Bond energies

In a reaction, bonds in the reactants break and new bonds form in the products. Breaking bonds takes in energy (endothermic) and making bonds gives out energy (exothermic). The bond energy is the energy needed to break one mole of a bond. Adding up the bonds broken and the bonds made gives the overall energy change of the reaction.

Key facts

  • Breaking bonds is endothermic: energy is taken in. Making bonds is exothermic: energy is given out.
  • Bond energy: the energy needed to break one mole of a particular bond, in kJ/mol.
  • Energy change = energy needed to break bonds − energy released making bonds.
  • If more energy is released making bonds than is needed to break them, the reaction is exothermic and the energy change is negative.
  • Count every bond: draw the displayed formulae, and multiply by the numbers in front of each formula.
  • Bond energies are mean values, so the answer is an estimate.

Worked examples

Example 1

Hydrogen reacts with chlorine: H2 + Cl2 → 2HCl. Bond energies in kJ/mol: H–H 436, Cl–Cl 243, H–Cl 432. Calculate the energy change.

  1. Bonds broken: one H–H and one Cl–Cl. 436 + 243 = 679 kJ
  2. Bonds made: two H–Cl. 2 × 432 = 864 kJ
  3. Energy change = broken − made. 679 − 864 = −185 kJ/mol
  4. More energy is given out than taken in, so the reaction is exothermic.

Answer: −185 kJ/mol (exothermic)

Example 2

Methane burns: CH4 + 2O2 → CO2 + 2H2O. Bond energies in kJ/mol: C–H 413, O=O 498, C=O 805, O–H 464. Calculate the energy change.

  1. Bonds broken: four C–H in methane and two O=O. 4 × 413 + 2 × 498 = 1652 + 996 = 2648 kJ
  2. Bonds made: two C=O in carbon dioxide and four O–H in two water molecules. 2 × 805 + 4 × 464 = 1610 + 1856 = 3466 kJ
  3. Energy change = broken − made. 2648 − 3466 = −818 kJ/mol

Answer: −818 kJ/mol

Example 3

For N2 + 3H2 → 2NH3 the energy change is −93 kJ/mol. N≡N is 945 kJ/mol and H–H is 436 kJ/mol. Find the N–H bond energy.

  1. Bonds broken: one N≡N and three H–H. 945 + 3 × 436 = 2253 kJ
  2. Bonds made: two NH3 molecules, each with three N–H bonds: 6 N–H. −93 = 2253 − 6x
  3. Rearrange. 6x = 2253 + 93 = 2346
  4. Divide by 6. x = 391 kJ/mol

Answer: 391 kJ/mol