Specification Point 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 600-700°c)
Long-chain hydrocarbons: large number of hydrocarbon molecules. More viscous and less flammable so less useful.
Short-chain hydrocarbons: small numbers of hydrocarbon molecules.
Cracking is simply splitting of larger molecules to simpler ones.
Industrial catalytic cracking site
- Cracking allows large hydrocarbon molecules to be broken down into smaller, more useful hydrocarbon molecules.
- Fractions containing large hydrocarbon molecules are heated at 600 – 700°c to vaporise them.
- Vapours will then pass over a hot catalyst of silica or alumina
- This process breaks covalent bonds in the molecules, causing thermal decomposition reactions.
- As a result, cracking produces smaller alkanes and alkenes. The molecules are broken up in a random way which produce a mixture of alkanes and alkenes.
Specification Point 4.18:
Explain why cracking is necessary, in terms of the balance between supply and demand for different fractions
Why Cracking is Necessary
Although there is use for each fraction after the fractional distillation of crude oil, the amount of longer chain hydrocarbons produced is far greater than needed.
However, the amount of shorter chain hydrocarbons produced is far less than needed (e.g. gasoline fraction) and there is a higher demand for shorter chain hydrocarbons. This is why cracking is necessary to increase the supply of shorter chain hydrocarbons.
- Short chain hydrocarbons burn well and flow well.
- Cracking is used to produce petrol for cars.
- Cracking also produces alkenes which are raw material in the plastic industry.
- Cracking is used to produce hydrogen gas which is a raw material in manufacture of ammonia in Haber process.
Example: The Cracking of hexane (C6H14) to produce butane (C4H10) and ethene
C6H14 (g) → C4H10 (g) + C2H4 (g)
Ethene: The ethene can be used to make poly(ethene), a type of plastic.
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