AQA A-Level Chemistry Paper 3, June 2018: Question 9

✅ Correct Answer

D: CH₃ĊFCl + Cl• → CH₃CFCl₂

A termination step removes free radicals from the reaction mixture. In this step, two radicals collide and combine to form a single, stable non-radical molecule:

Radical + Radical → Molecule

💡 Key Knowledge: The "Radical Count" Rule

You can instantly categorise any free-radical step simply by counting the radical dots (•):

• Initiation: 0 radicals → 2 radicals
  Molecule → Radical + Radical (homolytic fission driven by UV light)
• Propagation: 1 radical → 1 radical
  Molecule + Radical → New Radical + New Molecule (chain continues)
• Termination: 2 radicals → 0 radicals
  Radical + Radical → Molecule (chain ends)

🧠 Exam Technique: Analysis of Incorrect Options

• A: CH₃CH₂CH₃ + Br• → CH₃ĊHCH₃ + HBr
  1 radical on left, 1 radical on right. This is a propagation step (hydrogen abstraction from propane).
• B: ClO• + O₃ → Cl• + 2O₂
  1 radical on left, 1 radical on right. This is a propagation step in the catalytic destruction of ozone.
• C: RO• + CH₂=CH₂ → ROCH₂ĊH₂
  1 radical on left, 1 radical on right. This is a propagation step (radical addition to an alkene in polymerisation).

❌ Common Errors & Pitfalls

• Confusing C for termination: Students often see two reactants joining to form one product and assume it is termination. However, the product in C is still a radical ( ROCH₂ĊH₂ ), meaning the chain reaction is still active!
• Misidentifying ozone depletion steps: Reactions involving ClO• often cause confusion. Remember that because a chlorine radical ( Cl• ) is regenerated, step B is propagation, allowing chlorine to act as a catalyst.
• Overlooking the radical dot: Always scan both sides of the arrow carefully to count the total number of single dots representing unpaired electrons.