Combustion of Alkanes (Oxford AQA International A Level Chemistry)

Combustion of Fuels

• Alkanes are combusted (burnt) on a large scale for their use as fuels

• Depending on the amount of oxygen available, alkanes can undergo complete or incomplete combustion

Complete combustion

• Complete combustion occurs when alkanes are burnt in excess (plenty of) oxygen

• All the carbon and hydrogen in an alkane 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

• Incomplete combustion occurs, when alkanes are burnt in a limited supply of oxygen

• This means that the hydrogen is fully oxidised to water, but not all of 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

Incomplete combustion forming carbon monoxide

• Incomplete combustion often takes place inside a car engine due to a limited amount of oxygen present

• With a greatly reduced supply of oxygen, carbon will be produced in the form of soot

Incomplete combustion forming carbon (soot)

Combustion & The Environment

• Car exhaust fumes include toxic gases such as:

  • Carbon monoxide, CO

  • Nitrogen oxides, NO_x

  • Unburned hydrocarbons

  • Sulfur dioxide, SO₂

• When released into the atmosphere, these pollutants have serious environmental and health consequences

Carbon monoxide

• Carbon monoxide is formed by the incomplete combustion of the carbon contained with fuels

2C₈H₁₈ (g) + 17O₂ (g) → 16CO (g) + 18H₂O (g)

• Carbon monoxide is then released in the exhaust fumes into the atmosphere

• Carbon monoxide is a toxic, odourless gas which can cause dizziness, loss of consciousness and eventually death

• It binds to haemoglobin and reduces the transport of oxygen and carbon dioxide

  • Oxygen is transported to organs

  • Carbon dioxide is removed as waste material from organs

 How carbon monoxide acts in the body

Oxides of nitrogen

• Normally, nitrogen is too unreactive to react with oxygen in air

• However, the high temperatures and pressures inside a car engine cause the oxidation of nitrogen:

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

• 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

Unburned hydrocarbons

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

• In air, nitrogen oxides can react with these to form photochemical smog

• They are also harmful to the lungs, eyes and plant-life

Catalytic removal of pollutants

• 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

Reactions inside a catalytic converter

• 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

  • For example, octane:

C₈H₁₈ + 25[O] → 8CO₂ + 9H₂O

Sulfur dioxide

• Fuels are often contaminated with sulfur impurities

• When these impurities undergo combustion, the sulfur is oxidised to sulfur dioxide

 S (s) + O₂ (g) → SO₂ (g)

• The sulfur dioxide is then released in the exhaust fumes into the atmosphere

• Similar to nitrogen oxides, sulfur dioxide can also dissolve and react in water with oxygen to form sulfuric 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

Reducing sulfur dioxide emissions

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

• 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 sulfite

  • This 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

Pollutants, their Effect & Removal Table