AQA A Level Chemistry

Revision Notes

3.2.3 Combustion of Alkanes

Combustion

  • Alkanes are combusted (burnt) on a large scale for their use as fuels

Complete combustion

  • 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
    • For example, the complete combustion of octane to carbon dioxide and water

Hydrocarbons Complete Combustion, downloadable AS & A Level Chemistry revision notes

The complete combustion of alkanes

Incomplete combustion

  • 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
    • For example, the incomplete combustion of octane to form carbon monoxide

 Hydrocarbons Incomplete Combustion, downloadable AS & A Level Chemistry revision notes

The incomplete combustion of alkanes

  • 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 in the form of soot:Incomplete Combustion of Alkanes, downloadable IB Chemistry revision notes

Combustion & The Environment

  • Car 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 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

 

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 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 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 car 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 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.

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