Fractional Distillation
Petroleum or crude oil what comes straight out of the ground, it is a mixture of hydrocarbons (mainly alkanes) of differing lengths and hence differing chemical properties. To make more use of them we need to seperate out the different lengths.
It works on the principal that the longer chain hydrocarbons have a higher boiling point. The petroleum is heated, and then pumped into the bottom of the distilation column. There is a temperature gradient where it is cooler at the top, this means that the smaller molecules will keep rising as a gas until they condense at a point below their boiling point. This is then collected as a fraction. For a diagram of this see the crude oil page.
Great!! We have seperated the fractions ... but there is a problem. There is alot more of the larger alkanes than there is demand for and not enough of the smaller alkanes. So the larger hydrocarbons need to be broken into smaller ones by the following process:
Cracking
To make up this larger demand for smaller alkenes the larger ones are cracked. It is defined as breaking down long alkanes into shorter alkanes and alkenes.
There are two different ways that cracking can be done, thermal cracking which uses heat and pressure to break up the long alkanes; and catalytic cracking which uses a catalyst to do the work. These two are compared in the below table.
| . | Thermal Cracking | Catalytic Cracking |
|---|---|---|
| Temperature | 400-900oC | 450oC |
| Pressure | 7000 kPa | Slight |
| How? | Increased vibrations in atoms from heat splits the alkane. The higher the temperature, the further towards the end of the chain it splits at. | Zeolite catalyst has pores in its crystal where the rection takes place. branched alkanes cannot fit into this. |
| Product Uses | Smaller alkanes can be sold easily, and alkenes can be used to make plastics in a further process. | This process produced branched alkanes, which burn more easily and therefore make good fuels. |
Combustion
These products make very good fuels as their combustion is very exothermic. There are two way that a combustion might take place depending on how much oxygen there is.
Complete combustion takes place in a plentiful supply of oxygen, its products are carbon dioxide and water; see the example below.
C4H10 + 6½O2 ® 4CO2 + 5H2O
Incomplete combustion takes place in a limited supply of oxygen, its products are water, and carbon or carbon monoxide water, an example of the reaction is.
CH4 + O2 ® C + 2H2O
CH4 + 1½O2 ® CO + 2H2O
However, if you think about this as happening in your car engine, then because of impurities in the fuel, other chemicals are produced. Notably nitrous oxides and sulphur dioxide, which is responsible for acid rain. Fortuneately we have a way of removing the more harmful chemicals, by use of a catalytic converter.
The catalytic converter removes carbon monoxide, nitrogen oxide and hydrocarbons from the exhaust. They contain a thin layer of metals such as platinum and rhodium they act as a catalyst to convert the harmful gases into less harmful ones.
Chlorination
This is the process where chloromethane is produced from chlorine and methane. If you did this experiment outside at night, nothing would happen; but do it in a sun bed and it will react explosively. This is because UV light provides the energy to break the Cl - Cl bonds.
The overall equation of this reaction is as above, however there are three stages to it. And at each one we shall study the mechanism.
The first stage is initiation where the UV light breaks the bond in the Chlorine Molecule to form two chlorine free radicals this merely means it has an unpaired electron and is therefore very reactive.
The second step is propagation where a hydrogen is removed from the methane and bonds with the free radical; but this in turn produces a methyl free radical.
The final step is termination and this is when two free radicals combine. Mostly this means the Cl and CH3 because these collisions are more probable. However, it is also possible for two oCH3 to join and make ethane.