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Cracking for Alkenes
Polymers and Fuels
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The chemical process of breaking long alkane molecules into smaller alkanes and alkenes.


A hydrocarbon is unsaturated if it contains any carbon-carbon double bonds.


Small molecules that react together in a repeating pattern to make long chain molecules.


A long chain molecule made up of a repeating pattern of small units called monomers.


A reaction where water is chemically added to a chemical.


Some of the heavier hydrocarbons removed during fractional distillation aren't very useful. They are either thick liquids or have very high boiling points (so are solid), worst yet they don't burn very easily. We can 'crack' them into smaller compounds to make something much more useful.

Large hydrocarbons can be cracked by:

  1. passing them over a hot catalyst, or
  2. heating to a high temperature and high pressure


Alkenes are similar to alkanes, but with one big difference. There is at least one double bond between two carbon atoms in an alkene. There can be more, but for when you are asked to draw them you will only need to draw one. Alkenes can be made during the process of cracking.

General Formula of an Alkene
Table showing the first four alkenes and their structures.
Table showing the first five alkanes, and first four alkenes and a tip on how to remember them.

Testing for Unsaturation

Reacting ethene with bromine causes a colour change from orange to colourless.

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Combining many smaller (alkene) molecules together can make a polymer. These chemicals have very useful properties, and you can bet that the clothes you're wearing, and the seat you're sat on have some kind of polymer in them.

An infographic on how to write the polymerisation reaction of an alkene.


Polymers with some cross-linking bonds between polymer chains. These allow for water to be trapped, and can act as wound dressing, contact lenses, and disposable nappy liners.


This is a 'smart polymer' as it changes to the world around it. For example, stiches that change shape as they warm up (i.e. they get tighter around a cut keeping it together with the right amount of force).


This polymer has been used for a while and currently is used for drinks bottles as it's lighter than glass and it is very tough. Recycled PET is being used for clothing.

For interest:

Plastic Waste

Non-biodegradable plastics take hundreds of years to break down, taking up space in landfill sites, and damaging environments. Some plastics even make their way into the ocean killing marine life.

  • Biodegradable plastics

Plastics made from cornstarch can break down much quicker than traditional polymers such as poly(ethene) and poly(propene). However, using a food crop to make plastics causes a similar to the issue of using crops for biofuel.

  • Recycling plastics

Until we can make better biodegradable plastics, we should recycle our current ones. It saves some energy, but most importantly we don't need more crude oil.

Recyclable Plastics


We can make ethanol from ethene. See the previous topic for reasons why we would want ethanol.


We can make ethanol by heating ethene with steam. The addition of water across the carbon-carbon double bond means this is a hydration reaction.

Ethene can be reacted with steam to make ethanol

Do you ever feel like a plastic bag, drifting through the wind - wanting to start again?

Do you ever feel like a plastic bag, drifting through the wind - wanting to start again?

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Cracking for Alkenes
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Page last updated: 16/04/2017