Bond Energy – IGCSE Chemistry Definition
IGCSE Chemistry definition of bond energy: the energy needed to break one mole of a particular covalent bond. Covers calculations to determine exothermic or endothermic.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
Bond energy is the energy needed to break one mole of a specific covalent bond. It is a Supplement topic on the 0620 syllabus and is used to calculate whether a reaction is exothermic or endothermic. Bond energy calculations are a common Paper 4 question worth 3-4 marks.
The 0620 definition
Bond energy is the energy required to break one mole of a particular covalent bond in the gaseous state, measured in kJ/mol. It is also the energy released when one mole of that bond is formed.
Breaking bonds is endothermic (requires energy input). Making bonds is exothermic (releases energy).
Common bond energies
| Bond | Bond energy (kJ/mol) |
|---|---|
| H-H | 436 |
| C-H | 412 |
| C-C | 348 |
| C=C | 612 |
| O-H | 463 |
| O=O | 496 |
| C=O (in CO₂) | 803 |
| C-O | 360 |
| Cl-Cl | 242 |
| H-Cl | 431 |
| N-H | 388 |
(Values given in the exam — you do not need to memorise them)
The calculation method
- List all bonds broken in the reactants and add their energies (total energy in)
- List all bonds formed in the products and add their energies (total energy out)
- Overall energy change = total energy to break bonds - total energy to form bonds
- If the answer is negative: exothermic. If positive: endothermic.
Worked example
H₂ + Cl₂ → 2HCl
Bonds broken: 1 x H-H (436) + 1 x Cl-Cl (242) = 678 kJ
Bonds formed: 2 x H-Cl (2 x 431) = 862 kJ
Energy change = 678 - 862 = -184 kJ/mol
Negative value, so the reaction is exothermic — more energy is released forming bonds than is used breaking bonds.
Worked exam question
Use the bond energies below to calculate the energy change for: CH₄ + 2O₂ → CO₂ + 2H₂O. C-H = 412, O=O = 496, C=O = 803, O-H = 463. (3)
Mark scheme
Bonds broken: 4 x C-H + 2 x O=O = 4(412) + 2(496) = 1648 + 992 = 2640 kJ [1]
Bonds formed: 2 x C=O + 4 x O-H = 2(803) + 4(463) = 1606 + 1852 = 3458 kJ [1]
Energy change = 2640 - 3458 = -818 kJ/mol (exothermic) [1]
Common exam mistakes
- Forgetting to count all the bonds. CH₄ has four C-H bonds, not one. 2H₂O has four O-H bonds, not two.
- Subtracting the wrong way round. Always: bonds broken - bonds formed. A negative answer = exothermic.
- Saying “breaking bonds releases energy”. Breaking bonds always requires energy (endothermic). Making bonds releases energy (exothermic).
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