3.2.2Organic Chemistry
Modification of Alkanes by Cracking
5 min read 5 parts Lesson 8 of 55
Section 1 of 5Why Crack
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.
Section 2 of 5Types of Cracking
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 |
Section 3 of 5Equations
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.
Section 4 of 5Worked Example
Question
- Dodecane, C₁₂H₂₆, is cracked to give ethene and one other alkane only.
- Write the equation if two molecules of ethene form.
- Two ethene molecules contain 2 × C₂H₄ = C₄H₈.
- Subtract from C₁₂H₂₆carbon 12 − 4 = 8, hydrogen 26 − 8 = 18.
- The remaining product is C₈H₁₈ (octane), which is an alkane because it fits CₙH₂ₙ₊₂.
Answer C₁₂H₂₆ → C₈H₁₈ + 2C₂H₄
Section 5 of 5Exam Checklist & Tips
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.