Useful Products from Oil

Crude Oil

Crude oil is found in the Earth's crust.

It is a very important raw material from which useful products can be separated or made.

Crude oil is a mixture of many compounds.

A mixture is made up of two or more elements or compounds which are not chemically bonded together. It is possible to separate the substances in a mixture by physical methods, for example, distillation.

Most of the molecules in crude oil are hydrocarbons.

A hydrocarbon contains hydrogen and carbon atoms only.

One family of hydrocarbons is called the alkanes. All the carbon atoms forming the spine of the molecule are joined together by single covalent bonds. Compounds containing only single bonds are said to be saturated.

Other hydrocarbons have double covalent bonds in their structure. They are said to be unsaturated. This family is called the alkenes.

The alkenes are very reactive due to the double bond in their structure. They are used to make other substances.

Test for unsaturation

Bromine water is added to the hydrocarbon. If it is saturated, the bromine water stays yellow-brown. If it is unsaturated, it becomes colourless because the double bond opens and the bromine atoms attach.

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Fractional Distillation

The hydrocarbon molecules in crude oil vary in size. They can be separated into groups of similar sized molecules by fractional distillation.

The groups produced are called fractions. Many of the fractions are used as fuels.

When any hydrocarbon is burnt, carbon dioxide and water vapour are released into the atmosphere.

Many fuels also contain sulphur. When this is burnt, it forms sulphur dioxide.

Carbon dioxide increases the greenhouse effect and can lead to global warming.

Sulphur dioxide can cause acid rain.

Fractional distillation involves heating the oil in a tall tower until it evaporates.

Compounds with short carbon chains are more volatile and boil off first, because they have lower boiling points. They rise to the top of the tower and are collected. These ignite easily and are most flammable.

Compounds with longer carbon chains have a higher boiling point and are collected lower down the tower. They are more viscous (don't flow easily) and are less flammable.

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Cracking

The longer chain hydrocarbons are not as useful as fuels because they don't ignite easily. However, they can be broken down into shorter chain hydrocarbons, which are more useful. This process is called cracking.

Cracking involves heating the hydrocarbons and passing the vapours produced over a hot catalyst.

Some of the products of cracking are used as fuels, but others are used to make plastics.

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Plastics

Plastics are very long molecules called polymers, which are made of many small molecules called monomers that are joined together in a process called polymerisation.

For example, when many ethene molecules are joined together, polyethene, or polythene, is formed.

Polyethene is used for making plastic bags and bottles.

When many propene molecules are joined together, the polymer polypropene is formed.

Polypropene is used for making crates and ropes.

When polymers are made by joining monomers together, with no other substance being produced, the process is called addition polymerisation. The equation represents the formation of an addition polymer.

The double bonds in unsaturated monomers are broken open so they can join together, forming a saturated polymer. Notice that there are free bonds at the ends of the polymer.

Plastics are very useful because they can be moulded, they are lightweight and they do not corrode.

However, they are difficult to dispose of because they are unreactive and do not rot away. They are not biodegradable. If they are burnt, they release toxic gases into the atmosphere.

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