Long chain hydrocarbons are found at the bottom of the fractionating column. These hydrocarbons have higher boiling points, a lower flammability and are more viscous. They are not as useful as the short chain hydrocarbons, but cracking solves this.
How is cracking done?
- Liquid long chain hydrocarbon put in test tube with a catalyst (usually pieces of ceramic)
- The ceramic is heated strongly, and a little heat is added to the hydrocarbon.
- The hydrocarbon evaporates and when it meets the catalyst, it thermally decomposes.
Reaction: Alkane >> (thermal decomposition) >> Alkane ( but shorter hydrocarbon chain) + Alkene
Formula for Alkene = CnH2n.
Alkenes contain at least one double bond and are unsaturated. The rest for unsaturated hydrocarbons is using bromine water and adding the hydrocarbon. If the hydrocarbon is unsaturated, the bromine water will turn colourness. This shows if the chemical had double bonds.