Cracking

Key Stage 4

Meaning

Cracking is a Thermal Decomposition process in which large hydrocarbon molecules are broken into smaller hydrocarbon molecules.

About Cracking

Cracking is often done because Crude Oil contains more large hydrocarbon molecules than can be used and not enough short hydrocarbon molecules than are needed.

When Crude Oil fractions are cracked the long alkanes are broken down into smaller alkanes and alkenes.

Cracking is done at very high temperatures (500°C) and uses either a catalyst to aid the reaction or steam.

When an alkane is cracked into smaller pieces there are not enough Hydrogen [[atom]s to produce two saturated hydrocarbons. One of the hydrocarbons must be unsaturated and therefor will have a double bond.

Examples

If butane were cracked.		An alkane and an alkene are produced.

Cracking a hydrocarbon can produce many possible products:

Hexadecane → Decane + Hexene

<chem>C16H34 -> C10H22 + C6H12</chem>

Hexadecane → Decane + Butene + Ethene

<chem>C16H34 -> C10H22 + C4H8 + C2H4</chem>

Decane → Pentane + Pentene

<chem>C10H34 -> C5H12 + C5H10</chem>

Decane → Hexane + Butene

<chem>C10H34 -> C6H14 + C4H8</chem>

References

AQA

Cracking (of hydrocarbons), page 149, GCSE Combined Science; The Revision Guide, CGP, AQA

Cracking (of hydrocarbons), page 226, 227, GCSE Chemistry, CGP, AQA

Cracking (of hydrocarbons), page 77, GCSE Chemistry; The Revision Guide, CGP, AQA

Cracking (of hydrocarbons), pages 194, 195, GCSE Combined Science Trilogy; Chemistry, CGP, AQA