Glossary:
A long chain molecule that is formed by joining lots of smaller molecules (monomers) together.
A small molecule that can be joined together with other small molecules to form a polymer.
A process that can be used to separate the substances in a mixture according to their boiling points.
The process that is used to break long-chain hydrocarbons down into shorter, more useful hydrocarbons.
A non-renewable resource, a fossil fuel.
A saturated hydrocarbon with the general formula CnH2n+2. E.g. methane.
An unsaturated hydrocarbon that contains a carbon-carbon double bond and has the general formula CnH2n. E.g. ethene.
Reactions where the double bond in an alkene opens up to leave a single bond and a new atom is added to each carbon.
Hydrocarbons are compounds that are made up of the elements hydrogen and carbon only.
Crude oil
Crude oil is a finite resource found in rocks. Crude oil is the remains of an ancient biomass consisting mainly of plankton that was buried in mud. Crude oil is a mixture of a very large number of compounds. Most of the compounds in crude oil are hydrocarbons.
Fractional Distillation
The many hydrocarbons in crude oil may be separated into fractions, each of which contains molecules with a similar number of carbon atoms, by fractional distillation.
The fractions can be processed to produce fuels and feedstock for the petrochemical industry.
Many of the fuels on which we depend for our modern lifestyle, such as petrol, diesel oil, kerosene, heavy fuel oil and liquefied petroleum gases, are produced from crude oil.
Many useful materials on which modern life depends are produced by the petrochemical industry, such as solvents, lubricants, polymers, detergents.
C7: Organic Chemistry
Fractional Distillation – How it works?
The crude oil is first heated so that it evaporates (turns into a gas) and is piped in at the bottom of a fractional column. The gas rises up the column and gradually cools. Different compounds in the mixture have different boiling points, so they condense (turn back into liquid) at different temperatures. This means they condense at different levels in the fractionating column.
The group of hydrocarbons that condense together are called fractions.
For example:
Hydrocarbons with lots of carbon atoms have high boiling points, so they condense near the bottom of the column. Hydrocarbons with small number of carbon atoms have low boiling points, so they condense near the top of the column.
Cracking and Alkenes
Hydrocarbons can be broken down (cracked) to produce smaller, more useful molecules. Cracking can be done by various methods including catalytic cracking and steam cracking.
In both methods, first step is to heat the long-chain hydrocarbons to vaporise them (turn them into a gas).
In catalytic cracking, the vapour is then passed over a hot, powdered catalyst. Aluminium oxide is one of the catalysts used. The long-chain molecules split apart or ‘crack’ on the surface pf the specks of catalyst.
In steam cracking, the vapour can be mixed with steam and heated to a very high temperature. This will also lead to thermal decomposition of long-chain hydrocarbon molecules to form smaller ones.
Alkenes
The products of cracking include alkanes and another type of hydrocarbon called alkenes. Alkenes are more reactive than alkanes and react with bromine water, which is used as a test for alkenes.
There is a high demand for fuels with small molecules and so some of the products of cracking are useful as fuels.
Alkenes are used to produce polymers and as starting materials for the production of many other chemicals.
Combustion of Hydrocarbons
The combustion of hydrocarbon fuels releases energy. During combustion, the carbon and hydrogen in the fuels are oxidised. The complete combustion of a hydrocarbon produces carbon dioxide and water.
Incomplete combustion of a hydrocarbon produces water and carbon or carbon monoxide.
C7: Organic Chemistry