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Combustion of Alkanes (CIE AS Chemistry)

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Combustion of Alkanes & the Environment

  • 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 a lack of enough oxygen in the engine, some of the carbon is only partially oxidised to CO instead of carbon dioxide (CO2)

Word equation for incomplete combustion

Hydrocarbons Incomplete Combustion_Combustion of Alkanes & the Environment, downloadable AS & A Level Chemistry revision notes

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 effect of carbon monoxide on haemoglobin

Hydrocarbons Toxic Effect CO, downloadable AS & A Level Chemistry revision notes

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 the 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, endanger plant and aquatic life (as lakes and rivers become too acidic) and directly damage 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:

CnH2n+2 + (3n+1)[O] → nCO2 + (n+1)H2O

Pollutants, their effect & removal table

  Formation  Environmental Consequence  Catalytic Removal
Carbon Monoxide Incomplete combustion of alkanes in car engines Toxic gas

Oxidation to CO2

2CO + O2 → 2CO2

OR

2CO + 2NO → 2CO2 + N2

Oxides of Nitrogen Oxidation of nitrogen in car engines Dissolve in and react in water with oxygen to form acid rain

Reduction to nitrogen gas:

2CO + 2NO → 2CO2 + N2

VOCs  Unburnt hydrocarbons from fuels and their oxides formed in car engines React with oxides of nitrogen in the atmosphere to produce PAN

Oxidise unburnt hydrocarbons to carbon dioxide and water

CnH2n+2 + (3n + 1)[O] → nCO2 + (n + 1)H2O

PAN From the photochemical reaction of VOCs and nitrogen oxides in the atmosphere Photochemical smog Oxidise unburnt hydrocarbons and reduce nitrogen oxides to prevent the formation of PAN in the atmosphere

Examiner Tip

Although CO2 is not a toxic gas, it is still a pollutant causing global warming and climate change

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Philippa

Author: Philippa

Expertise: Chemistry

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.