AS 2.3 Chemistry Alkanes

Saturated Hydrocarbon

A compound which contains no C=C bond or C triple bond C

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Physical properties of alkanes

Non-polar, insoluble in water
Low melting and boiling points increasing with increasing relative molecular mass
All less dense than water, density increasing with increasing relative molecular mass
Colourless when pure
Branching reduces the contact theref

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What colour flame do alkanes burn with

Clean blue-yellow flame
As number of carbon atoms in alkane increases the flame becomes orange and burns with more 'smoke'

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When alkane fuels combust what pollutants are formed

Carbon dioxide
Carbon monoxide
Carbon
Nitrogen oxides
Sulfur oxides
Unburned hydrocarbons

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Percentage of CO2 in air and relation to global warming

Increased from 0.03% to 0.04% due to increased combustion of organic compounds and this us believed to have cause global warming

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How do catalytic converters help

Catalytic converts in an exhaust system reduce the environmental impact of these pollutants by converting them to less harmful products

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Chemically what happens inside a catalytic converter

Two steps:
Gases form weak bonds with the metal atoms in the converter - this is called adsorption. This holds the gases in just the right position for them to react
When they react the products then break away from the metal atoms - desorption. This free

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Pollutants Converted

CO = CO2
Oxides of Nitrogen = Nitrogen
Unburnt hydrocarbon = CO2 and H20

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Substitution

A reaction in which an atom or group of atoms in a molecule is replaced by a different atom or group of atoms

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Alkanes with CL2 and Br2

Alkanes react with chlorine and bromine to produce halogenoalkanes in a substitution reaction
UV light is required

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Homolytic Fission

Bond breaking in which one of the two shared electrons goes to each atom (resulting in free radicals)

The radicals have no overall change as the protons equal the e-s

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Heterolytic Fission

This is bond breaking in which both of the shared electrons go to just one of the atoms
(Usually goes to most electronegative one, it then becomes negatively charges as it has one more e-

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When do you find Homolytic Fission and Heterolytic Fission

Heterolytic Fission is more common when a bond is polar
So non-polar alkanes will undergo homolytic fission but polar organic molecules like halogenoalkanes can break heterolytically

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AS 2.3 Chemistry Alkanes

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