3.2.6 Combustion of Alkanes

• Cars’ 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 drastic environmental consequences damaging nature and health

Carbon monoxide

• Carbon monoxide is formed in the incomplete combustion of alkanes inside a car engine
• Due to lack of enough oxygen in the engine, some of the carbon is only partially oxidised to CO instead of carbon dioxide (CO₂)

Incomplete combustion of alkanes is caused by a limited supply of oxygen

• CO is a toxic and odourless gas which can cause dizziness, loss of consciousness and eventually death
  • The CO binds 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’s 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 car’s 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

Catalytic removal

• To reduce the amount of pollutants released in cars’ exhaust fumes, many cars are now fitted with catalytic converters
• Precious metals (such as platinum) are coated on a honeycomb to provide a large surface area
• The reactions that take place in the catalytic converter include:
  • Oxidation of CO to CO₂:

2CO + O₂ → 2CO₂

or

2CO + 2NO → 2CO₂ + N₂

• • Reduction of NO/NO₂ to N₂:

2CO + 2NO → 2CO₂ + N₂

• • Oxidation of unburnt hydrocarbons:

C_nH_2n+2 + (3n+1)[O] → nCO₂ + (n+1)H₂O

Pollutants, their effect & removal table