Cracking of Hydrocarbons
Cracking is a chemical process used to break down long-chain hydrocarbons into shorter, more useful ones. It also produces alkenes, which are important for making plastics.
Why do we crack hydrocarbons?
- Long-chain hydrocarbons (from crude oil) are less useful
- Shorter chains are more flammable and make better fuels
- Cracking helps meet demand for substances like petrol and ethene
💡 Cracking turns large, less useful molecules into smaller, more useful ones
What does cracking produce?
- A shorter-chain alkane (useful fuel)
- An alkene (used in making polymers/plastics)
Example:
C₁₀H₂₂ → C₈H₁₈ + C₂H₄
(decane → octane + ethene)
Types of cracking
Type | Conditions | Notes |
Thermal cracking | High temperature (~750°C) and high pressure | Produces lots of alkenes |
Catalytic cracking | Lower temperature, uses zeolite catalyst | Faster and more efficient |
Test for alkenes
- Alkenes are unsaturated hydrocarbons (contain C=C double bonds)
- You can test for alkenes using bromine water:
Bromine water turns from orange to colourless if an alkene is present.
Questions
- Why do we crack hydrocarbons?
- What two types of products are made from cracking?
- What is the test for alkenes?
- Which type of cracking uses a catalyst?
- Are alkenes saturated or unsaturated?
Summary
- Cracking breaks long-chain alkanes into short alkanes and alkenes
- Makes more useful hydrocarbons like petrol and ethene
- Done by thermal or catalytic cracking
- Alkenes are used to make plastics
- Alkenes decolourise bromine water