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Course: Oxford AQA International A Level Chemistry
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Oxford AQA International A Level Chemistry

1. Physical Chemistry

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2. Inorganic Chemistry

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3. Organic Chemistry

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3.2.2 Modification of Alkanes by Cracking

3.2.2 Modification of Alkanes by Cracking: Explain why cracking is carried out, compare thermal and catalytic cracking, and write balanced equations for cracking reactions.

3.2.2 Modification of Alkanes by Cracking
Why Crack
Supply and demand
  • Fractional distillation gives more long-chain hydrocarbons than the market needs, but demand for short-chain fuels is higher.
Surplus fractions
  • Long-chain fractions are less valuable, so they are broken into smaller, more useful molecules.
Alkenes
  • Cracking also produces alkenes, which are reactive and are the starting point for polymers and other chemicals.
Cracking defined
  • The breaking of C–C bonds in long-chain alkanes to give shorter alkanes and alkenes.
Thermal cracking
  • High temperature (up to about 1000 °C) and high pressure (up to about 70 atm), with no catalyst, favour many alkenes.
Catalytic cracking
  • A zeolite catalyst at about 450 °C and slight pressure produces motor fuels and aromatic hydrocarbons.
Mechanism of thermal
  • Bonds break homolytically to form radicals, which is why a high temperature is required.
Benefits of catalyst
  • Lower temperatures and pressures save energy, so catalytic cracking has lower running costs.
Products of catalytic
  • Branched alkanes, cycloalkanes and aromatic compounds improve the quality of petrol.
Feature Thermal Catalytic
Temperature up to ~1000 °C ~450 °C
Pressure up to ~70 atm slight
Catalyst none zeolite
Main products alkanes and many alkenes motor fuels, aromatics
1

Cracking produces at least one alkene and one shorter alkane, so the total number of atoms must balance on both sides.
2

Decane cracks as C₁₀H₂₂ → C₈H₁₈ + C₂H₄, giving octane and ethene.
3

A long chain may crack into one alkane and two alkene molecules, for example C₁₄H₃₀ → C₈H₁₈ + 2C₃H₆.
4

Check carbon and hydrogen: 14 carbons and 30 hydrogens on the left match 8 + 6 = 14 carbons and 18 + 12 = 30 hydrogens on the right.

Question: Dodecane, C₁₂H₂₆, is cracked to give ethene and one other alkane only. Write the equation if two molecules of ethene form.

  1. Two ethene molecules contain 2 × C₂H₄ = C₄H₈.
  2. Subtract from C₁₂H₂₆: carbon 12 − 4 = 8, hydrogen 26 − 8 = 18.
  3. The remaining product is C₈H₁₈ (octane), which is an alkane because it fits CₙH₂ₙ₊₂.

Answer: C₁₂H₂₆ → C₈H₁₈ + 2C₂H₄

3.2.2 – Key Points for the Exam

  • Cracking: Breaking long-chain hydrocarbons into shorter ones.
  • Zeolite: A porous aluminosilicate used as a catalyst in catalytic cracking.
  • Alkene: A hydrocarbon with a C=C double bond.
  • Demand: How much of a product customers want to buy.

Common Mistakes

  • Writing cracking equations that do not balance for carbon or hydrogen.
  • Saying cracking makes only alkanes; at least one alkene is produced.
  • Stating that catalytic cracking needs higher temperature than thermal.
  • Confusing cracking, a chemical change, with fractional distillation, a physical process.
YOUR STUDY PATH

3.2.2 Modification of Alkanes by Cracking

Oxford AQA International A Level Chemistry / 3. Organic Chemistry

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