Atmospheric Pollutants (OCR Gateway GCSE Chemistry: Combined Science)

Major Sources of Atmospheric Pollutants

• 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

Complete versus Incomplete Combustion

• A fuel is a substance which releases energy in an exothermic reaction
• When the fuel is a hydrocarbon then water and carbon dioxide are the products formed
• Hydrocarbon compounds undergo complete and incomplete 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

The Problems Caused by Pollutants

Carbon Monoxide & Soot

• Carbon monoxide is an extremely poisonous gas as it combines with haemoglobin in blood and prevents it from carrying oxygen
• It is particularly malevolent as it is colourless and odourless, making it difficult to detect
• A lack of oxygen supply to the brain can lead to fainting, coma or in worst case scenarios, even death
• The carbon particles released from incomplete combustion clump together to form soot which gradually falls back to the ground
• Soot causes respiratory problems and covers buildings and statues, making them look unclean and accelerating corrosion

Statues and monuments in very polluted areas become blackened over time from soot

Sulfur Dioxide

• Sulfur dioxide is a colourless, pungent smelling gas that is a major air pollutant responsible for acid rain
• The sulfur dioxide released mixes with clouds and readily dissolves in rainwater
• SO₂ is a non-metal oxide so it forms an acidic solution in water, hence forming acid rain
• Acid rain causes corrosion to metal structures, buildings and statues made of carbonate rocks
• It causes damage to aquatic organisms, pollutes crops and water supplies, and irritates lungs, throats and eyes

Oxides of sulfur and nitrogen contribute to the formation of acid rain

Oxides of Nitrogen

• Oxides of nitrogen come largely from vehicle exhausts and also contribute to acid rain
• Aside from acid rain, oxides of nitrogen produce photochemical smog and breathing difficulties, in particular for people suffering from asthma