Bond Enthalpies and Calculating Reaction Enthalpy

At the molecular level, chemical reactions are the breaking and making of bonds. Breaking a bond requires energy because the attractive forces holding atoms together must be overcome, so bond breaking is always endothermic. Making a bond releases energy as atoms fall into a lower-energy arrangement, so bond making is always exothermic. The overall enthalpy change of a reaction is the net result of these two opposing processes, and this insight lets us estimate $\Delta H$ from tabulated bond energies.

The bond enthalpy (or bond dissociation energy) is the energy required to break one mole of a particular covalent bond in the gaseous state, averaged over many different molecules. For example, the C–H bond enthalpy of $+414 \text{ kJ mol}^{-1}$ is a mean value because the exact energy to brea