= Kirchhoff's law of thermochemistry
{c}
{title2=$(\partial\Delta H/\partial T)_p=\Delta C_p$}
At fixed <pressure>, the <temperature> <derivative> of a reaction or transformation <enthalpy> is the corresponding difference in <heat capacity at constant pressure>:
$$
\left(\frac{\partial\Delta H}{\partial T}\right)_p=\Delta C_p,\qquad
\Delta H(T_2)=\Delta H(T_1)+\int_{T_1}^{T_2}\Delta C_p(T)\,dT.
$$
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>.
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