Aqueous Electrolyte Solutions

In dealing with the thermodynamic properties of ions we have one difficulty in addition to those encountered in dealing with compounds and elements. For compounds and elements we found that although we could measure absolute values for some properties, others such as enthalpy and the other energy terms contained an undetermined constant. We got around this by using the concept of "formation from the elements." It would of course be very convenient to also have thermodynamic properties of individual ions, but because positively and negatively charged ions cannot be separated from each other to any significant extent, their individual properties cannot be measured. To get around this, we need an additional convention, while retaining the formation from the elements convention. In addition we have certain problems in dealing with the activities and activity coefficients of electrolytes and individual ions. In the following section we discuss the problems of activities of ionic species. We follow the presentation of Klotz (1964), and include the HC1 example used by Pitzer and Brewer (1961), and an expanded consideration of the choice of solute components. Following that we discuss the conventions used to obtain numerical values for the state variables of individual ions. We begin by demonstrating that the basic approach is not arbitrarily chosen by chemists with a view to confusing students, nor is it dictated by the electrically charged nature of ions. It is dictated by the algebraic consequence of the fact that when neutral solute molecules dissociate into charged particles, the number of solute particles is increased. For example, when one mole of the undissociated solute AB(aq), which can be treated using Henry's Law, Raoult's Law, and the rest of the equations developed in previous chapters, becomes instead one mole of A(aq) plus one mole of B(aq), certain consequences develop that have nothing to do with whether A(aq) and B(aq) are electrically charged or not.