AQA A-Level Chemistry AS Paper 2, June 2025: Question 21

✅ Correct Answer

B: C₂H₆ + Cl₂ → C₂H₅Cl + HCl

When ethane is in excess, monochlorination is the major pathway. Each substitution replaces one hydrogen atom with a chlorine atom and forms hydrogen chloride ( HCl ), not hydrogen gas ( H₂ ).

💡 Key Knowledge

• Role of UV Light: Provides energy for homolytic fission of the Cl–Cl bond in the initiation step:
  Cl₂ → 2Cl•
• Propagation Steps:
  1) C₂H₆ + Cl• → •C₂H₅ + HCl
  2) •C₂H₅ + Cl₂ → C₂H₅Cl + Cl•
• Excess Alkane Effect: A high concentration of unreacted ethane makes chlorine radicals far more likely to collide with C₂H₆ than with already-formed C₂H₅Cl , favouring monosubstitution ( C₂H₅Cl ).
• Excess Halogen Effect: An excess of halogen would lead to further successive substitutions, ultimately forming hexachloroethane ( C₂Cl₆ ).

🧠 Exam Technique & Elimination

Quickly eliminate wrong answers through systematic checks:

• Check by-product identity: Radical substitution of alkanes always produces hydrogen halide ( HCl ), never elemental hydrogen ( H₂ ). This immediately eliminates A and C.
• Check reactant conditions: Look for the keyword "excess ethane". Excess ethane prevents poly-substitution. Therefore, the product is chloroethane ( C₂H₅Cl ), eliminating D (which represents reaction in excess chlorine).

❌ Common Errors & Misconceptions

• The H₂ Trap (Options A & C): Confusing substitution with dehydrogenation. The removed hydrogen atom combines with a halogen radical to yield HCl , not H₂ .
• Ignoring Reaction Conditions (Option D): Equation D is chemically balanced ( C₂H₆ + 6Cl₂ → C₂Cl₆ + 6HCl ) and represents full substitution, but this requires an excess of chlorine, directly contradicting the question prompt.