This exam tips covers Exam Tips: 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 13 of 14 in this topic. Treat this as a marking guide for what examiners are looking for, not just a fact list.
💡 Exam Tips: Cracking
🎯 Common Question Types:
- Write a cracking equation given one product (2 marks)
- Describe the bromine water test for alkenes (2 marks)
- Explain why cracking is economically important (2 marks)
- Compare thermal and catalytic cracking (3 marks HT)
📝 Key Command Words:
- Write: Balance by atom conservation (C then H)
- Describe: State colour change and what it means
- Explain: Link to supply/demand and polymer production
⚠️ Common Mistakes to Avoid:
- Forgetting to include an alkene in cracking products
- Confusing cracking with combustion (no oxygen in cracking)
- Not checking atom balance on both sides of equation
Quick Check: Decane (C₁₀H₂₂) is cracked to form pentene (C₅H₁₀) and another product. Identify the other product.
C₁₀H₂₂ → C₅H₁₀ + ?. Carbons remaining: 10 - 5 = 5. Hydrogens remaining: 22 - 10 = 12. C₅H₁₂ — check with alkane formula: 2(5)+2 = 12 ✓. The other product is pentane (C₅H₁₂), an alkane.
Quick Check: Bromine water is added to two hydrocarbons, A and B. A decolourises the bromine water; B does not. What can you conclude about A and B?
A contains a C=C double bond — it is an alkene (unsaturated). It undergoes an addition reaction with bromine, decolourising it. B has no C=C double bond — it is an alkane (saturated). It does not react with bromine water at room temperature.
Practice questions for Cracking (HT)
What is cracking in chemistry?
Describe the conditions used in thermal cracking and state the types of product formed.