Pressure and Temperature Dependence of AG

When studying geological problems, we are particularly interested in reactions that take place over a wide range of pressures and temperatures. Therefore, we are 

If Equation (7.64) is integrated from a reference temperature and pressure of 298 K and P = P° to any temperature T and pressure P, we obtain 

Erom Equation (7.65), we can see that the first integral on the right side is integrated with respect to temperature at constant pressure P° and that the second integral on the right side is integrated with respect to pressure at constant temperature T. Thus, the temperature dependence and the pressure dependence can be dealt with independently. 

Temperature Dependence. A simple expression for the temperature dependence is found if we remember that [Equation (7.26)] 

It can be seen from Equation (7.70) that to calculate AG at any temperature and pressure we need to know values of AH and AS at standard conditions (P= 100 kPa, T = 298 K), the value of ACp as a function of temperature at the standard pressure, and the value of AEj- as a function of pressure at each temperature T. Thus, the temperature dependence of AC/ and the temperature and pressure dependence of AVj-are needed. If such data are available in the form of empirical equations, the required integrations can be carried out analytically. If the data are available in tabular form, graphical or numerical integration can be used. If the data are not available, an approximate result can be obtained by assuming ACp and AVp are constant 

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The pressure and temperature dependence of AG (AG free energy between U and N is given by... 

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