1.9.3 Combustion of Alkanes

• 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