From the definition of enthalpy as , the enthalpy change at constant pressure . However for most chemical reactions, the work term is much smaller than the internal energy change which is approximately equal to . As an example, for the combustion of carbon monoxide 2 CO(g) + O2(g) → 2 CO2(g), = −566.0 kJ and = −563.5 kJ. Since the differences are so small, reaction enthalpies are often loosely described as reaction energies and analyzed in terms of bond energies.