Kirchhoff's law of thermochemistry ($(\partial \Delta H/\partial T{)}_p=\Delta C_p$)

At fixed pressure, the temperature derivative of a reaction or transformation enthalpy is the corresponding difference in heat capacity at constant pressure:Use a consistent molar or mass basis and retain each phase's range of stability when applying the relation. A latent enthalpy evaluated along a pressure-dependent phase coexistence curve can also have a pressure-variation contribution; the fixed-pressure derivative must not automatically be identified with that full derivative.