Teaching Thermodynamics of Ideal Solutions: An Entropy-Based Approach To Help Students Better Understand and Appreciate the Subtleties of Solution Models

Abstract

The thermodynamic formalism of ideal solutions is developed in most of the textbooks postulating a form for the chemical potential of a generic component, which is adapted from the thermodynamics of ideal gas mixtures. From this basis, the rest of useful thermodynamic properties can be derived straightforwardly without further hypothesis. Although formally elegant, this approach to ideal solutions does not allow appreciation of subtle concepts embodied in the model such as requirements of molecular size and shape or the fact that equations that contain the universal gas constant (R) can be applied to describe liquid or solid solutions. As alternative, it is discussed here an approach centered on the behavior of the partial molar entropy of the component using the framework provided by the concept of accessible volume. It is shown that this way of presenting the topic allows a more natural flow and, particularly, analytical justification of all the hypothesis and ideas behind many fundamental solution models, including that of ideal solutions, with the extra advantage that it can almost entirely carried out from a macroscopic point of view.