AQA Chemistry, Unit 1, Alkanes

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Rebecca Pinder Alkanes
Alkanes are saturated hydrocarbons, with no double bonds.
Petroleum is a mixture of hydrocarbons, mainly alkanes
Cracking - The breaking of long-chain alkane molecules (obtained from crude old) into shorter chain
hydrocarbons, some of which are alkenes. This involved the breaking of c-c bonds.
Thermal Cracking
High temperature
High pressure
Gain lots of alkenes
Catalytic Cracking
High Temperature
Slight Pressure
Zeolite catalyst
Produces more branches molecules ­ used to produce motor fuels and aromatic
hydrocarbons
Advantages of Catalytic Cracking
Produces more branched alkanes, which burn more easily and are an important component in
petrol.
Lower temperature and pressure mean it's cheaper then thermal cracking
Never just say `cracking' ALWAYS SPECIFY
Economic reasons for cracking:
Demand for lower boiling point fractions is greater than the proportions found in crude old.
Increased amount of gasoline and other economically important fractions
Converts heavy fractions into higher value products
Produces ethene used for poly(ethene)
Combustion
Complete:
In excess oxygen, short chain alkanes undergo complete combustion
Alkane + oxygen carbon dioxide + water
Incomplete:
In limited oxygen:
Alkane + oxygen carbon monoxide + water
OR
Alkane + oxygen carbon + water
Combustion of alkanes is highly exothermic, so alkanes make good fuel!
Internal combustions engines
Chains with 5-10 carbons are used as fuel in internal combustion engines.

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Rebecca Pinder Alkanes
This can produce a number of pollutants:
CO ­ nonane + oxygen carbon monoxide + water
NO ­ N2 + O2 2NO
NO2 ­ 2NO + O2 2NO2
Unburned hydrocarbons can also beb released
These can be removed using a catalytic converter
2CO +2NO 2CO2 + N2
C8H16+25NO 8CO2 + 25/2 N2 + 9H2O
Catalytic converters have a ceramic honeycomb coated in a thin layer of catalyst metal (Pt, Pd, Rh) to
give a large surface area so more will react.…read more

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