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.