CIE AS Chemistry (9701) exams from 2022

Revision Notes

3.2.6 Combustion of Alkanes

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


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


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 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


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

Hydrocarbons Table 1_Combustion of Alkanes & the Environment, downloadable AS & A Level Chemistry revision notes

Exam Tip

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

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.

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