For example, hexane can be cracked to form butane and ethene:
hexane → butane + ethene
C6H14 → C4H10 + C2H4
The starting compound will always fit the rule for an alkane, CnH2n+2. The first product will also follow this rule. The second product will contain all the other C and H atoms. The second product is an alkene, so it will follow the rule CnH2n.
C16H34 is an alkane which can be used as the starting chemical in cracking. One of the products of cracking this compound is an alkane which has 10 carbon atoms in it. Write a balanced symbol equation for this cracking reaction.
C16H34 → C10H22 + C6H12
Various methods can be used for cracking, eg catalytic cracking and steam cracking:
The slideshow describes the process of catalytic cracking.
Cracking is important for two main reasons:
The supply is how much of a fraction an oil refinery produces. The demand is how much of a fraction customers want to buy. Very often, fractional distillation of crude oil produces more of the larger hydrocarbons than can be sold, and less of the smaller hydrocarbons than customers want.
Smaller hydrocarbons are more useful as fuels than larger hydrocarbons. Since cracking converts larger hydrocarbons into smaller hydrocarbons, the supply of fuels is improved. This helps to match supply with demand.
Alkanes and alkenes both form homologous series of hydrocarbons, but:
Alkenes will react with bromine water and turn it from orange/brown to colourless. This is the way to test for a double C=C bond in a molecule.