Worked Example 2: Hydrogen + Chlorine → Hydrogen Chloride
Part of Bond Energies (HT) — GCSE Chemistry
This worked example covers Worked Example 2: Hydrogen + Chlorine → Hydrogen Chloride within Bond Energies (HT) for GCSE Chemistry. Revise Bond Energies (HT) in Energy Changes for GCSE Chemistry with 20 exam-style questions and 15 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 6 of 15 in this topic. Treat this as a marking guide for what examiners are looking for, not just a fact list.
Topic position
Section 6 of 15
Practice
20 questions
Recall
15 flashcards
🧮 Worked Example 2: Hydrogen + Chlorine → Hydrogen Chloride
Calculate ΔH for: H₂ + Cl₂ → 2HCl
Bond energies given:
- H-H bond = 436 kJ/mol
- Cl-Cl bond = 242 kJ/mol
- H-Cl bond = 431 kJ/mol
Step 1: Identify bonds broken (reactants)
- 1 × H-H bond = 1 × 436 = 436 kJ
- 1 × Cl-Cl bond = 1 × 242 = 242 kJ
- Total energy in: 436 + 242 = 678 kJ
Step 2: Identify bonds formed (products)
- 2HCl contains 2 × H-Cl bonds = 2 × 431 = 862 kJ
- Total energy out: 862 kJ
Step 3: Calculate ΔH
ΔH = 678 − 862 = −184 kJ/mol
Conclusion: ΔH is negative, so the reaction is exothermic. More energy is released forming H-Cl bonds than is needed to break the H-H and Cl-Cl bonds. This reaction can proceed spontaneously once initiated (e.g., by UV light or a spark).
Compare with the first example: Both are exothermic, but ΔH for H₂ combustion (−482 kJ/mol) is much larger than for H₂ + Cl₂ (−184 kJ/mol). This is because the O-H bonds formed in water (463 kJ/mol each) are much stronger than H-Cl bonds (431 kJ/mol).
Quick Check: Is breaking bonds exothermic or endothermic? What about making bonds?
Breaking bonds is ENDOTHERMIC — it always requires energy input. Making (forming) bonds is EXOTHERMIC — it always releases energy. Remember: Breaking = Bad (needs energy in), Making = Magic (releases energy out). A common exam misconception is getting these the wrong way round.