Pollutants That Affect Air Quality

Section 1 of 7Complete and incomplete combustion

Type	Oxygen supply	Products	Example equation
Complete	Plenty	Carbon dioxide and water	C3H8 + 5O2 → 3CO2 + 4H2O
Incomplete	Limited	Carbon monoxide and water	2CH4 + 3O2 → 2CO + 4H2O
Very limited oxygen	Very little	Carbon (soot) and water	CH4 + O2 → C + 2H2O

Section 2 of 7Where each pollutant comes from

Carbon monoxide

• Carbon monoxide Formed when there is not enough oxygen, so carbon is only partly oxidised, for instance in engines and badly ventilated heaters.

Soot (particulates)

• Soot (particulates) Tiny carbon particles and unburnt hydrocarbons are released when oxygen is very short, often as a black smoke from diesel engines.

Sulfur dioxide

• Sulfur dioxide Sulfur impurities in fossil fuels burn to form SO₂: S + O₂ → SO₂.

Oxides of nitrogen

• Oxides of nitrogen At the high temperature in engines, nitrogen and oxygen from the air react to form NO and NO₂: N₂ + O₂ → 2NO.

Section 3 of 7Harm caused

1Carbon monoxide

• Carbon monoxide A colourless, odourless, toxic gas that binds to haemoglobin and cuts oxygen carried in the blood,
  • causing headaches, unconsciousness and death.

2Particulates

• Particulates Cause global dimming by reflecting sunlight, and lung problems when breathed in.

3SO₂ and NO_x

• SO₂ and NO_x Dissolve in rain to form acid rain, damaging buildings, trees and water life, and cause respiratory illness such as asthma.

Section 4 of 7Reducing air pollution

1

Remove sulfur from fuels before burning, or remove SO₂ from flue gases.

2

Fit catalytic converters to vehicles to turn CO and NO_x into less harmful gases such as carbon dioxide and nitrogen.

3

Use cleaner energy sources and improve public transport so fewer fuels are burnt.

4

Make sure fuel-burning appliances are well ventilated and serviced, and fit carbon monoxide alarms in homes.

Section 5 of 7Worked example: balancing combustion

Propane burning in plenty of oxygen

Write the formulae

• Write the formulae C₃H₈ + O₂ → CO₂ + H₂O

Balance carbon and hydrogen

• Balance carbon and hydrogen Three carbon atoms need 3CO₂ and eight hydrogen atoms need 4H₂O.

Balance oxygen last

• Balance oxygen last The products contain 6 + 4 = 10 oxygen atoms, so 5O₂ are needed on the left.

Final equation

• Final equation C₃H₈ + 5O₂ → 3CO₂ + 4H₂O, with the same number of each atom on both sides.

Section 6 of 7Key terms

Term	Meaning
Fossil fuel	A fuel formed from ancient remains, mainly hydrocarbons.
Complete combustion	Burning in plenty of oxygen to give carbon dioxide and water.
Incomplete combustion	Burning in limited oxygen so that carbon monoxide or carbon is made.
Particulates	Tiny solid particles, such as soot, in the air.
Acid rain	Rain made acidic by dissolved sulfur dioxide and nitrogen oxides.
Catalytic converter	A device in a car exhaust that converts toxic gases into less harmful ones.

Section 7 of 7Exam technique

1

Saying nitrogen oxides come from nitrogen in the fuel. They form from nitrogen and oxygen in the air at high temperature.

2

Writing that sulfur dioxide comes from carbon in fuels. It comes from sulfur impurities.

3

Forgetting that carbon monoxide is toxic because it prevents haemoglobin carrying oxygen.

4

Balancing equations incorrectlycheck atoms of each element on both sides.

5

Describing carbon monoxide as having a smell. It is odourless, which is why it is so dangerous.