Estimating Temperature Effects on Heat Capacity and Other Thermodynamic Properties

Estimating Temperature Effects on Heat Capacity and Other Thermodynamic Properties 

In the previous section, the heat capacity was treated as being independent of temperature during equilibrium constant calculations. Although most thermodynamic property data is measured at 25 C, industrial applications demand accurate data for higher temperatures. The desirability of a method for accurately predicting elevated temperature thermodynamic properties in order to avoid the need for experimental measurements is obvious. In 1964, Criss and Cobble (9, 10) proposed the correspondence principle as such a method. They summarized it as foUows (9)  

Criss and Cobble noted that the entropies of ions can be expressed as functions of mass, charge and ionic size, and that the functional dependences appear to be the same at 25°C and higher temperatures. This suggested that it would not be necessary to know the complete functional dependences and the entropies at elevated temperatures could be determined from 25°C entropies. They suggested that the following equation could be used to determine the standard state entropy at tj having an experimental standard state entropy at temperature ti  

In testing their theory they experienced the problem of the lack of reliable data at elevated temperatures. They noted that the best thermodynamic function for calculating the partial molal entropies are the partial molal heat capacities, Cp°, and that few systems had been studied at temperatures greater than 100°C. Acceptable estimates of could be obtained given the partial molal heat 

Based on the experimental partial molal entropies. Criss and Cobble presented values for the a and b parameters of equation (3.32), which are tabulated in Appendix 3.1. They note a surprising accuracy of 0.5 cal. mole l deg l for simple ions up to 150 C  

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