO2 and oxides of nitrogen cause acid rain
- CO2 is linked to climate change
- Particulates cause global dimming
- Complete combustion (plenty of oxygen) gives CO2 and water; incomplete combustion (limited oxygen) gives CO and/or soot
- The elements present in the fuel (and reaction with air) determine which products form
That's the notes covered.
Carry on to the next subtopic.