Thermodynamic partial derivatives constant, different values

Our analysis using standard state values implies dP = 0, and under that constraint, the second term in Equation 6.44 is zero. The remaining partial derivative is the constant-pressure heat capacity of the fth substance, Cp. To evaluate the enthalpy (or other thermodynamic state functions) at a temperature, call it T2, different from a temperature Ti for which there are known enthalpies, we invoke Hess s law, treating the temperature changes of reactants and products as reaction steps. Here is an example ... 

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