Alkanes Revision Notes

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  • Created on: 11-04-16 08:44
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Saturated hydrocarbons.
Petroleum is a mixture that consists of mainly alkane hydrocarbons that can be separated by
fractional distillation.
Cracking involves breaking CC bonds in alkanes.
Thermal cracking happens at high pressure and high temperatures. It produces a high % of
Catalytic cracking takes place at a slight pressure, high temp. and with a zeolite catalyst. It is used
mainly to produce motor fuels and aromatic hydrocarbons.
Economic reasons for cracking:
So that supply meets demand
It cuts costs because reaction can be done at low pressure and lower temp.
Catalyst speeds up rate so saving time and time is money.
Alkanes are used as fuels.
Combustion of alkanes can either be complete:
e.g. C
3H8(g) + 5O
2(g) 2(g) + 4H
Or incomplete:
3H 8(g) + 3.5O
3CO(g) + 4H2O(g)
Carbon monoxide is poisonous, it binds to haemoglobin in red blood cells like oxygen would so
oxygen can't get around body. Soot causes breaking problems and can build up in engines to stop
them working properly.
Internal combustion of engine produces lots of pollutants including NOx (toxic and poisonous), CO,
carbon and unburned hydrocarbons. These can be removed using a catalytic converter.
NO x ­ very toxic and poisonous, produced when high pressure and temp in engine cause N and O
from air to react. Nitrogen monoxide can further react to produce nitrogen dioxide:
N2(g) + O
2NO(g) + O2(g) 2NO2
Combustion contains sulphur leads to sulphur dioxide that causes air pollution.
Sulphur released when fossil fuels burnt which reacts to form sulphur dioxide gas. When sulphur
dioxide in atmosphere it dissolves in moisture and converted to sulphuric acid which causes acid
rain. Same happens with nitrogen. Acid rain destroys vegetation and corrodes buildings and
statues. It also kills fish in lakes.
Powdered calcium carbonate (limestone) or calcium oxide is mixed with water to make a alkaline
slurry. When flue gases mix with alkaline slurry the acidic sulphur dioxide gas reacts with calcium
compounds to form harmless salt (calcium sulphate).

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CaO(s) + SO CaSO
2(g) 3(s)
Chlorination of Alkanes
Halogens react with alkanes in photochemical reactions to form halogenoalkanes. Photochemical
reactions are reactions started by UV.
A H atom substituted by Cl or Br. This is a free radical substitution.
A free radical is particle with unpaired e.
Free radical form when covalent bond splits equally, gives 1 e to each species. Unpaired e makes
them very reactive.…read more


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