Cracking is the breaking of C-C bonds in long-chain alkanes to form shorter, more useful molecules. It is needed because fractional distillation gives more long-chain fractions than are needed and not enough petrol and alkenes.Thermal cracking uses a high temperature, about 400 to 900 °C, and a high pressure, up to about 70 atm. The molecules are only heated for a very short time to stop them breaking down completely to carbon.The C-C bonds break by homolytic fission to form free radicals. Thermal cracking produces a high percentage of alkenes, along with some shorter alkanes.For example: C10H22 → C6H14 + 2C2H4The alkenes, such as ethene, are very valuable because they are used to make polymers and many other chemicals.
Thermal Cracking
Read the text
Read the text and highlight anything you think is important. When you go on, the text is hidden and you answer from memory.
Specification: AQA Chemistry (7404), AS-level, 3.3.2.2 Modification of alkanes by cracking. Topic: Alkanes.
Key terms in this lesson
- free radical
- A species with an unpaired electron, often formed when a covalent bond breaks by homolytic fission.
- homolytic fission
- The breaking of a covalent bond in which each atom keeps one electron from the shared pair, forming two free radicals.
More in Alkanes
- Structural Isomers
- Physical Properties
- Fractional Distillation of Crude Oil
- Fracking
- catalytic Cracking
- Combustion
- Polluting the Atmosphere
- Flue Gas Desulfurisation
All 20 lessons in Alkanes · All AQA AS-level Chemistry topics