Cracking and alkenes
The fractions from crude oil don't match demand — there's too much long-chain (e.g. heavy fuel oil) and not enough short-chain (e.g. petrol). Cracking breaks long-chain hydrocarbons into shorter, more useful molecules.
What is cracking?
Cracking is thermal decomposition of long-chain alkanes into a smaller alkane + an alkene.
General: long alkane → smaller alkane + alkene(s)
Example: C₁₀H₂₂ → C₈H₁₈ + C₂H₄ (decane → octane + ethene).
Two methods
1. Thermal cracking
- Heat long alkane to ~700 °C and high pressure.
- No catalyst required.
- Used in petrochemical industry for some products.
2. Catalytic cracking
- ~550 °C, hot vapour passed over a hot zeolite catalyst.
- Lower temperature → less energy.
- Faster, more controlled.
Why we crack
- More petrol/diesel to meet demand.
- Produces alkenes (ethene, propene) — vital feedstocks for polymers and other chemicals.
Alkenes
Alkenes have at least one C=C double bond and the general formula CₙH₂ₙ.
| Name | Formula |
|---|---|
| Ethene | C₂H₄ |
| Propene | C₃H₆ |
| Butene | C₄H₈ |
| Pentene | C₅H₁₀ |
Alkenes are unsaturated (the C=C can break to add more atoms).
Test for alkenes — bromine water
Bromine water is orange/yellow. Add to:
- An alkene: turns colourless (decolourised) — Br₂ adds across the double bond.
- An alkane: stays orange — no reaction.
Reaction: C₂H₄ + Br₂ → C₂H₄Br₂ (1,2-dibromoethane).
Uses of alkenes
- Polymers: ethene → polyethene (plastic bags, bottles); propene → polypropene; chloroethene → PVC.
- Starting materials for many other chemicals (alcohols, solvents).
✦Worked example— Worked example — predicting cracking products
C₁₂H₂₆ is cracked. Suggest two possible products.
A long alkane → smaller alkane + alkene. Possibilities:
- C₁₂H₂₆ → C₈H₁₈ + 2C₂H₄ (octane + 2 ethene)
- C₁₂H₂₆ → C₁₀H₂₂ + C₂H₄ (decane + ethene)
Check atom balance: 12 C, 26 H on each side.
⚠Common mistakes
- Forgetting alkenes are unsaturated — that's why they decolourise bromine water.
- Imbalanced cracking equation — atoms must balance on both sides.
- Saying cracking gives only short alkanes — there must be at least one alkene to balance H atoms.
- Mixing up bromine water test — alkenes DECOLOURISE; alkanes don't.
Links
Builds on C7.1, C7.2. Sets up C7.5 (alkene reactions), C7.7 (polymerisation HT).
AI-generated · claude-opus-4-7 · v3-deep-chemistry