Refining equilibrium concepts

Zone refining is a dynamic nonequilibrium separation process. However, to understand its essential features it is important to understand the equilibrium concepts on which it is based. Of central importance is the notion of the equilibrium distribution coefficient, k0 Cg/, where Cg and Cl are the concentrations of solute at equilibrium in the solid and liquid phases respectively. Since the value of ko may be dramatically different from unity, it is clear that at equilibrium a solute may distribute itself between solid and liquid phases with a great preference for one or the other at a given temperature. On this basis the relative amount of solute in each phase can be controlled, and a separation can be carried out. 

In natural waters organisms and their abiotic environment are interrelated and interact upon each other. Such ecological systems are never in equilibrium because of the continuous input of solar energy (photosynthesis) necessary to maintain life. Free energy concepts can only describe the thermodynamically stable state and characterize the direction and extent of processes that are approaching equilibrium. Discrepancies between predicted equilibrium calculations and the available data of the real systems give valuable insight into those cases where chemical reactions are not understood sufficiently, where nonequilibrium conditions prevail, or where the analytical data are not sufficiently accurate or specific. Such discrepancies thus provide an incentive for future research and the development of more refined models. 

The refined treatment of surface equilibria considers different kinds of surface sites with respect to their reactivity [25-27]. Formally one may solve this problem by using any of the three above concepts by introducing two (or more) different values of the corresponding equilibrium constants. In doing so the mass balance of surface sites should be corrected appropriately. 

The concepts of an acid, a base, and a salt are ancient ones that modern chemical science has adopted and refined. Our treatment of the subject at this stage will be mainly qualitative, emphasizing the definitions and fundamental ideas associated with acids and bases. The quantitative treatment of acid-base equilibrium systems is treated in another unit. 

On the other hand, I have not attempted any refined analysis of conceptions such as probability, or statistical equilibrium, since these are not difficult to have a working knowledge of, though purists could subject to harassing criticism almost any treatment save a very elaborate one. Excessively detailed analysis however, would have obscured the general plan which I was anxious to try and depict. 

Kekuld described the exchange of double and single bonds to be an equilibrium process. Over time, this view was refined by the advent of resonance theory and molecular orbital concepts of delocalization. The two drawings above are now viewed as resonance structures, not as an equilibrium process. 

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