Chemical bonds hold atoms together in molecules. To break or form bonds, energy is transferred. We can use bond energy values to calculate the overall energy change in a reaction.

What is bond energy?

• Bond energy is the energy needed to break one mole of a specific bond
• It is measured in kJ/mol
• All chemical reactions involve breaking bonds in reactants and forming bonds in products

Energy changes in reactions

• Breaking bonds = takes in energy → endothermic
• Forming bonds = releases energy → exothermic

💡 If more energy is released than absorbed → exothermic reaction

💡 If more energy is absorbed than released → endothermic reaction

Bond energy calculation formula

Energy change = total energy in (breaking bonds) – total energy out (making bonds)

Worked example

Hydrogen + chlorine → hydrogen chloride
H₂ + Cl₂ → 2HCl

Bond energies:

• H–H = 436 kJ/mol
• Cl–Cl = 243 kJ/mol
• H–Cl = 431 kJ/mol

Step 1: Energy in (bonds broken)
H–H = 436
Cl–Cl = 243
Total in = 679 kJ

Step 2: Energy out (bonds formed)
2 × H–Cl = 2 × 431 = 862
Total out = 862 kJ

Step 3: Energy change
679 – 862 = –183 kJ

✅ This is exothermic (negative energy change)

Tips for bond energy calculations

• Only use the bonds shown in the balanced equation
• Multiply bond energies by the number of bonds
• Subtract: energy in – energy out
• A negative answer = exothermic, positive = endothermic