Causes & Effects of Pollution
- Alkanes are combusted (burnt) on a large scale for their use as fuels
Complete combustion
- When alkanes are burnt in excess (plenty of) oxygen, complete combustion will take place and all carbon and hydrogen will be oxidised to carbon dioxide and water respectively
- For example, the complete combustion of octane to carbon dioxide and water
The complete combustion of alkanes
Incomplete combustion
- When alkanes are burnt in only a limited supply of oxygen, incomplete combustion will take place and not all the carbon is fully oxidised
- Some carbon is only partially oxidised to form carbon monoxide
- For example, the incomplete combustion of octane to form carbon monoxide
The incomplete combustion of alkanes
- Incomplete combustion often takes place inside a car engine due to a limited amount of oxygen present
- With a reduced supply of oxygen, carbon will be produced in the form of soot:
- Car exhaust fumes include toxic gases such as carbon monoxide (CO), oxides of nitrogen (NO/NO2) and volatile organic compounds (VOCs)
- When released into the atmosphere, these pollutants have serious environmental consequences damaging nature and health
Carbon monoxide
- CO is a toxic and odourless gas which can cause dizziness, loss of consciousness and eventually death
- The CO binds well to haemoglobin which therefore cannot bind oxygen and carbon dioxide
- Oxygen is transported to organs
- Carbon dioxide is removed as waste material from organs
The high affinity of CO to haemoglobin prevents it from binding to O2 and CO2
Oxides of nitrogen
- Normally, nitrogen is too unreactive to react with oxygen in air
- However, in a car engine, high temperatures and pressures are reached causing the oxidation of nitrogen to take place:
N2(g) + O2(g) → 2NO(g)
N2(g) + 2O2(g) → 2NO2(g)
- The oxides of nitrogen are then released in the exhaust fumes into the atmosphere
- Car exhaust fumes also contain unburnt hydrocarbons from fuels and their oxides (VOCs)
- In air, the nitrogen oxides can react with these VOCs to form peroxyacetyl nitrate (PAN) which is the main pollutant found in photochemical smog
- PAN is also harmful to the lungs, eyes and plant-life
- Nitrogen oxides can also dissolve and react in water with oxygen to form nitric acid which is a cause of acid rain
- Acid rain can cause corrosion of buildings, endangers plant and aquatic life (as lakes and rivers become too acidic) as well as directly damaging human health
Particulates
- Solid carbon particles (or particulates) released from incomplete combustion clump together to form soot which gradually falls back to the ground
- Particulates can cause several problems:
- If they are inhaled they can damage the lungs and cause respiratory problems
- They can cover buildings and statues, making them look unclean and accelerating corrosion
- They can reflect sunlight back into space reducing the amount of light reaching the earth, this is called global dimming
Pollutants, their Effect & Removal Table