Alkenes

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  • Alkenes
    • They are hydrocarbons with a double bond between two of the carbon atoms
    • The general formula is CnH2n
    • They have the fewer hydrogens than alkanes with the same number of carbon atoms.
      • This makes them unsaturated
    • The first four alkenes are:
      • Ethene
      • Butene
      • Propene
      • Pentene
    • The functional group is C=C
    • Reactions of alkenes
      • Burning
        • They burn the same as other hydrocarbons but with a smoky flame
          • This is because they usually burn with incomplete combustion
      • Addition reactions
        • They usually react by addition reactions as the double bond opens up to leave a single bond and space for a new atom.
      • All alkenes react in similar ways because of the double bond
      • Hydrogenation
        • This is the addition of hydrogen
        • Alkenes are reacted with hydrogen in the reaction of a catalyst
      • Steam reactions
        • When alkenes react with steam water is added across the double bond
          • This creates an alcohol
        • It is done across a catalyst
        • This is one way of making ethanol industrially.
          • After the reaction the mixture is passed into a condenser
            • Ethanol and water have a higher boiling point to ethene so they condense but unreacted ethene gas is recycled back to the machine.
      • Halogen reactions
        • Alkenes also react in addition reactions with halogens like chlorine, bromide and iodine.
    • To test for alkenes add bromine water to the alkene
      • The bromine water is bright orange and when it reacts with an alkene it opens the double bond and makes it colourless.
    • Addition polmerisation
      • Alkenes are used to make polymers such as poly(ethene)
      • To do this monomers join together to make polymers
      • They alkene is put into a bracket and the double bond opened.
        • This is called the repeating unit.
    • Cracking
      • Longer hydrocarbons can be broken down into smaller more useful molecules
        • The products can be very useful as fuels
      • Cracking can be done in a number of ways including  catalytic cracking and steam cracking.
      • The products of cracking include alkanes and alkenes
      • Catalytic cracking involves:
        • Vaporising the long chain carbons
          • Passing the vapour over a hot powered aluminium catalyst.
            • The long chain carbons split apart.
      • Steam cracking involves:
        • Vaporising the long chain carbons
          • Then mix them with steam.
            • Then heat them to a very high temperature.

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