Fuels made from oil mixtures containing large hydrocarbon molecules are not efficient as they do not flow easily and are difficult to ignite. Crude oil often contains too many large hydrocarbon molecules and not enough small hydrocarbon molecules to meet demand. This is where cracking comes in.

Cracking allows large hydrocarbon molecules to be broken down into smaller, more useful hydrocarbon molecules. Fractions containing large hydrocarbon molecules are heated to vaporise them. They are then:

  • heated to 600-700°C
  • passed over a catalyst of silica or alumina

These processes break covalent bonds in the molecules, causing thermal decomposition reactions. Cracking produces smaller alkanes and alkenes.

Cracking of decane

There is a greater demand for smaller hydrocarbons than larger ones.

decane → octane + ethene

C10H22 → C8H18 + C2H4

The process is as follows:

Structural formula of the process of using heat to crack decane, resulting in octane and ethene.

Some of the smaller hydrocarbons formed by cracking are used as fuels (eg large chains are often cracked to form octane for petrol, which is in high demand), and the alkenes are used to make polymers in the manufacturing of plastics. Sometimes, hydrogen is also produced during cracking.

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