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:

C_nH_2n+2 + (3n+1)[O] → nCO₂ + (n+1)H₂O

Pollutants: formation, effect & catalytic removal table

• 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