Oil is thought to have formed over millions of years from the break down of tiny dead creatures. Natural gas is formed alongside oil.
The dead organisms sank to the bottom of lakes or seas and became trapped in muddy sediments. As the sediments built up, the lower layers were under pressure. They eventually turned to rock. If there was no oxygen in the sediments, heat and pressure turned the remains of the organisms into oil and natural gas.
Some rocks are porous - they have a network of tiny holes in them.Sandstone and limestone are examples. Oil is a liquid so it seeps into porous rocks. Gas also diffuses into these rocks.
Porous rocks may also contain water. Gas and oil do not mix with water. They are less dense than water. This means they form layers above the water.
Sometimes the rock layers form so that the oil and gas are trapped under the rock such as shale that is not porous. Large amounts of oil and gas may collect in a porous rock. The pressure on the oil may build up so much that when a hole is drilled through the rock cap, oil gushes out.
Crude oil is a mixture of many thousands of different compounds with different properties. They are called hydrocarbons because they only contain the elements hydrogen and carbon.
To make crude oil useful, batches of similar compounds with similar properties need to be sorted. These batches are called fractions and they are separated by fractional distillation.
The theory behind this technique is that some of the compounds in crude oil are easily vaporised, for example, they are volatile due to their low boiling points. Others are less volatile and have higher boiling points.
In fractional distillation, the crude oil is heated to make it vaporise. The vapour is then cooled. Different fractions of the oil are collected at different temperatures.
| Fraction: | No. of carbon atoms: | Colour: | Boiling point range oC: | Uses: |
| Refinery gas | 1 - 4 | Colourless | Below room temp. | Gaseous fuel, making chemicals. |
| Gasoline (petrol) | 4 - 12 | Colourless to pale yellow | 32-160oC | Motor car fuel, making chemicals. |
| Kerosine (paraffin) | 11 - 15 | Colourless to yellow | 160-250oC | Heating fuel, jet fuel. |
| Diesel oil | 15 - 19 | Brown | 220-350oC | Diesel fuel for lorries, trains, etc. and heating fuel. |
Residue
| C | Dark brown | Above 350oC | Fuels for power stations, ships etc. Some is distilled further to give lubricating oils, waxes, etc. |
| 20 - 30 | ||||
| 30 - 40 | ||||
| 50 and above |
As the hydrocarbon molecule chain increases its boiling point increases, it becomes more viscous, becomes more difficult to light, the flame becomes sootier and it develops a stronger smell.
2.Products from crude oil
Physical properties:
The chemistry of carbon compounds is called organic chemistry. There are millions of organic chemicals, but they can be divided into groups called homologous series. All members of a particular series will have similar chemical properties and can be represented by a general formula.
The alkane series is the simplest homologous series. The main source of alkanes is from crude oil.
Alkanes are covalent compounds. They are hydrocarbons, which means they contain hydrogen and carbon. The general formula for an alkane is CnH2n+2.
Properties and uses of alkanes:
| Name of alkane: | Melting point oC: | Boiling point oC: | Density g/cm3: | State at room temperature: |
| Methane CH4 | -182 | -162 | 0.42 | Gas |
| Ethane C2H6 | -183 | -88 | 0.55 | Gas |
| Propane C3H8 | -188 | -42 | 0.58 | Gas |
| Octane C8H18 | -57 | 126 | 0.72 | Liquid |
The first four alkanes are gases at room temperature.
Alkanes with 5-17 carbon atoms are liquids.
Alkanes with 18 or more carbon atoms are solids.
As the number of carbon atoms increases, the melting points, boiling points and densities increases.
They are insoluble in water but dissolve in organic solvents such as benzene.
Their chemical reactivity is poor. The C-C bond and C-H bond are very strong so alkanes are not very reactive.
They will carry out combustion. Burning alkanes in air (oxygen) produces water and carbon dioxide. The reactions are very exothermic (give out heat energy), so alkanes in crude oil and natural gas are widely used as heating fuels.
For example:
If alkanes combust in too little air, carbon monoxide may form. This is dangerous and can cause death.
The lighter fractions (for example, petrol) are in large demand. The heavier fractions are not so useful but unfortunately chemists have to be able to convert these heavier fractions into petrol and other useful products, due to supply and demand, by a method known as cracking.
Cracking breaks down molecules into smaller ones. Catalysts or heat may be used to crack the alkane chain into smaller ones.
Note, that one of the products that is formed when we crack naphtha contains a double bond between two carbon atoms. A hydrocarbon that possesses one double bond belongs to the next homologous series called alkenes.
Another reaction that often occurs after fractional distillation is reforming. Hydrocarbons of the same formula have different boiling points. Straight-chained alkanes have greater boiling points than the branched version. This means they catch light more easily - but this can be too much for the hot cylinder of the car engine. Reforming converts straight-chained alkanes to branched.
The members of this series contain a double bond. They are hydrocarbons.
The general formula of the alkenes is CnH2n Most alkenes are formed when fractions from the fractional distillation of crude oil are cracked.
Properties of alkenes:
Like alkanes, the boiling point, melting point and densities increase with larger size molecules.
They are insoluble in water.
They combust like alkanes to produce carbon dioxide and water. However, they burn with sootier flames due to their higher percentage of carbon content to hydrogen.
Chemically, alkenes are more reactive than alkanes. This is because they possess a double bond that can be broken open and added to in a reaction.
For example:
These reactions are called addition reactions.
Saturated and unsaturated:
Organic compounds, like alkanes, which have four single covalent bonds to all their carbon atoms are described as saturated.
Alkenes are hydrocarbons with a double bond between two carbon atoms and are described as unsaturated. This is because they do not have the maximum number of atoms attached to their four bonds, as one is double!
Polyunsaturated margarines and vegetable oils contain many C=C bonds.
3. Polymerisation
Facts about plastics:
Polythene (polyethene) is made by forming a long chain of ethene molecules. Many other compounds are made in a similar way. A compound made like this is called a polymer.
Polymers are long chains of monomers. A monomer is the building block or in other words the repeating unit that is used to make the polymer. In the above example, ethene is the monomer and polythene the polymer.
Polystyrene (many styrene molecules) is another well-known polymer.
Many polymers can be easily moulded into many shapes - these are called plastics.
Polymerisation is the name given to the reaction that produces polymers.
Remember: alkenes can become polymers but alkanes cannot. This is because alkanes are saturated whereas alkenes are unsaturated which means that they can carry out addition reactions, required for polymerisation.
This type of polymerisation is called addition polymerisation.
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