Hess's law is useful in the determination of enthalpies of the following:
#Heats of formation of unstable intermediates like CO and NO.
#Heat changes in phase transitions and allotropic transitions.
#Lattice energies of ionic substances by constructing Born–Haber cycles if the electron affinity to form the anion is known, or
#Electron affinities using a Born–Haber cycle with a theoretical lattice energy.