Knowledge Organiser: Cracking
Key Terms
- Cracking — thermal decomposition of long alkanes
- Products — shorter alkane + alkene (always)
- Thermal cracking — high T (700-900°C) + high P
- Catalytic cracking — lower T, zeolite catalyst
Bromine Water Test
- Orange bromine water + alkene → colourless
- Orange bromine water + alkane → stays orange
- Addition reaction across C=C double bond
Equation Balancing
- Count C atoms: must balance
- Count H atoms: must balance
- Alkene check: CₙH₂ₙ formula
- Always at least one alkene product
Why Crack?
- More petrol (high demand)
- More alkenes for polymers
- Low-value → high-value products
- Match supply to demand
Key Equations
- C₁₀H₂₂ → C₈H₁₈ + C₂H₄ (example cracking: decane → octane + ethene)
- Long alkane → shorter alkane + alkene (always)
- Alkene test: decolourises orange bromine water → colourless
Common Mistakes
- Saying cracking produces only alkanes: Cracking always produces at least one ALKENE — the C=C double bond formed is what makes the products useful for polymers
- Confusing thermal and catalytic cracking: Thermal cracking uses high temperature (700-900°C) and high pressure; catalytic cracking uses a zeolite catalyst at lower temperature
- Not balancing the cracking equation: Total carbons and hydrogens must balance on both sides — count all C and H atoms carefully
- Saying alkanes decolourise bromine water: Only the ALKENE product decolourises bromine water — alkanes do not react with bromine water
Memory Aid
- Big biscuit = long alkane (e.g., C₁₀H₂₂)
- Break it = crack it with heat/catalyst
- Smaller pieces = shorter alkane + alkene
- More useful = petrol + polymer material