cracking hydrocarbons

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Long-chain hydrocarbons form thick liquids like trashing aren't useful.

Cracking turns them into shorter molecules, more useful.

Thermal decomposition, breaking them down into simpler molecules by heating them.

 A lot of the long molecules produced from fractional distillation, cracked into smaller ones because there is more demand for products like petrol than for diesel. 

Cracking produces alkenes, which then make plastics

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In industry, vaporized hydrocarbons are passed over a powdered catalyst at about 400°C. Silicon dioxide and aluminium oxide are used. 

In the lab, you can used porcelain as a catalyst.

  • Start by heating the paraffin, in mineral wool, after a few seconds to heat the porcelain chips until the paraffin vaporized and the porcelain glows red. 
  • The heated paraffin vapour cracks as it passes over the porcelain. 
  • The smaller alkanes and alkenes travel down the delivery tube.
  • The smallest ones are gases that collect in the gas jar.
  • You can show the alkenes because bromine water decolourises.
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Alkenes can be used to make polymers.

This means joining together lots of monomers to form very large molecules, long chain molecules are called polymers.

Ethene molecules can be joined up to produce polyethene. They open up their double bonds.

Polyethene is stretchy so it can make plastic bags to hose pipes.

If you join lots of propane together, you have polypropene that is tough and can be used as carpets to plastic containers.

Chloroethene molecules together, pvc, flexible and resistant to wear so it can do clothing, pipes.

Tetrafluoroethene becomes polytetrafluoroethene, ptfe, interactive, flame resistant so it's used as non - stick coating for pans.

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Don't rot

Most plastics are non-biodegradable, they're not broken down by microorganisms.

When you burn some plastics, they give off toxic gases.

It's best to recycle them as this helps to conserve resources. There are different types of plastics so they have to be separated. This makes it expensive and difficult.

Developing biodegradable polymers will help with the problems. 

Some polythene bags are now made with starch granules in them. If the pIastic is buried the starch is broken down by microorganisms in the soil.

You can also get plastics that break down in sunlight, used in agriculture.

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