This worked example covers Worked Example: Combustion of Hydrogen within Bond Energies (HT) for GCSE Chemistry. Revise Bond Energies (HT) in Energy Changes for GCSE Chemistry with 25 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 5 of 15 in this topic. Treat this as a marking guide for what examiners are looking for, not just a fact list.
🧮 Worked Example: Combustion of Hydrogen
Calculate ΔH for: 2H₂ + O₂ → 2H₂O
Bond energies given:
- H-H bond = 436 kJ/mol
- O=O bond = 498 kJ/mol
- O-H bond = 463 kJ/mol
Step 1: Identify bonds broken (reactants)
- 2 × H-H bonds = 2 × 436 = 872 kJ
- 1 × O=O bond = 1 × 498 = 498 kJ
- Total energy in: 872 + 498 = 1370 kJ
Step 2: Identify bonds formed (products)
- 2H₂O contains 4 × O-H bonds = 4 × 463 = 1852 kJ
- Total energy out: 1852 kJ
Step 3: Calculate ΔH
ΔH = 1370 − 1852 = −482 kJ/mol
Conclusion: ΔH is negative, so the reaction is exothermic. More energy is released making bonds than is needed to break bonds.
Practice questions for Bond Energies (HT)
Which statement correctly describes the energy change when chemical bonds are broken?
Explain how you would determine, from a bond energy calculation, whether a reaction is exothermic or endothermic.