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5.4 Bond energy calculations

How to use bond energies to calculate the overall energy change of a reaction, why the answer is negative for exothermic reactions and positive for endothermic reactions, and worked examples.

Higher only AQA Edexcel OCR 1 min read

Bond energies

Key terms

  • Bond energy: the energy needed to break one mole of a particular covalent bond, measured in kJ/mol.
  • Breaking bonds takes in energy (endothermic).
  • Making bonds releases the same amount of energy (exothermic).
energy change = sum of bonds broken − sum of bonds made
  • A negative answer means the reaction is exothermic.
  • A positive answer means the reaction is endothermic.
Bars comparing the 679 kJ/mol needed to break the H–H and Cl–Cl bonds with the 864 kJ/mol released making two H–Cl bonds, giving an energy change of −185 kJ/mol (exothermic)
Bars comparing the 679 kJ/mol needed to break the H–H and Cl–Cl bonds with the 864 kJ/mol released making two H–Cl bonds, giving an energy change of −185 kJ/mol (exothermic)

Method

  1. Draw the displayed formula of every molecule so you can count the bonds.
  2. Add up the energy needed to break all the bonds in the reactants.
  3. Add up the energy released making all the bonds in the products.
  4. Subtract: bonds broken − bonds made.

Example: hydrogen + chlorine

H2 + Cl2 → 2HCl (bond energies: H–H 436, Cl–Cl 243, H–Cl 432 kJ/mol)

  • broken: 436 + 243 = 679 kJ/mol
  • made: 2 × 432 = 864 kJ/mol
  • energy change = 679 − 864 = −185 kJ/mol (exothermic)

Example: burning methane

CH4 + 2O2 → CO2 + 2H2O (C–H 413, O=O 498, C=O 805, O–H 464 kJ/mol)

  • broken: (4 × 413) + (2 × 498) = 1652 + 996 = 2648 kJ/mol
  • made: (2 × 805) + (4 × 464) = 1610 + 1856 = 3466 kJ/mol
  • energy change = 2648 − 3466 = −818 kJ/mol
Some bond energies
BondBond energy in kJ/mol
H–H436
Cl–Cl243
H–Cl432
C–H413
O=O498
C=O805
O–H464

Test yourself on this lesson

5 exam questions · 12 marks · with mark schemes and model answers

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