4.1 cracking hydrocarbons
Fractions from crude oil can be broken down by thermal decomposition in a catalytic cracker. The fraction is vaporised and passed over a hot catalyst, which causes the molecules to split apart and from smaller molecules.
Some of the smaller molecules are alkanes, but some are alkenes that contain carbon-carbon double bonds.
Alkenes are unsaturated because they contain fewer hydrogen atoms than alkanes with the same number of carbon atoms
Their general formula is CnH2n
They are hydrocarbons so like alkanes burn in air
However they are more reactive than alkanes. So alkenes will react with bromine water turning the yellow-orange solution colourless (not clear!).
4.2 making polymers form alkenes
Polymers are very large molecules made form many small molecules that have joined together. The small molecules used to make polymers are called monomers.
Lots of ethane molecules can join together in long chains to form poly(ethene), commonly called polythene. This reaction is called addition polymerisation because the molecules simply add together and only the polymer is produced.
We can react other alkenes together in a similar way to form polymers such as poly(propene. Many of the plastics we use in bags, bottles, containers and toys are make from alkenes.
Polymers have long molecules that form a tangled mass, rather like spaghetti, this gives them flexibility and strength.
4.3 properties of plastics
Using different monomers produces polymers with different properties.Thermosoftening polymers have weak intermolecule forces between their molecules, so when they are heated they become soft. Then when they cool down they become hard again this means they can be heated into shape and then reheated into another shape.
When thermosetting polymers are heated for the first time chemical bonds form between the polymer molecules, they link together in a giant network. These strong bonds make the plastic set hard and it can’t be softened again on heating.We use thermosetting plastics where flexibility is important and where they are not exposed to very high temperatures. Thermosetting plastics are more rigid and can withstand higher temperatures.Chemical bonds- are strong and join atoms together to make moleculesIntermolecular forces- act between molecules and are weaker than chemical bonds.
4.4 new and useful polymers
We can design the properties of polymers by choosing different monomers and by changing the conditions used to make them.
Polymers are widely used for food packaging to keep food in good condition. Some of these polymers are not biodegradable and cause problems with waste disposal.
The polymers used for drinks bottles are strong, flexible, lightweight, clear and non-porous.
Polymers have been developed to coat fabrics that make then waterproof but able to let gases through (breathable) new polymers have been developed for medical use, including hydrogels which are also used in agriculture and food.
Smart polymers can be used to control the release of drugs and shape memory polymers are used for stitching wounds.