Cracking hydrocarbons

Cracking breaks large hydrocarbon molecules into smaller ones. It is useful because long-chain hydrocarbons obtained from crude oil are often less useful or less in demand than smaller molecules.

The hydrocarbon is vaporised and heated so that some of its carbon–carbon bonds break. The products are a mixture of smaller hydrocarbons, including alkanes, which have only carbon–carbon single bonds, and alkenes, which contain a carbon–carbon double bond.

Methods of cracking

In catalytic cracking, hydrocarbon vapour passes over a hot catalyst. The reaction takes place at a high temperature with the help of the catalyst.

In steam cracking, hydrocarbon vapour is mixed with steam and heated to a very high temperature.

Equations for cracking

A cracking equation shows one possible set of products. For example, a molecule of dodecane can crack to form a molecule of octane and a molecule of butene:

The equation is balanced: each side has 12 carbon atoms and 26 hydrogen atoms. Atoms are rearranged during cracking, so the total number of each type of atom must be the same on both sides of the equation.

Uses of cracking products

Cracking breaks large hydrocarbon molecules into smaller molecules. It is useful because some large hydrocarbons are less useful, while many of the smaller products are in greater demand.

Cracking products as fuels

Small hydrocarbon molecules can be used as fuels. Burning them releases energy for transport, heating and other uses. Cracking therefore helps increase the supply of fuels that people need.

Alkenes as manufacturing materials

Cracking also produces alkenes. These are valuable raw materials because they can be used to make polymers, including plastics. Alkenes are also starting materials for making many other chemicals used in everyday products.

Hydrocarbons therefore supply more than energy: through products such as alkenes, they also provide raw materials for manufacturing.

Alkenes and the bromine water test

Alkenes are hydrocarbons: compounds containing only carbon and hydrogen. They contain a carbon–carbon double bond, whereas alkanes contain only carbon–carbon single bonds.

Alkenes are more reactive than alkanes because their double bond can react more readily than the single bonds in alkanes. Alkenes can be produced by cracking, in which larger hydrocarbon molecules are broken down into smaller molecules, including alkenes.

Testing for an alkene with bromine water

Bromine water is orange. When it is added to an alkene, bromine reacts at the carbon–carbon double bond. The bromine is used up, so the orange colour disappears and the mixture becomes colourless. This is called decolourisation.

Under normal test conditions, an alkane does not react with bromine water, so the orange colour remains. The colour change therefore distinguishes an alkene from an alkane.