Energy ChangesWorked Example

Worked Example: Combustion of Hydrogen

Part of Bond Energies (HT) — GCSE Chemistry

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 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 5 of 13 in this topic. Treat this as a marking guide for what examiners are looking for, not just a fact list.

Topic position

Section 5 of 13

Practice

20 questions

Recall

15 flashcards

🧮 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.

Quick Check: Is breaking bonds exothermic or endothermic? What about making bonds?

Practice Questions for Bond Energies (HT)

Which statement correctly describes the energy change when chemical bonds are broken?

A. Energy is released to the surroundings
B. Energy is absorbed from the surroundings
C. No energy change occurs
D. Energy may be released or absorbed depending on the bond

1 markfoundation

Explain how you would determine, from a bond energy calculation, whether a reaction is exothermic or endothermic.

2 marksstandard

Worked Example: Combustion of Hydrogen

🧮 Worked Example: Combustion of Hydrogen

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Practice Questions for Bond Energies (HT)

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