9.3.1Chemistry of the Atmosphere
Atmospheric pollutants from fuels — AQA GCSE Combined Science Trilogy 8464 - Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 10 of 11
Part 1 of 4Fuels and what they contain
Hydrocarbons
- Hydrocarbons Most fossil fuels (coal, crude oil, natural gas) are mainly hydrocarbons, which contain only carbon and hydrogen.
Impurities
- Impurities Many fuels also contain small amounts of sulfur, which leads to sulfur dioxide when burned.
Combustion
- Combustion Combustion is burning in oxygen, a reaction that releases energy.
- The products depend on how much oxygen is available.
Part 2 of 4Complete or incomplete combustion
| Feature | Complete combustion | Incomplete combustion |
|---|---|---|
| Oxygen supply | Plentiful | Limited |
| Carbon in the fuel becomes | Carbon dioxide, CO2 | Carbon monoxide, CO, and/or carbon (soot), C |
| Hydrogen in the fuel becomes | Water, H2O | Water, H2O |
| Example | CH4 + 2O2 → CO2 + 2H2O | 2CH4 + 3O2 → 2CO + 4H2O |
| Flame | Hot and blue | Cooler and yellow, may be sooty |
Part 3 of 4Where each pollutant comes from
Burning fuels
Carbon monoxide
- Carbon monoxide Forms when carbon is only partly oxidised because oxygen is limited, e.g. in faulty boilers, poorly ventilated stoves and engines.
Soot (particulates)
- Soot (particulates) With very little oxygen, hydrocarbons break up and tiny solid particles of carbon are released, seen as black smoke.
Sulfur dioxide
- Sulfur dioxide Sulfur impurities react with oxygen: S + O2 → SO2.
Oxides of nitrogen
- Oxides of nitrogen Nitrogen and oxygen from the air do not react in normal burning,
- but do at the very high temperatures inside car engines and power stations, e.g. N2 + O2 → 2NO.
Part 4 of 4Predicting the products
1
Identify the elements in the fuel: carbon and hydrogen, and possibly sulfur.
2
Decide whether oxygen is plentiful (complete combustion) or limited (incomplete combustion).
3
Carbon forms CO2 when oxygen is plentiful, but CO or C when it is limited; hydrogen always forms water.
4
Add SO2 if the fuel contains sulfur, and oxides of nitrogen if the temperature is very high and air is present.
Key terms
| Term | Meaning |
|---|---|
| Hydrocarbon | A compound containing hydrogen and carbon only. |
| Complete combustion | Burning in plentiful oxygen so that the products are carbon dioxide and water. |
| Incomplete combustion | Burning in limited oxygen, producing carbon monoxide and/or soot as well as water. |
| Particulates | Tiny solid particles, such as soot, released into the air. |
| Oxides of nitrogen | Gases (NO and NO2) formed when nitrogen and oxygen in the air react at high temperature. |
Avoid these mistakes
1
Saying the nitrogen oxides come from the fuel. They come from nitrogen and oxygen in the air reacting at high temperature.
2
Forgetting that water is a product of both complete and incomplete combustion of hydrocarbons.
3
Stating that incomplete combustion produces only carbon monoxide. Soot can be formed as well.
4
Mixing up the source of sulfur dioxide: it comes from sulfur impurities in the fuel, not from the hydrocarbon itself.
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