Affinity and chemical equilibrium in solutions

The affinity of a reaction taking place in a non-reactive solvent is given by (7.74) and the law of mass action by (7.75). If the reaction occurs between dissolved molecules in very dilute solution, the affinity is readily expressed as a function of the molar concentrations (20.47) or the molalities (20.51). Instead of (7.27) we now have to define two new equilibrium constants and  

The quantities and are related to the equilibrium constant by the following relations which follow from (20.49) and (20.53), 

The derivatives of with respect to T or p in an ideal system are given by (7.37) and (7.36). In a non-ideal solution these equations become 

From equation (20.70 ) for the affinity, we can define a standard affinity just as in (7.79), and calling this we have 

This standard affinity is related to the standard affinity by the relation 

---