Example 1: Crack C₁₂H₂₆ to form octane and an alkene
1. Write what we know: C₁₂H₂₆ → C₈H₁₈ + ?
2. Find missing carbons: 12 - 8 = 4 carbons
3. Find missing hydrogens: 26 - 18 = 8 hydrogens
4. Unknown product: C₄H₈ (butene — an alkene! CₙH₂ₙ check: 2×4=8 ✓)
Answer: C₁₂H₂₆ → C₈H₁₈ + C₄H₈
1. Write what we know: C₁₂H₂₆ → C₈H₁₈ + ?
2. Find missing carbons: 12 - 8 = 4 carbons
3. Find missing hydrogens: 26 - 18 = 8 hydrogens
4. Unknown product: C₄H₈ (butene — an alkene! CₙH₂ₙ check: 2×4=8 ✓)
Answer: C₁₂H₂₆ → C₈H₁₈ + C₄H₈
Example 2: Crack C₁₆H₃₄ to form ethene and an alkane
1. Write what we know: C₁₆H₃₄ → ?alkane + C₂H₄
2. Find remaining carbons: 16 - 2 = 14 carbons
3. Find remaining hydrogens: 34 - 4 = 30 hydrogens
4. Check if C₁₄H₃₀ is an alkane: 2(14)+2 = 30 ✓
Answer: C₁₆H₃₄ → C₁₄H₃₀ + C₂H₄
1. Write what we know: C₁₆H₃₄ → ?alkane + C₂H₄
2. Find remaining carbons: 16 - 2 = 14 carbons
3. Find remaining hydrogens: 34 - 4 = 30 hydrogens
4. Check if C₁₄H₃₀ is an alkane: 2(14)+2 = 30 ✓
Answer: C₁₆H₃₄ → C₁₄H₃₀ + C₂H₄
Key Rule: In cracking, you ALWAYS get at least one alkene. Alkenes have the general formula CₙH₂ₙ (two fewer hydrogens than the corresponding alkane CₙH₂ₙ₊₂).
This worked example covers Writing and Balancing Cracking Equations 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 6 of 14 in this topic. Treat this as a marking guide for what examiners are looking for, not just a fact list.
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