C1bL2 Alkenes and Making Plastics

Key Words

 

Alkane - Compound that consists of hydrogen and carbon in which the carbons are linked by single bonds.

Alkene - Compound that consists of hydrogen and carbon in which some of the carbons are linked by double bonds.

Carbon double bond - two bonds between neighbouring carbon atoms instead of one.

Chemical formula - a way of writing down the numbers of each kind of atom in a substance.

Cracking - breaking up long chain alkanes into smaller molecules, usually with a catalyst

Ethene - an alkene with two carbons.

General Formula - a formula that links the number of carbons to the number of hydrogen atoms.

Hydrocarbons - compounds consisting of hydrogen and carbon.

Monomers - unit molecule from which polymers are made.

Plastics - materials made from polymers that can be easily moulded into shape.

Polymer - very long chain molecule made up of many repeating sub-units.

Polymerisation - making polymers.

Polythene - a polymer made up from ethene.

Reactive - takes part readily in chemical reactions.

Recycle - to use materials again when they have been disposed of.

Smart materials - materials that change shape or colour when conditions are changed

Thermal decomposition - compound breaking down when heated.

Unsaturated - hydrocarbons with double bonds.

Test Yourself

Homework
Chemistry GCSE
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Grade E

Alkenes are hydrocarbons with double bonds.  This molecule is ethene (C2H4):

 Remember that the black circle represents carbon, and the white circles are hydrogen.  In each of these molecules there is a double bond.

This molecule is propene:

The chemical formula for propene is C3H6.  Because there are fewer hydrogen atoms in alkenes than there are in alkanes, alkenes are described as unsaturated.  In other words, an unsaturated hydrocarbon has a carbon double bond in it.

Double bonds are more reactive.

Ethene is used to make polythene.  It comes from cracking longer chain alkanes.  Polythene is a polymer made up of ethene sub-units or monomer.  Ethene is compressed and heated in the presence of a catalyst.  The double bonds are "sticky", and one bond joins to the next molecule.

Here are 3 ethene molecules:

And here they are having linked together in a chain:

Note that there are no longer any double bonds.

 

Grade C

Alkanes and alkenes have similar sounding names, like ethane and ethene.  However their chemistry is quite different.  Propene is much more reactive than propane, because of the double bond.

 

Propane                                                            Propene

The general formula for alkenes is CnH2n. 

The alkene series is:

Alkene Molecular Formula
Ethene C2H4
Propene C3H6
Butene C4H8
Pentene C5H10
Hexene C6H12

Note that there is no such thing as methene, as the single carbon cannot make a double bond onto itself.

Like polythene, polypropene (polypropylene) is a polymer made up of monomers, this time propene.

Propene loses its double bond to form a polymer like this:

It's quite similar to polythene, except that there is a CH3 (methyl) group.

Polymers are very strong along the chain, as it's hard to pull the C-C bonds apart.  They are also quite flexible.

 

Grade A

Ethene is a very simple plant hormone involved in the ripening of fruit.  If you want to ripen tomatoes, you can put a banana skin near to the plants.  The ethene gas is absorbed by the tomato plants and starts off the ripening process.

Polymers have some useful features:

  • Low density;
  • Resistant to corrosion;
  • Flexible;
  • Soften when heated, so can be recycled and remoulded.

Some polymers can conduct electricity, or change shape when placed near a magnetic field.  Some change colour when heated or when electricity is passed through them.  They are called smart polymer materials.

The TFT computer screen is actually made of a polymer film.