This topic summary covers Knowledge Organiser: Cracking within Cracking (HT) for GCSE Chemistry. Revise Cracking (HT) in Organic Chemistry for GCSE Chemistry with 24 exam-style questions and 15 flashcards. This topic appears less often, but it can still be a useful differentiator on mixed-topic papers. It is section 14 of 14 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
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
Practice questions for Cracking (HT)
What is cracking in chemistry?
Describe the conditions used in thermal cracking and state the types of product formed.
Quick recall flashcards
Why is cracking needed?
Long alkanes are less useful. Cracking produces shorter alkanes for fuel and alkenes for polymers
What is cracking?
Breaking down long-chain hydrocarbons into shorter, more useful molecules