Cracking Hydrocarbons

Longer chain Hydrocarbons,can be broken down into shorter more useful hydrocarbons. This process is called Cracking.

Long chain hydrocarbons ----------------------------> Short chain hydrocarbons.

                                            (heat + catalyst)

Cracking involves...

• heating the hydrocarbons until they vaporise
• passing the vapour over a hot catalyst.(or mixed with steam)

A thermal decomposition reaction then takes place

The products of cracking include alkanes and alkenes. Some of the products are useful as fuels. Alkenes react with bromine water turning it from orange to colorless.

As well as forming single bonds with other atoms, carbon atoms can form double bonds. This means that not all the carbon atoms are linked to four other atoms; a double carbon-carbon bond is present instead.

Some of the products of cracking are hydrocarbon molecules with at least one double bond (alkenes)

• The General formula for alkenes is CnH2n
• The simplest alkene is ethene C2H4

Ethene is made up of four hydrogen atoms and two carbon atoms, and contains on double carbon-carbon bond.

The 'spine' of a hydrocarbon is made up of a chain of carbon atoms. When the chains are joined togther by single carbon-carbon bonds the hydrocarbon is saturated and is known as an alkane.

• Hydrogen atoms can make one bond each.
• Carbon atoms can make four bonds each.
• The simplest alkane, methane, is make up of four hydrogen atoms  and one carbon atom

The General formula for alkanes is C2H2n+2

The carbon atoms in alkene are linked to four other atoms by single bonds. This means that the alkane is saturated. This explains why alkanes are fairly unreactive, but they do burn well.