Dependence of the Gibbs Function on External Field

In our exposition of the properties of the Gibbs function G (Chapter 7), we examined systems with constraints on them in addition to the ambient pressure. We found that changes in Gibbs function are related to the maximum work obtainable from an isothermal transformation. In particular, for a reversible transformation at constant pressure and temperature [Equation (7.79)], 

Chemical Thermodynamics Basic Concepts and Methods, Seventh Edition. By Irving M. Klotz and Robert M. Rosenberg 

Of course, the equahty in Equation (7.79) is symmetric that is, the equation may be read in the mirror-image direction If we perform reversible (non-PV) work DWnei on a system at constant pressure and temperamre, we increase its Gibbs function by the amount dGx,p. For example, if we reversibly change the position x of a body in the gravitational field of the earth [Fig. 21.1(a)], we perform an amount of work given by 

If we consider lowering a body down a shaft [Fig. 21.1(h)] it is convenient to change our convention regarding the positive direction of x to downward. Hence, 

For a system of constant composition in which fields are absent, we found in Chapter 7 that because the Gibbs function G is a function of pressure and temperature, 

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DEPENDENCE OF THE GIBBS FUNCTION ON EXTERNAL FIELD 501 More generally, for constraints other than gravity, we can also state that... 

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