4.17 describe how long-chain alkanes are converted to alkenes and shorter-chain alkanes by catalytic cracking (using silica or alumina as the catalyst and a temperature in the range of 600–700ºC)

- In cracking, a large alkane is cracked to produce smaller, more useful molecules.

• longer chained alkane --> shorter chained alkane + alkene
• OR: longer chained alkane --> alkene + hydrogen
• OR: longer chained alkane --> alkane + alkane + alkene

Why do we crack alkanes?
- shorter chained alkanes are used for fuels
- shorter chained alkenes are used for plastic production

Explanation:
- cracking allows large hydrocarbon molecules to be broken down into smaller, more useful hydrocarbon molecules
- fractions containing large hydrocarbons are heated at 600-700℃ to vaporize them
- vapours will pass over the hot catalyst of silica/alumina
- this process breaks covalent bonds in molecules, causing thermal decomposition reactions
- as a result, cracking produces smaller alkanes and alkenes.
e.g. decane --> octane + ethene

Conditions of Cracking;
2 ways of cracking:
- catalytic cracking: using heat and a catalyst. The catalyst speeds up the reaction
- steam cracking: using heat and steam.