Cracking & Alkenes

Cracking

• Cracking breaks large hydrocarbon molecules into smaller, more useful ones
• Why: there is high demand for small molecules (petrol) and low demand for large ones, so cracking the surplus large fractions matches supply to demand. It also makes alkenes for polymers
• Two methods, both needing a high temperature:
  • Catalytic cracking: the vapour is passed over a hot catalyst
  • Steam cracking: the vapour is mixed with steam at a very high temperature
• The products are a shorter alkane and an alkene:

$$\text{C}_{10}\text{H}_{22} \rightarrow \text{C}_8\text{H}_{18} + \text{C}_2\text{H}_4$$

• Balance cracking equations by counting carbons and hydrogens: the atoms on the right must add up to the molecule on the left
• Modern life depends on hydrocarbons: fuels for transport and heating, feedstock for plastics, medicines, detergents, and solvents