Bond Enthalpy

It requires an input of energy to break apart bonds (endothermic). Forming bonds, therefore, releases energy (exothermic) because it is the reverse reaction. Breaking bonds should always be a positive amount of energy and making bonds should always be a negative amount of energy.

Bond Enthalpy is the energy released by breaking one mole of a specific type of bond. Note that the table below shows only positive energy values for breaking bonds. Bond enthalpy changes depending on which elements are bonded together.

Remember that Enthalpy is a state based function and it doesn't matter the pathway that is taken, but only the starting and end conditions. Therefore we can apply Hess's Law to determine the enthalpy of a reaction by knowing the standard bond enthalpies and the number of bonds in the reactants and products.

We calculate the energy required to break each bond in the reactants (positive because it is endothermic) and then subtract the energy released by forming the bonds in the products (negative because it is exothermic).