There are many different ways in which a large hydrocarbon chain can be broken into smaller hydrocarbons. Within fractional distillation the products obtained in the larger fractioning columns, i.e. the one containing the heavier C12 to C20 will subsequently be more beneficial if it is converted into more useful smaller hydrocarbon products. In the HSC course you will be required to know two methods of cracking a hydrocarbon as well as detailed knowledge on fractional distillation.

Thermal Cracking


As shown to the right, thermal cracking is the process of cracking larger molecules by using heat and this case steam. The gas or fine mist of the substance is passed through heat coils, normally this is enough to break bonds between carbons and produce alkyne radicals. Cracking may be cycled until the desired product is acheived.

Cracking of Decane

Chemical Equations

C10H22-> 2C5H11 (radicals)

C5H11 (radical) -> C3H7 (radical) + C2H4

2C3H7 (radical) -> C6H14

Word Equations

Decane -> Pentyl radicals

Pentyl radicals -> Propyl radical + Ethene

Propyl radical -> Hexane

Cracking of Ethane

C2H6 (g) -> C2H4 (g) + H2 (g)

$\Delta$ H = +138 kJ/mol

Word Equations

Ethane -> Ethene + Hydrogen

Catalytic Cracking

Catalytic cracking is acheived by the use of a catalyst and lower temperatures. The catalyst used in cracking is zeolite, which is an aluminium silicate. Zeolite has a high internal surface area due to small open channels and an open-cage structure, this allows the temperature to be around 500oC instead of the regular 600-800oC required to thermally crack the same product. Although this saves a lot of energy, only small molecular products1 due to the small open channel and cage-like structure of zeolite may be obtained.

Catalytic Cracking of Decane

Chemical Equations

C10H22 (l) -(zeolite)> C2H4 (g) + C8H18 (l)

Word Equations

Decane -(zeolite)> Ethene + Octane

HSC Question:

Justify the necessity of cracking and describe the purpose in your answer you may include chemical equations and diagrams2.

Suggested Answer:

Justify is to support an argument or conclusion, you must provide evidence for the argument.

Cracking is the process of breaking large hydrocarbon molecules (such as products C15-C20 into smaller hydrocarbon molecules such as propene and ethylene. There are two methods of cracking, thermal cracking and catalytic cracking.
Thermal cracking involves the use of pylysis coils (heating coils) from which ethane is passed through to produce ethene.

C2H6 (g) -> C2H4 (g) + H2 (g)

$\Delta$ H = +138 kJ/mol

Catalytic cracking involves the use of a catalyst in order to reduce the temeperature required to crack the hydrocarbon. Hence a lower temperature of 500oC is used as opposed to 750-900oC required for thermal cracking. A catalyst such as zeolite is used as it's small open channels and cage-like structure gives a high internal surfae area, however due to this structure only small molecular products such as ethene and propene can be obtained.

Only a small amount of large hydrocarbon products are used directly as they are produced from fractional distillation. Cracking allows the excess of large hydrocarons to be converted to smaller hydrocarbons and hence cracking is necessary in order to maximise useful products from crude oil.
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