• 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

   

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

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:

• 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

   

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