Products from Oil

What is the Chemistry of the Cracking of Hydrocarbons?

Cracking of hydrocarbons involves thermal decomposition.
This means that the large hydrocarbon molecules
 break into smaller molecules when they are heated.
The hydrocarbons are boiled and the hydrocarbon gases
 are passed over a hot powdered aluminium oxide (Al2O3)
catalyst. The catalyst works by providing
a convenient surface for the cracking to take place.

For example,
decane (an alkane with 10 carbons)
can be cracked to produce octane and ethene.

decane       arrow      octane    +   ethene.
 C10H22(g)    arrow      C8H18(g)  +   C2H4(g)

Octane is used as petrol.
Ethene is used in the manufacture of polymers.

Cracking an alkane produces a smaller alkane plus an alkene.
If you add up the number of hydrogen atoms in the above
reaction, you will see that there are 22 on each side.
An alkene is produced because there are not enough
hydrogen atoms to produce two alkanes, after the original
hydrocarbon is cracked. Cracking takes place at about 600 °C.

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