Pollution from Combustion (Edexcel AS Chemistry)

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Causes & Effects of Pollution

  • 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

  • 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

Particulates 

  • Solid carbon particles (or particulates) released from incomplete combustion clump together to form soot which gradually falls back to the ground
  • Particulates can cause several problems:
  • If they are inhaled they can damage the lungs and cause respiratory problems
  • They can cover buildings and statues, making them look unclean and accelerating corrosion
  • They can reflect sunlight back into space reducing the amount of light reaching the earth, this is called global dimming

Pollutants, their Effect & Removal Table

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

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Richard

Author: Richard

Expertise: Chemistry

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.