Chemguide: Support for CIE A level Chemistry

Learning outcome 14: Hydrocarbons

14.1: Alkanes

Statement 14.1.4

This statement is about the use of cracking to make smaller and more useful hydrocarbons from larger hydrocarbons. There are two aspects of this - industrial, and in the lab. CIE may ask you about it in either context.

Before you go on, you should find and read the statement in your copy of the syllabus.

Industrial cracking

You will find this discussed on two Chemguide pages in different parts of the site. I suggest you start with the page about cracking alkanes. If you follow the link in the second green box to the page about catalysis in the petrochemical industry, you will find catalytic cracking repeated with a bit more detail. You don't have to know about isomerisation or reforming.

There is more information on those pages than you will need. CIE are likely to expect you to know:

  • Cracking is carried out at high temperatures (500°C or more) in the presence of a zeolite catalyst.

  • Cracking produces a mixture containing alkenes amongst other molecules. Alkenes are industrially useful molecules.

  • The mixture also contains smaller alkanes which are used in fuels such as petrol.

Cracking in the lab

You can crack various large molecules in the lab by passing their vapour over a variety of hot catalysts. One CIE multiple-choice question, for example, shows vapour being passed over a strongly heated catalyst named as "silica, alumina or baked clay". Silica is silicon dioxide; alumina is aluminium oxide.

Another question asks about a build-up of black on such a catalyst. That would be carbon. Although we don't tend to discuss it much, cracking a hydrocarbon can result in the formation of carbon.

You will find a diagram showing the sort of apparatus you might use on a page about dehydrating alcohols. You would simply replace the alcohol in the horizontal test tube by, say, liquid paraffin, or some other large hydrocarbon.

Because something like liquid paraffin isn't as volatile as ethanol, you would have to heat that as well (but fairly gently) to make it vaporise.

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© Jim Clark 2020