AQA A Level chemistry - AS Unit 1: Section 3.1.6 Alkanes - Modification of alkanes by cracking

3.1.6 Alkanes - Modification of alkanes by cracking

Students should:

understand that cracking involves the breaking of C-C bonds in alkanes
know that thermal cracking takes place at high pressure and high temperature and produces a high percentage of alkenes (mechanism not required)
know that catalytic cracking takes place at a slight pressure, high temperature and in the presence of a zeolite catalyst and is used mainly to produce motor fuels and aromatic hydrocarbons (mechanism not required)
understand the economic reasons for the cracking of alkanes (e.g. ethene used for poly(ethene); conversion of heavy fractions into higher value products)

Alkenes

These structures will be similar to those of the alkanes, except two hydrogens on adjacent carbons are replaced by a double bond between those carbons. The number '1' in the names refers to the position of the carbon starting the double bond. No numbering is needed in the first two members as there can be no ambiguity.

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Cracking

Cracking is the process whereby long and medium chain hydrocarbons are heated and break apart giving short chain alkanes, alkenes and hydrogen. The actual products are fairly random. There are three fundamental kinds of cracking:

1 Thermal cracking
2 Hydrocracking
3 Catalytic cracking

Thermal cracking

This is expensive as the hydrocarbons are subject to high temperature and pressure. The mechanism is free radical and it is difficult to control the composition of the products, which are mixtures of short chain alkanes and alkenes.

Hydrocracking

Steam is added allowing lower temperatures and adding other advantages:

Lower partial pressure of hydrocarbon is needd helping the equilibrium to the right hand side.
Collision with the steam molecules helps instigate the cracking process and lower temperatures are needed
The presence of steam helps to prevent coking - i.e. deposition of carbon on the walls of the cracker.

Catalytic cracking

This has the advantage of much lower temperatures. Zeolites (aluminates and silicates) are used which promote an ionic mechanism by removing a hydrogen atom as a hydride ion leaving a carbonium ion, which then reforms or breaks apart into different products. The products include high percentages of branched and cyclic alkanes and alkenes. This is useful for the petroleum industry.

The products of cracking in all cases are then returned to be fractionally distilled once again and separated according to need.

Ethene is probably the most economically useful product of cracking, as it is used in many different applications in society, as plastics (polythenes)

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