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Alkanes and Alkenes

- Hydrocarbons that contain single bonds only are called alkanes.

- Hydrocarbons that have at least two carbon atoms joined together by a double bond are called alkenes.

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Displayed formula of alkanes

H - C - H, with an H above and below the C. (http://www.bbc.co.uk/schools/gcsebitesize/science/images/methane_chem_struc_2.gif)

four C's and ten H's atoms (http://www.bbc.co.uk/schools/gcsebitesize/science/images/butane_chem_struc.gif)

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Bromine water

Bromine water is an orange solution of bromine. It becomes colourless when shaken with an alkene. Alkenes can decolourise bromine water, while alkanes cannot.

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Polymer displayed formula

Polymer molecules are very large compared with most other molecules, so the idea of a repeating unit is used when drawing a displayed formula. When drawing one, starting with the monomer:

  • change the double bond in the monomer to a single bond in the repeating unit
  • add a bond to each end of the repeating unit.
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What does polymerisation need?

- High pressure

- Catalyst

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Saturated and unsaturated

Alkanes are saturated - they have only single bonds.

Alkenes have a double bond - they are unsaturated. 

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Displayed formula of saturated hydrocarbons

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Addition polymerisation

Lots of unsaturated monomer molecules (alkenes) can open up their double bonds and join together to form polymer chains

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