Bond Energy

Bond energy is the energy needed to break one mole of a particular bond in gaseous molecules, measured in kJ/mol. It is always a positive number because pulling bonded atoms apart always costs energy. A stronger bond has a higher bond energy: a C≡C triple bond is harder to break than a C=C double bond, which is harder than a C–C single bond.

Breaking bonds is endothermic - energy must be put in. Making bonds is exothermic - energy is given out. The energy to break a bond is exactly equal to the energy released when that same bond reforms. So a reaction is a tug of war: energy in to break the reactant bonds, energy out as the product bonds form.

To find the overall energy change, add up the energy needed to break all the bonds in the reactants, then subtract the energy released making all the bonds in the products. ΔH = (sum of bonds broken) − (sum of bonds made). If more energy comes out than went in, ΔH is negative and the reaction is exothermic. If more goes in than comes out, ΔH is positive and it is endothermic.

An energy-level (reaction profile) diagram shows the journey. Reactants start at one level, climb over an energy hill (the activation energy), then settle at the product level. If products sit lower than reactants the reaction is exothermic; if higher, endothermic. The height of the hill is the activation energy - the minimum energy needed to get the reaction going.