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