Cracking
Fractions that are produced by the distillation of crude oil can go through a process
called cracking, a chemical reaction which produces smaller hydrocarbons, including
alkanes and alkenes. Ethene and other alkenes are unsaturated hydrocarbons and can
be used to make polymers.
Fuels made from oil mixtures containing large hydrocarbon molecules are not efficient: they do not
flow easily and are difficult to ignite. Crude oil often contains too many large hydrocarbon molecules
and not enough small hydrocarbon molecules to meet demand. This is where cracking comes in.
Cracking allows large hydrocarbon molecules to be broken down into smaller, more useful
hydrocarbon molecules. Fractions containing large hydrocarbon molecules are heated to vaporise
them. They are then either:
passed over a hot catalyst, or
mixed with steam and heated to a very high temperature.
These processes break chemical bonds in the molecules, causing thermal decomposition reactions.
Cracking produces smaller alkanes and alkenes (another type of hydrocarbon).
Some of the smaller hydrocarbons formed by cracking are used as fuels, and the alkenes are used to
make polymers in plastics manufacture.
The products of cracking include alkenes (for example ethene and propene). The alkenes are a family
of hydrocarbons that share the same general formula:
CnH2n
The general formula means that the number of hydrogen atoms in an alkene is double the
number of carbon atoms. For example, ethene is C2H4 and propene is C3H6.
Alkene molecules can be represented by displayed formulas in which each atom is shown as its symbol
(C or H) and the chemical bonds between them by a straight line.
Structure of alkenes
Alkenes are unsaturated hydrocarbons. They contain a double covalent bond, which is shown as two
lines between two of the carbon atoms. The presence of this double bond allows alkenes to react in
ways that alkanes cannot. They can react with oxygen in the air, so they could be used as fuels. But they
are more useful than that: they can be used to make ethanol and polymers (plastics) - two crucial
products in today's world.
