Can someone please explain to me the process of cracking oil to produce ethene or propene.

high temperature and catalyst. thats pretty much it. and the catalyst is optional if you can get really high temperatures.

 

basically you heat the molecules up enough so they shake themselves appart.

Don't you also throw some hydrogen at it?

When you crack a long chain alkane into two shorter chains, yo get an alkane and an alkene as there aren't enough hydrogen atoms in the starting chain to make two saturated molecules. So yes, normally hydrogen is added as well to reduce the alkene product down to the alkane.

Catalytic cracking is different from hydro cracking. And thermal cracking is again different.

 

Catalytic cracking is the process of heating up the oil, provide a (heterogeneous = solid) catalyst and breaking up the C-C bonds, and reshuffling the hydrogen atoms.

 

Since a typical alkane is basically:

 

[ce]H3C-CH2-CH2-CH2 - (etc) - CH3[/ce]

 

When you break up a C-C bond you cannot form another alkane - there is simply not enough hydrogen. Somewhere a double bond is needed to make a stable molecule, with less hydrogens than an alkane. So, breaking up a C-C bond, and some reshuffling of hydrogen atoms will give:

 

[ce]H3C-CH=CH2 [/ce] and [ce]CH3 - (etc) - CH3[/ce]

 

In hydro cracking, there is enough hydrogen, so more alkanes are produced.

 

Thermal cracking has about the same products as catalytic cracking, but it needs to be heated to a higher temperature to achieve this.