Combustion of Alkanes (CIE A Level Chemistry)

• 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 a lack of enough oxygen in the engine, some of the carbon is only partially oxidised to CO instead of carbon dioxide (CO₂)

Word equation for incomplete combustion

Incomplete combustion of alkanes is caused by a limited supply of oxygen

• 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

 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
• However, 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 table

	Formation	Environmental Consequence	Catalytic Removal
Carbon Monoxide	Incomplete combustion of alkanes in car engines	Toxic gas	Oxidation to CO2 2CO + O2 → 2CO2 OR 2CO + 2NO → 2CO2 + N2
Oxides of Nitrogen	Oxidation of nitrogen in car engines	Dissolve in and react in water with oxygen to form acid rain	Reduction to nitrogen gas: 2CO + 2NO → 2CO2 + N2
VOCs	Unburnt hydrocarbons from fuels and their oxides formed in car engines	React with oxides of nitrogen in the atmosphere to produce PAN	Oxidise unburnt hydrocarbons to carbon dioxide and water CnH2n+2 + (3n + 1)[O] → nCO2 + (n + 1)H2O
PAN	From the photochemical reaction of VOCs and nitrogen oxides in the atmosphere	Photochemical smog	Oxidise unburnt hydrocarbons and reduce nitrogen oxides to prevent the formation of PAN in the atmosphere