Other conditions of equilibrium

The most important conditions of equilibrium are those concerning the temperature, pressure, and chemical potential given in the preceding sections. However, other equivalent conditions are also useful, and it is well to develop them here. 

We may be interested in the sign of the change of enthalpy for variations of a system that is already at equilibrium. We do have the knowledge that the pressure is uniform throughout the system. We may use Equation (4.23), from which 

Using the same arguments as in Section 5.3, we find that 0 

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