Combustion of Alkanes (AQA A Level Chemistry): Revision Note

Combustion

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

Complete combustion

• When alkanes burn in excess oxygen, complete combustion occurs

  • All carbon atoms are oxidised to carbon dioxide, and all hydrogen atoms are oxidised to water.

  • For example, the complete combustion of octane is shown by the balanced equation:

Incomplete combustion

• When alkanes burn in limited oxygen, incomplete combustion occurs, and the carbon is not fully oxidised

• Instead of forming only carbon dioxide, some carbon is partially oxidised to carbon monoxide

• For example, the incomplete combustion of octane, forming carbon monoxide, can be represented by the balanced equation:

• 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 contain toxic gases such as carbon monoxide (CO), nitrogen oxides (NO and NO₂, often referred to as NOx), and volatile organic compounds (VOCs)

• When released into the atmosphere, these pollutants contribute to environmental problems such as acid rain, photochemical smog, and global warming

  • They can also harm human health

Carbon monoxide

• Carbon monoxide (CO) is a toxic, colourless, and odourless gas that can cause dizziness, unconsciousness, and, in severe cases, death

• CO binds strongly to haemoglobin in red blood cells, reducing its ability to bind and transport oxygen

• Normally, haemoglobin carries oxygen from the lungs to body tissues and transports carbon dioxide from tissues back to the lungs for removal

Oxides of nitrogen

• Nitrogen is normally too unreactive to react with oxygen in the air

• However, in a car engine, very high temperatures and pressures are reached

• Under these conditions, nitrogen reacts with oxygen to form nitrogen monoxide:

N₂(g) + O₂(g) → 2NO(g)

• The nitrogen monoxide can then be further oxidised in air to form nitrogen dioxide:

2NO(g) + O₂(g) → 2NO₂(g)

• 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

• These nitrogen oxides (NO and NO₂, collectively called NOx) are released into the atmosphere in car exhaust fumes

• Car exhaust fumes also contain unburnt hydrocarbons, which are volatile organic compounds (VOCs)

  • In sunlight, nitrogen oxides react with VOCs to form peroxyacetyl nitrate (PAN), a major component of photochemical smog

  • PAN can irritate the lungs and eyes, and can also damage plants

Catalytic removal

• To reduce the emission of harmful gases, most modern cars are fitted with catalytic converters

• Precious metals such as platinum, palladium, and rhodium are coated onto a ceramic honeycomb structure

  • This provides a large surface area for the catalytic reactions to take place.

• The main reactions occurring in a three-way catalytic converter are:

  • Oxidation of carbon monoxide to carbon dioxide:

2CO + O₂ → 2CO₂

• Reduction of nitrogen monoxide to nitrogen:

2CO + 2NO → 2CO₂ + N₂

• Oxidation of unburnt hydrocarbons to carbon dioxide and water (for an alkane):

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

• These reactions convert toxic carbon monoxide, nitrogen oxides, and unburnt hydrocarbons into less harmful gases such as carbon dioxide, nitrogen, and water

Reducing sulfur dioxide emissions

• Sulfur dioxide emissions from coal-fired power stations can be reduced by treating the waste gases before they are released into the atmosphere

• The waste gases are passed through a scrubbing tower, where a slurry containing calcium oxide or calcium carbonate is sprayed into the gases

  • This process is known as flue gas desulfurisation (FGD), or sulfur scrubbing

• When calcium oxide is used, it first reacts with sulfur dioxide and water to form calcium sulfite

  • The calcium sulfite is then oxidised to form calcium sulfate (gypsum):

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

• Calcium carbonate can also be used

  • It reacts with sulfur dioxide and oxygen to form calcium sulfate and carbon dioxide:

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

• This process removes sulfur dioxide from flue gases and reduces the formation of acid rain

Sulfur dioxide scrubber

Pollutants, their effects, and removal