Combustion of Alkanes (AQA A Level Chemistry)

Combustion

• 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:

  

Combustion & The Environment

• 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:

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

Reducing sulfur dioxide emissions

• The main way to reduce sulfur dioxide emissions is to treat the waste gases from coal fired power stations 

• The waste gases are passed into a scrubbing chamber which sprays a wet slurry of calcium oxide and calcium carbonate into the gases

  • This process is also known as sulfur scrubbing or flue gas desulfurisation

• Calcium oxide can be used:

  • Calcium oxide and water reacts with sulfur dioxide to initially produce calcium sulfiite, which is then further oxidised to calcium sulfate or gypsum:

CaO (s) + 2H₂O (l) + SO₂ (g) + ½O₂ (g) → CaSO₄.2H₂O (s)

• Calcium carbonate can also be used

CaCO₃ + ½O₂ (g) + SO₂ (g) → CaSO₄ (s) + CO₂ (g)

Sulfur dioxide scrubber

The scrubber sprays a lime slurry over the waste gases to remove 90 - 95% of the sulfur dioxide

Pollutants, their Effect & Removal Table