Fractional Distillation separates compounds mixed together in Crude Oil into fractions containing Hydrocarbons of a similar length. It doesn't make any compounds that weren't there already.

Shorter Hydrocarbons evaporate easily (they are volatile) and so burn well without giving off much soot.

So these shorter Hydrocarbons make better fuels (like petrol) and there is a very high demand.

Longer Hydrocarbons contain more energy but are difficult to burn. There is a much lower demand for these.

Fortunately, it is possible to "crack" long Alkanes to produce shorter Alkanes (for fuels) and Alkenes (for making Polymers)

One way to crack long alkanes is to pass them at high temperature (actual temp not needed) over a suitable catalyst (we use unglazed pottery but knowledge of actual catalyst not needed.)

In general

Long Alkane --> Short Alkane + Alkene

Though we're not in control of where the alkane cracks.

So, we can't control which alkanes & alkenes we make

Examiners like to give you part of a cracking equation and ask for the rest.

You don't need to learn these - it's simple Maths.

eg

C₂₀H₄₂ --> C₄H₈ + ___________?

There are 16 missing Carbon atoms and 34 missing H atoms on the righthand side.

And only one product - so, all the missing atoms go into the one product.

Answer: C₂₀H₄₂ --> C₄H₈ + C₁₆H₃₄

eg 2

C₃₀H₆₂ --> C₁₆H₃₄ + 2 ___________?

There are 14 missing Carbon atoms and 28 missing H atoms on the righthand side.

But there are two lots of product - so, divide the missing atoms by two.

Answer: C₃₀H₆₂ --> C₁₆H₃₄ + 2 C₇H₁₄