Combustion of Alkanes (Cambridge International A-Level Chemistry)

Combustion of Alkanes & the Environment

• 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

• When oxygen supply is limited, carbon monoxide (CO) forms instead of carbon dioxide:

fuel + oxygen → carbon monoxide + water

• For example, Incomplete combustion of propane:

propane + oxygen → carbon monoxide + water

C₃H₈(l) + 3½O₂(g) → 3CO (g) + 4H₂O (l)

• Carbon monoxide is extremely dangerous because it is:

  • Colourless and odourless (can’t be seen or smelled)
  • Hard to detect without a sensor

• It is a toxic and poisonous gas that binds irreversibly to haemoglobin in the blood.

  • This prevents haemoglobin from carrying oxygen.

• Lack of oxygen transport leads to:

  • Dizziness
  • Loss of consciousness
  • Potentially death if not treated

The effect of carbon monoxide on haemoglobin

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
• In a car’s 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 car’s exhaust fumes into the atmosphere

• Car exhaust fumes also contain unburnt hydrocarbons from fuels and their oxides (VOCs)

• In the 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, endanger plant and aquatic life (as lakes and rivers become too acidic) and directly damage human health

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, their effect & removal summary

• Carbon Monoxide (CO)

  • Formation:

    • Incomplete combustion of alkanes in car engines

  • Environmental consequence:

    • Toxic gas

  • Catalytic removal:

    • Oxidised to carbon dioxide (CO₂):

2CO + O₂→ 2CO₂

2CO + 2NO → 2CO₂+ N₂

• Oxides of Nitrogen (NO, NO₂etc.)

  • Formation:

    • Oxidation of nitrogen in car engines (due to high temperatures)

  • Environmental consequence:

    • Dissolve in water and react with oxygen to form acid rain

  • Catalytic removal:

    • Reduced to nitrogen gas:

2CO + 2NO → 2CO₂+ N₂

• Volatile Organic Compounds (VOCs)

  • Formation:

    • Unburnt hydrocarbons from fuels
    • Oxides of these hydrocarbons formed in car engines

  • Environmental consequence:

    • React with oxides of nitrogen in the atmosphere to form PAN

  • Catalytic removal:

    • Oxidised to CO₂ and H₂O
    • General formula reaction:

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

• PAN (peroxyacyl nitrate)

  • Formation:

    • From the photochemical reaction of VOCs and nitrogen oxides in the atmosphere

  • Environmental consequence:

    • Contributes to photochemical smog

  • Catalytic removal:

    • Oxidise unburnt hydrocarbons
    • Reduce nitrogen oxides to prevent PAN formation

Exam Hint Although CO₂ is not a toxic gas, it is still a pollutant causing global warming and climate change