Cracking breaks large alkanes into smaller molecules

1. A large, saturated hydrocarbon molecule is broken into smaller ones.
2. The large molecule comes from one of the heavier fractions of crude oil.
3. Breaking it produces a smaller alkane and at least one alkene.
4. Decane can crack to give octane and ethene: $$C_{10}{H}_{22}\to C_8{H}_{18}+C_2{H}_4$$
5. The atoms are only rearranged, so the equation balances on both sides.

Some of the products are unsaturated

1. There are not enough hydrogen atoms to make every product a saturated molecule.
2. One product therefore forms a carbon to carbon double bond.
3. A molecule with such a double bond is unsaturated, and is an alkene.
4. Ethene is the alkene produced most often, and it is the most useful.
5. More than one alkene can form, and hydrogen is sometimes a product as well.

Why cracking is necessary

1. Fractional distillation gives a fixed proportion of each fraction from the oil.
2. Demand for petrol and other short-chain fuels is far higher than that proportion.
3. The heavier fractions are produced in greater amounts than anyone needs.
4. Cracking converts that surplus into the shorter molecules that are in demand.
5. It also produces the alkenes used to make polymers, which distillation cannot supply.

The conditions used

1. The heavy fraction is first heated until it vaporises.
2. The vapour is passed over a hot catalyst, which breaks the molecules up.
3. A high temperature alone will also crack the molecules without a catalyst.
4. Cracking is a thermal decomposition, since heat breaks one substance into others.
5. The products are separated afterwards, since a mixture is always formed.