CIE AS Chemistry (9701) 2019-2021

Revision Notes

3.2.11 Hydrocarbons as Fuels

Combustion of Alkanes & the Environment

Alkanes as fuels

  • Alkanes can be burnt in oxygen in a process called combustion
    • Complete combustion is when alkanes burn in excess oxygen to form carbon dioxide and water

Alkanes + oxygen (excess) → carbon dioxide + water

    • Incomplete combustion is when alkanes burn in a limited supply of oxygen to form the toxic carbon monoxide and carbon (soot)

Alkanes + oxygen (limited) → carbon monoxide + water (+ carbon)

  • The longer the alkane chains, the more energy is required to burn them when used as fuels
    • The bigger the alkane molecules, the stronger the van der Waals forces between the molecules
  • Alkanes are suitable to be used as fuels in industry, in the home and in transport as:
    • They are readily available
    • Burn cleanly in the presence of excess oxygen
    • Have high enthalpy changes of combustion and thus release a lot of energy when burnt

 Environmental consequences

  • 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)
  • 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

Oxides of nitrogen & unburnt hydrocarbons

  • Normally, nitrogen is too unreactive to react with oxygen in air
  • However, in a car’s engine, high temperatures and pressures are reaches 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 can cause acid rain
    • This can cause corrosion of buildings, endangering plant-life, aquatic life as lakes and rivers get too acidic and damaging human health

Enhanced greenhouse effect

  • When sunlight hits the Earth’s surface, some of the light gets reflected back into the atmosphere as infrared (IR) radiation
  • Molecules such as water vapour, CO2 and hydrocarbons absorb this IR radiation and pass it onto other molecules during collisions
  • As a result of this the atmosphere warms up
  • This process is known as the greenhouse effect and keeps the Earth’s temperature suitable for life
  • However, burning excessive fossil fuels increases the amount of CO2 in the atmosphere which increase the greenhouse effect causing the Earth to warm up
  • This is known as the enhanced greenhouse effect

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: formation, effect & catalytic removal table

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

Use of Infrared Spectroscopy in Monitoring Air Pollution

  • Air pollutants can be detected by infrared (IR) spectroscopy
  • IR spectroscopy identifies particular bonds in a molecule
  • Therefore, each pollutant will show a different pattern of absorptions
  • The concentrations of the pollutants can also be determined by IR spectroscopy by looking at the intensities of the peaks
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