Chemistry- Alkanes AQA Unit 1.6

Covers all on the specification for AQA As level chemistry on Alkanes aka Unit 1 section 6.

Includes-Alkanes, Fractional distillation, Industrial cracking, Combustion of Alkanes

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  • Created on: 19-12-11 13:50
Preview of Chemistry- Alkanes AQA Unit 1.6

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Chemistry unit 1 section 6 Alkanes
Alkanes are saturated hydrocarbons.
They contain only carbon-carbon and carbon-hydrogen single bonds.
They are the least reactive of the organic compounds.
The main source is crude oil.
Hydrocarbon- a compound consisting only of hydrogen and carbon
Saturated- Contain single carbon -carbon bonds only
Unsaturated-contains a C=C bond
Petroleum fraction- mixture of hydrocarbons with a similar chain length and
boiling range
The general formula for is CnH2n+2 (for ring alkanes is CnH2n)
Physical properties
They are almost non-polar because the electronegitivities of carbon(2.5) and
hydrogen(2.1) are so similar.
The only intermolecular forces between their are weak. However the larger the
molecule the stronger the Van der Waals forces ) .
Boiling points
Because of the increasing intermolecular forces the boiling points of alkanes
increase as the length of the carbon chain increases.
Alkanes with branched chains have lower melting points because they cannot pack
together as closely and the van der waals forces are not as effective.
Alkanes are insoluble in water
Water molecules are held together by hydrogen bongs which are much stronger
than the van der waals forces in the the alkane molecule.
However they do mix well with other relatively non-polar liquids.
The are relatively unreactive. Due to strong carbon-carbon and carbon-hydrogen
They do not react with acids, bases, oxidizing and reducing agents.
They do burn and react with halogens under certain conditions.
Fractional distillation of crude oil
This is a Physical Process that includes the splitting of Van der Waals forces
Petroleum is a mixture mainly of alkane hydrocarbons
The process
· The crude oil is pre-heated in a furnace
· the mixture of liquid and vapor passes into a column
· The fractions condense at different heights
· The temperature of the column is cooler at the top than it is at the bottom
· The separation depends on boiling point
· Boiling point depends on size of molecules( length of carbon chain)
· The larger the molecule that larger the Van der Waals forces
· Similar molecules( size, mass and bp) condense together
· Smaller molecules condense at the top(lower temp) and larger molecules
condense at the bottom( higher temp)
· The liquid that condenses at each tray is piped off
Vacuum Distillation Unit
Allows heavier fractions to be further separated without high temperatures which
could break them down
· Heavy Residues from the fractioning column are distilled again under a vacuum
· Lowering the pressure over a liquid will lower its boiling point

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Chemistry unit 1 section 6 Alkanes
Cracking- conversion of large hydrocarbons to smaller molecules by breakage of
C-C bonds
High MR alkanes -> Smaller Mr alkanes + alkenes
Thermal Cracking
High presser- 7000kPA
HIgh temperature- 400- 900
Mainly alkenes( which can be used to make polymers and alcohol)
Hydrogen( used in the Haber process and in margarine production)
· Carbon-Carbon bonds break down so that initially the make two shorter chains
ending with a free radical
· These free radicals are…read more

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Chemistry unit 1 section 6 Alkanes
SO can be removed by flue gas desulpherisation. The gases pas through a scrubber
containing calcium oxide which reacts which the sulphur dioxide.…read more

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Chemistry unit 1 section 6 Alkanes…read more


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