Crude Oil - Cracking

But what exactly is this chemical reaction? Thermal cracking is a type of chemical reaction that uses heat to break down long chain molecules into smaller, more reactive, and therefore potentially more useful, molecules. In the school laboratory, you may have carried out cracking for yourself using liquid paraffin and broken pot. The broken pot acts as a catalyst, making this process happen in seconds rather than hours or days, so technically, we should call the process thermal catalytic cracking.

Crude oil is a mixture of hydrocarbons, mainly from the alkane family. The chemicals are separated by fractional distillation and each fraction has its uses. Unfortunately, some fractions are more useful than others but are not present in crude oil in sufficient quantities to meet demand - we call this under production. Other fractions, like the residue which contains the longest chain hydrocarbons, are overproduced so the problem then arises of what to do with these. The solution is to crack these large alkane molecules to produce the smaller molecules of the fractions that are under produced.

When the method of effective thermal cracking was developed, the scientists involved realized that it produced not only the smaller alkanes that they wanted, but also an alkene (in your cracking paraffin experiment the sweet smelling gas that discolored bromine water was ethene). These are highly reactive chemicals that can be used to manufacture many other useful chemicals like polymers.