Combustion of Alkanes (Edexcel International A Level Chemistry)

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Problems of Pollution - General

  • 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
  • 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
  • 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
  • Solid carbon particles (or particulates) released from incomplete combustion clump together to form soot which gradually falls back to the ground

Incomplete Combustion of Alkanes, downloadable IB Chemistry revision notes

  • Car exhaust fumes include toxic gases such as carbon monoxide (CO), oxides of sulfuroxides 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 sulfur

  • Some of the crude oil products from fractional distillation, cracking and reforming contain sulfur atoms
  • When these molecules are combusted, the sulfur atoms form sulfur dioxide and sulfur trioxide
    • Both of these sulfur oxides are acidic

S + O2 → SO2 

2SO2 + O2 → 2SO3 

  • When these acidic sulfur oxides dissolve into water in the atmosphere, they form sulfurous and sulfuric acid

SO2 + H2O → H2SO3 

SO3 + H2O → H2SO4 

  • Both of these acids contribute to acid rain, which is responsible for various environmental issues:
    • Damage / death of aquatic life
    • Damage / death to crops and forests
    • Release of carbon dioxide from carbonate rocks and building materials
    • Corrosion of metallic structures

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 + O2 → 2NO

N2 + 2O2 → 2NO2

  • 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 nitrous and nitric acid

3NO2 + O2 → HNO2 + 2HNO3

  • Both of these acids are a cause of acid rain, which can corrode buildings, endanger plant and aquatic life (as lakes and rivers become too acidic) as well as directly damaging human health

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.