Chemistry - Combustion of Alkanes

Chemistry - Combustion of Alkanes

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Combustion of Alkanes
Alkanes are quite unreactive - wont react with acids, bases, oxidising and reducing agents. They do,
however, burn in oxygen.
Shorter chained alkanes will burn completely with plenty of oxygen, producing CO2 and H2O.
E.g. Methane:
CH4 (g) + 2O2 (g) -> CO2 (g) + 2H2O (g) H = -890kJmol-1
These reactions are exothermic (give out heat). More carbons present mean greater heat output.
This is why hydrocarbons are used as fuels. They store a large amount of energy for a small amount
of weight.
Incomplete Combustion
In a limited supply of oxygen, carbon monoxide is produces.
E.g. propane:
2C3H8 (g) + 7O2 (g) -> 6CO(g) + 8H2O(g)
Carbon monoxide is poisonous.
Carbon produced when there is even less oxygen.
Polluting the Atmosphere
Carbon monoxide: poisonous gas produced by incomplete combustion.
Nitrogen Oxides: NO, NO2, N2O4 produced when there is enough energy for N2 and O2 to react in air.
This can happen in a petrol engine when the spark ignites the fuel. These oxides can react with water
vapour and oxygen in the air to form nitric acid. These contribute to acid rain and photochemical
Sulfur Dioxide: another contributor to acid rain. It is produced from the sulfur containing impurities in
crude oil. Sulfur Dioxide reacts with water vapour and oxygen to form acid rain:
SO2 + H2O -> H2SO3
H2SO3 + 0.5O2 -> H2SO4
Carbon particles: Carcinogenic and exacerbate asthma, produced when there is little oxygen.
Unburnt Hydrocarbons: may also enter the atmosphere. They contribute to photochemical smog and
can cause health problems. Also they are greenhouse gases.
Carbon dioxide and water vapour: Both produced in combustion and are "greenhouse gases".

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Combustion of Alkanes
Removing Sulfur
Calcium oxide(CaO) or limestone (CaCO3) are used to absorb the sulfur dioxide.
CaO (s) + SO2 (g) -> CaSO3 (s)
CaCO3 (s) + SO2 (g) -> CaSO3 (s)0 + CO2 (g)
Catalytic Converters
Modern cars often have catalytic converters in their exhaust systems. These reduce the output of
pollutants such as carbon monoxide, nitrogen oxides and unburnt hydrocarbons. The catalysts are a
honeycomb of a ceramic material coated in platinum.…read more


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