The standard enthalpies of many important reactions have been measured at different
temperatures. However, in the absence of this information, standard reaction
enthalpies at different temperatures may be calculated from heat capacities and the
reaction enthalpy at some other temperature (Fig. 2.20). In many cases heat capacity
data are more accurate than reaction enthalpies. Therefore, providing the information
is available, the procedure we are about to describe is more accurate than the
direct measurement of a reaction enthalpy at an elevated temperature.