Modification of Alkanes by Cracking (AQA A-Level Chemistry): Revision Notes

3.2.2 Modification of Alkanes by Cracking

What is Cracking?

Types of Cracking

There are two main types of cracking processes: thermal cracking and catalytic cracking.

Thermal Cracking

• Conditions: High pressure and very high temperature (around 700-1200 K).
• Products: A high percentage of alkenes (e.g., ethene, which is used to produce polymers).
• Mechanism: Involves breaking the bonds in a random manner, resulting in a variety of smaller hydrocarbons. Example:

The cracking of decane ($C_{10}H_{22}$) might produce ethene ($C_2H_4$) and octane ($C_8H_{18}$):

$$C_{10}H_{22} \rightarrow C_2H_4 + C_8H_{18}$$

Catalytic Cracking

• Conditions: Lower pressure, high temperature (around 720 K), and the presence of a zeolite catalyst.
• Products: Motor fuels (e.g., gasoline) and aromatic hydrocarbons.
• Mechanism: The catalyst speeds up the breaking of $C-C$ bonds, focusing on producing branched alkanes and aromatic compounds, which are important in motor fuel production.

2. Economic Reasons for Cracking

Cracking is economically important because it helps meet the demand for more useful hydrocarbons. Here's why cracking is essential:

High Demand for Short-Chain Hydrocarbons

• Short-chain alkanes are better fuels because they are more volatile and burn more easily. For example, propane and butane are widely used in heating and cooking.
• Short-chain alkenes are more reactive than alkanes and are used as feedstock for producing polymers like plastics.

Imbalance in Supply

• Fractional distillation of crude oil naturally produces a surplus of long-chain hydrocarbons and fewer short-chain hydrocarbons than are needed. Long-chain hydrocarbons are less useful and harder to ignite, making them less desirable for fuel. By converting the excess of long-chain hydrocarbons into more useful short-chain alkanes and alkenes, cracking maximises the utility of crude oil and aligns supply with industrial demand.