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What is the Chemistry of the Cracking of Hydrocarbons?

Cracking of hydrocarbons involves thermal decomposition.
This means that large hydrocarbon molecules break into
smaller molecules when they are heated. The hydrocarbons
are boiled and the hydrocarbon gases are either
mixed with steam and heated to a very high temperature or
passed over a hot powdered aluminium oxide catalyst.
The catalyst works by providing the hydrocarbon gases
with 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 the original alkane does
not have enough hydrogen atoms to produce two more alkanes.

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