Problems of Pollution - General
- 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
- 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
- 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
- Solid carbon particles (or particulates) released from incomplete combustion clump together to form soot which gradually falls back to the ground
- Car exhaust fumes include toxic gases such as carbon monoxide (CO), oxides of sulfur, 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 sulfur
- Some of the crude oil products from fractional distillation, cracking and reforming contain sulfur atoms
- When these molecules are combusted, the sulfur atoms form sulfur dioxide and sulfur trioxide
- Both of these sulfur oxides are acidic
S + O2 → SO2
2SO2 + O2 → 2SO3
- When these acidic sulfur oxides dissolve into water in the atmosphere, they form sulfurous and sulfuric acid
SO2 + H2O → H2SO3
SO3 + H2O → H2SO4
- Both of these acids contribute to acid rain, which is responsible for various environmental issues:
- Damage / death of aquatic life
- Damage / death to crops and forests
- Release of carbon dioxide from carbonate rocks and building materials
- Corrosion of metallic structures
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 + O2 → 2NO
N2 + 2O2 → 2NO2
- 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 nitrous and nitric acid
3NO2 + O2 → HNO2 + 2HNO3
- Both of these acids are a cause of acid rain, which can corrode buildings, endanger plant and aquatic life (as lakes and rivers become too acidic) as well as directly damaging human health
Pollutants, their Effect & Removal Table