Cracking – IGCSE Chemistry Definition
IGCSE Chemistry definition of cracking: breaking large hydrocarbons into smaller, more useful molecules. Covers methods, products, and exam tips for Cambridge 0620.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
Cracking connects the fractional distillation of crude oil to the production of alkenes for making polymers. It is one of the most important industrial processes in the syllabus and is regularly tested for 3–4 marks on Paper 2 and Paper 4.
The 0620 definition
Cracking is the thermal decomposition of large hydrocarbon molecules into smaller, more useful molecules. The products include shorter-chain alkanes and alkenes.
Why cracking is needed
Fractional distillation of crude oil produces fractions. The problem:
- Too much of the heavy fractions (long-chain hydrocarbons) — low demand
- Not enough of the light fractions (short-chain hydrocarbons) — high demand as fuels
- No alkenes are produced by distillation — alkenes must be made by cracking
Cracking solves both problems by converting surplus long chains into short-chain fuels and alkenes.
The products
Large alkane → smaller alkane + alkene(s)
Example: C₁₀H₂₂ → C₈H₁₈ + C₂H₄ (decane → octane + ethene)
Or: C₁₀H₂₂ → C₂H₄ + C₃H₆ + C₅H₁₂
Note: at least one alkene is always produced. The equations must balance for C and H atoms.
Methods of cracking
| Feature | Thermal cracking | Catalytic cracking |
|---|---|---|
| Temperature | ~800 °C | ~450 °C |
| Catalyst | None (heat + steam) | Zeolite (aluminium oxide / silicon dioxide) |
| Pressure | High | Low / atmospheric |
| Main products | Wide range of alkenes | Branched alkanes + aromatics + alkenes |
Uses of cracking products
- Short-chain alkanes (e.g. octane): petrol for vehicles
- Ethene and propene: raw materials for polymers (poly(ethene), poly(propene))
- Hydrogen: fuel cells, Haber process
Worked exam question
Decane (C₁₀H₂₂) is cracked. The products are octane and one other compound. (a) Name the type of reaction. [1] (b) Write the equation. [1] (c) Name the other product and state one use. [2]
Mark scheme
(a) Thermal decomposition / cracking [1]
(b) C₁₀H₂₂ → C₈H₁₈ + C₂H₄ [1]
(c) Ethene [1]; making poly(ethene) / plastic / polymers [1]
Common exam mistakes
- Writing a cracking equation that does not balance — check that the total C and H atoms on both sides are equal.
- Forgetting that at least one alkene is always produced — if the products were only alkanes, no hydrogen would be left over for the double bond.
- Confusing cracking with fractional distillation — distillation separates molecules by size, cracking breaks them apart.
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