The power of thermodynamic modeling; examples from molten halide mixtures
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. INTRODUCTION
a id NaCl 5 (XNa1)(XCl2)
IT is a great pleasure for me to present this article on thermodynamic modeling at the Milton Blander Symposium. Milton Blander has all his life been a strong promotor of the use of thermodynamic models. He also came early into my scientific life and one of his articles was Reference 6 in my Ph.D. Thesis. His book, Molten Salt Chemistry, from 1964[1] is still a classic. Since then, he has successfully modeled numerous partly very complicated systems. The present article will use simpler concepts than Blander often uses, although the final models may appear somewhat involved. The object of the present article is to demonstrate the descriptive and predictive power of structurally based models. Thermodynamics is the science of transformation of energy. The most common experimental methods to obtain thermodynamic information are measurement of enthalpy (H ) by calorimetry, free enthalpy (G) by electromotive force (EMF), or phase equilibria and PVT relationships. The link to structure is through entropy and statistical thermodynamics. A useful first approximation structural model for molten ionic systems is the so-called ideal Temkin model. Due to Coulombic forces, ionic crystals are ordered in a way that cations are only surrounded by anions and vice versa. An interchange of an anion and a cation in the lattice is energetically highly unfavorable. The Temkin model for molten salt mixtures transfers the ideas for a solid mixed crystal to the liquid. The molten salt has a “cation lattice” and an “anion lattice” so that
where n stands for number of mols and X for ion fraction. Another useful concept for molten salts has been the assumption of defined structural species, so-called complexes. The term “complex” has a less-defined meaning in molten salts, as the proposed entities are not isolated from each other by the solvent as for instance in water. Because of the somewhat unclear separation of possible complexes, a debate raged in the 1960s on the justification of the concept for molten salts. This discussion has ceased due to the usefulness of such models and because several of the proposed complexes have been identified by vibration spectroscopy. Usefulness means that the thermodynamic properties can be explained in a simple and logical way by the assumption of complexes. One should, however, always heed the warning professor Ha˚kon Flood gave us as students: “Thermodynamic experimental data can tell in principle only if a suggested structural model is possible or not, but they will frequently be unable to decide which one of several structures is correct or if a particular structure is the only one possible. It is also important to keep Hildebrands warning in mind that principally an insufficient model may still fit the activity (DG) data well, however, when applied to DS and DH-data much more serious disagreement will be evident.”
S 5 S+ 1 S2
[1]
The Temkin model was proposed by Temkin[2] and later applied by Flood et al.[3] Using the molten mixtures with NaCl as an example
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