Although fractional distillation of crude oil is an important part of the oil industry, it is dealt with in Part 1A of the specification and so is not relevant here (unless you are doing the whole of Module 1 together).
Hydrocarbons (carbon and hydrogen only) can be broken down into smaller units by cracking. This is particularly useful to convert the less-useful fractions of crude oil into more useful substances. It is done by heating the alkanes and passing them over a hot catalyst. This is an example of a thermal decomposition reaction.
When cracking takes place, a saturated molecule (alkane) becomes a smaller alkane and an unsaturated alkene. It is impossible to predict exactly which combination of alkane and alkene will be produced in each case but, just like in any other chemical reaction, the numbers of each kind of atom is maintained.
As an example, C10H22 could break down into C5H10 and C5H12
but just as easily, it could form C4H8 and C6H14 or C6H12 and C4H10.
Notice how in each case, the total number of carbon atoms is 10 and the total number of hydrogen atoms is 22. The products of cracking can be separated by fractional distillation.
The saturated products (alkanes) are usually used as fuels while the unsaturated products (alkenes) can be used to make polymers and other chemicals. One non-polymerisation use is to convert alkenes into alcohols by heating with steam in the presence of a catalyst.
Polymerisation looks to be quite complicated but try to remember it in simple terms first. Many small, alkene-like molecules (monomers) join together to become one large molecule (polymer). Think of two people facing each other, holding both holding the other’s right hand in their own left hand. Now have the same picture with lots of couples close together but still in their own pair. Now have each pair release one hand but keep the other together with their partner. The free hands can be used to join onto another pair. Eventually, one huge long chain is formed…..a polymer.
Ethene itself makes poly(ethene), propene makes poly(propene). Chloroethene makes poly(chloroethene) and styrene makes poly(styrene), even though nowadays it ought to be phenylethene making poly(phenylethene). Notice that although the polymers are saturated, they still have “-ene” in their name. This is because the name, “poly(ethene)” should be taken as meaning, “the polymer which is made from ethene”.
Polymers have many useful applications:
Low-density poly(ethene) is used to make plastic bags.
Rain gutters can be made from poly(chloroethene).
Dental polymers are used in tooth fillings.
New polymers are being developed.
Some recent innovations are “memory foam” which is used for mattresses. It moulds to the shape of your body as it gets warm and allows you to sleep more comfortably with proper support. Hydrogel wound dressings allow a wound to heal in controlled moist conditions.