Alkenes
- Created by: Abc312
- Created on: 26-04-18 19:57
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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
- They burn the same as other hydrocarbons but with a smoky flame
- 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.
- After the reaction the mixture is passed into a condenser
- When alkenes react with steam water is added across the double bond
- Halogen reactions
- Alkenes also react in addition reactions with halogens like chlorine, bromide and iodine.
- Burning
- 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.
- Passing the vapour over a hot powered aluminium catalyst.
- Vaporising the long chain carbons
- Steam cracking involves:
- Vaporising the long chain carbons
- Then mix them with steam.
- Then heat them to a very high temperature.
- Then mix them with steam.
- Vaporising the long chain carbons
- Longer hydrocarbons can be broken down into smaller more useful molecules
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