UNIT 2 CHEMISTRY · PEARSON AAQ · E1.8.2
Cracking of Long-Chain Alkanes
FREE CHEMISTRY REVISION · TOPIC 64
Cracking of Long-Chain Alkanes
Pearson BTEC Level 3 AAQ Applied Science
Unit 2: Principles and Applications of Chemistry · E1.8.2
What you will learn
- Explain why refineries crack large alkane molecules.
- Distinguish catalytic cracking, thermal cracking and steam cracking conditions.
- Predict possible smaller alkanes and alkenes and balance cracking equations.
1. Turning long chains into useful products
Cracking breaks large hydrocarbon molecules into smaller molecules by breaking carbon–carbon bonds and rearranging bonds. Long-chain alkane fractions are often less useful as transport fuels and are less in demand than shorter fractions. Cracking helps match refinery supply to demand and produces valuable alkene feedstocks. It is a chemical change; fractional distillation separates a mixture without changing its molecules.
The simplified pattern is a long-chain alkane → smaller alkane + alkene. Products are a mixture, and several different carbon-chain combinations are possible. An alkene has a C=C bond and can undergo addition reactions, including polymerisation. Industrial processes can also make branched hydrocarbons, other products and coke; one classroom equation represents just one possible outcome.
2. Conditions: specify the process
| Process | Representative conditions | Important distinction |
|---|---|---|
| Catalytic cracking | Vaporised feed, hot zeolite/aluminosilicate catalyst; commonly around 450–550°C and relatively low pressure | Catalyst provides a lower-activation-energy pathway; heating is still needed. Products include useful fuel-range hydrocarbons and alkenes. |
| Thermal cracking | High temperature without a catalyst; conventional thermal cracking can use high pressure | Typically around 450–750°C, depending on feed and process. High pressure is not a universal condition for every industrial cracking process. |
| Steam cracking | Feed diluted with steam; very high temperature, often roughly 750–900°C, short contact time and low hydrocarbon partial pressure; no catalyst | A thermal process designed to produce alkenes, especially ethene. Do not confuse steam dilution with catalytic hydration. |
Conditions are representative industrial ranges, not one universal recipe. In an exam, identify the named process: catalytic cracking needs heat plus a suitable catalyst; thermal cracking needs sufficiently high temperature and no catalyst. Cracking requires energy input overall. A catalyst is not a source of energy and does not make the bonds break without heating.
3. Predict and check products
- Count carbon atoms in the starting alkane. Distribute them between the proposed products.
- Use CₙH₂ₙ₊₂ for an acyclic alkane and CₙH₂ₙ for an acyclic monoalkene.
- Check the hydrogen count as well as carbon. A formula alone may not identify one structural isomer.
- An alkene needs at least two carbon atoms. Do not propose CH₂ as an ordinary stable alkene product.
- Do not add O₂ or H₂O to a simple cracking equation. Combustion and hydration are different reactions.
Worked and applied examples
Example 1 · Octane to hexane and ethene
C₈H₁₈ → C₆H₁₄ + C₂H₄. C: 8=6+2; H: 18=14+4. Ethene is unsaturated and can form poly(ethene). Hexane is a smaller alkane.
Example 2 · Another octane outcome
C₈H₁₈ → C₄H₁₀ + C₄H₈. C: 8=4+4; H: 18=10+8. But-1-ene is one possible C₄H₈ alkene. The molecular formula also permits other isomers; do not claim a unique structure without more information.
Example 3 · Find the missing product
C₁₂H₂₆ → C₁₀H₂₂ + X. Subtract atoms: X has C=2 and H=4, so X is C₂H₄, ethene. Check 12=10+2 and 26=22+4.
Example 4 · More than two molecules
C₁₀H₂₂ → C₆H₁₄ + 2C₂H₄. C: 10=6+4; H: 22=14+8. A single cracking event pattern is not restricted to exactly two product molecules.
Example 5 · Commercial choice
A refinery has excess heavy fractions but demand for petrol-range hydrocarbons and polymer feedstocks. Cracking increases the supply of smaller molecules. Shorter alkanes are generally more volatile and flow more easily than very long chains; ethene and propene supply polymer production.
Example 6 · Reject an unbalanced proposal
C₈H₁₈ → C₄H₁₀ + C₄H₁₀ is wrong: the right side has 20 H, not 18. Changing one product to C₄H₈ conserves atoms and gives an alkane–alkene combination.
4. Why the products matter
Smaller alkanes can be components of petrol and LPG or other fuels after separation and processing. Alkenes are chemical starting materials: ethene makes poly(ethene) and can be hydrated to ethanol; propene makes poly(propene). Product separation, process heating and catalyst regeneration cost energy. Cracking improves feedstock use but does not make fossil resources renewable or remove combustion emissions.
Common misconceptions and examination tips
- Cracking breaks molecules, not atoms. Both carbon and hydrogen must balance.
- Distillation is physical separation; cracking changes molecular structures.
- There is no single obligatory alkane–alkene product pair. State “one possible equation”.
- A catalyst permits a faster process under suitable conditions, but does not supply energy.
- Distinguish steam cracking from conventional high-pressure thermal cracking and from catalytic cracking.
Check your understanding
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