A bond energy is the energy needed to break one mole of a particular bond. It is measured in kJ/mol. You can use bond energies to work out the overall energy change of a reaction.
Step 1: add up the energy needed to break all the bonds in the reactants. Step 2: add up the energy released making all the bonds in the products. Step 3: subtract, using energy change = energy in (bonds broken) - energy out (bonds made).
Example: H2 + Cl2 → 2HCl. The bond energies are H-H 436, Cl-Cl 243 and H-Cl 432 kJ/mol. Bonds broken: 436 + 243 = 679 kJ/mol. Bonds made: 2 × 432 = 864 kJ/mol. Energy change = 679 - 864 = -185 kJ/mol.
A negative answer means more energy was released than taken in, so the reaction is exothermic. A positive answer means the reaction is endothermic.