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Cracking of Hydrocarbons - Chemistry.

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 good surface
for the cracking to take place - see catalysts.

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 broken.
Cracking takes place at about 600 °C.

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