Crude Oil - Cracking
Cracking is an example of a thermal decomposition chemical reaction.

Crude Oil - Cracking

You need to understand certain chemical reactions used in a variety of industrial processes for GCSE Chemistry. One of these is thermal cracking which is used on crude oil. The American chemist, William Burton, is usually credited with being the inventor of the thermal cracking process. The truth is that many others of different nationalities have been involved as the process has been developed and improved over time.

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 realised 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 discoloured bromine water was ethene). These are highly reactive chemicals that can be used to manufacture many other useful chemicals like polymers.

Which of the following shows only the products that could be directly obtained by cracking C10H24?
Alkanes up to C20H44, hydrogen and ethene
Alkanes up to C8H18, carbon dioxide and ethene
Propene, poly(ethene) and hydrogen
Alkanes up to C8H18, ethene and propene
Remember the smallest possible alkene is ethene. Therefore the largest alkane must be C8H18. You could dismiss the other three answers because at GCSE, you are expected to know that the products of thermal cracking of an alkane will produce only a shorter chain alkane and an alkene so any answer containing other products, like carbon dioxide and hydrogen, are false
The purpose of cracking is to...
split long chain hydrocarbons into more useful short chain molecules
split long chain hydrocarbons into their component molecules
split hydrocarbons into carbon and hydrogen
separate different sized hydrocarbons
It solves the issue of under production and over production at the same time
In the cracking process...
the hydrocarbon vapour is heated with iodine
the hydrocarbon is mixed with hydrogen and heated
the hydrocarbon vapour is passed over a hot catalyst
the hydrocarbon is liquefied, then passed over a catalyst
The catalyst used industrially is zeolite based
Cracking is an example of what type of chemical reaction?
Thermal decomposition
Thermal = heat; decomposition = break down; so thermal decomposition = break down using heat
The products of cracking are...
alkane + alkene
alkene + alkene
alkane + alcohol
alkane + alkane
Remember, nothing is being added, so the number and type of atoms must remain the same
An alkene molecule with one double bond contains 7 carbon atoms. How many hydrogen atoms does it contain?
This often comes up in GCSE questions related to crude oil
The alkene product of cracking...
is an unsaturated compound
has molecules in which each carbon atom has two bonds
is unreactive
will burn to produce carbon dioxide and hydrogen
Unsaturated means that it contains a double bond. In the laboratory, you test for it using bromine water. A positive result is that the bromine water is discoloured
The catalyst used in the cracking process in the lab is...
powdered ceramic pot
Broken pot can also be used. It needs to be powdered or broken to increase the surface area to maximise the speed of reaction
Select from the list the correct formula for ethene.
In organic chemistry, you need to know that any chemicals beginning with 'eth' contains two carbon atoms and you also need to know the general formulae for the alkanes and alkenes
What is the general formula for alkenes?
Alkenes contain twice as many hydrogen atoms as carbon atoms
You can find more about this topic by visiting BBC Bitesize - Crude oil, hydrocarbons and alkanes

Author:  Kate Gardiner

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