Читать книгу Geochemistry - William M. White - Страница 116

The assumption we made for ideal solution behavior was that interactions between molecules (species might be a better term in the case of electrolyte solutions) of solute and molecules of solvent were not different from those interactions between solvent ions only. In light of the discussion of aqueous solutions earlier, we can see this is clearly not going to be the case for an electrolyte solution. We have seen significant deviations from ideality even where the components have no net charge (e.g., water–ethanol); we can expect greater deviations due to electrostatic interactions between charged species.

The nature of these interactions suggests that a purely macroscopic viewpoint, which takes no account of molecular and ionic interactions, may have severe limitations in predicting equilibria involving electrolyte solutions. Thus, chemists and geochemists concerned with the behavior of electrolytes have had to incorporate a microscopic viewpoint into electrolyte theory. On the other hand, they did not want to abandon entirely the useful description of equilibria based on thermodynamics. We have already introduced concepts, the activity and the activity coefficient, which allow us to treat nonideal behavior within a thermodynamic framework. The additional task imposed by electrolyte solutions, and indeed all real solutions, therefore, is not to rebuild the framework, but simply to determine activities from readily measurable properties of the solution. The dependence of all partial molar properties of a solute on concentration can be determined once the activity coefficient and its temperature and pressure dependence are known.