The Concept of Chemical Equilibrium An Introduction

This chapter introduces very elementary, but fundamental, ideas and concepts of chemical equilibrium. Most of the topics mentioned will be developed in further chapters. However, it is very important that the simple concepts given in this chapter are understood before proceeding further. This chapter is more an explanatory revision exercise than a formal development of new ideas. 

An Introduction to Aqueous Electrolyte Solutions. By Margaret Robson Wright 2007 John Wiley Sons Ltd ISBN 978-0-470-84293-5 (cloth) ISBN 978-0-470-84294-2 (paper) 

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The usual emphasis on equilibrium thermodynamics is somewhat inappropriate in view of the fact that all chemical and biological processes are rate-dependent and far from equilibrium. The theory of non-equilibrium or irreversible processes is based on Onsager s reciprocity theorem. Formulation of the theory requires the introduction of concepts and parameters related to dynamically variable systems. In particular, parameters that describe a mechanism that drives the process and another parameter that follows the response of the systems. The driving parameter will be referred to as an affinity and the response as a flux. Such quantities may be defined on the premise that all action ceases once equilibrium is established. 

A more recent version of the learning package has been published in English (Ben-Zvi Silberstein, 1994). It is meant for students at an advanced level in secondary schools aiming at an introduction of sophisticated concepts like entropy and Gibbs energy, and the thermodynamic treatment of chemical equilibrium. 

The opening chapter An introduction to modelling of pollutants in the environment by Trevor M. Letcher demonstrates convincingly that equilibrium concepts and simple models lead to realistic predictions of, for example, the concentration of a polychlorinated biphenyl in the fishes of the St. Lawrence River. Relative solubilities expressed by octanol-water and air-water partition coefficients play a crucial role for estimating the distribution of chemicals in the environment. This is pointed out in the introductory chapter as well as in others such as Estimation of volatilization of organic chemicals from soil by Epaminondas Voutsas. 

With the introduction of the concept of activity, the equilibrium conditions for chemical reactions and phase transformations can be treated systematically. Assume, for example, an isothermal reaction or transformation of the following... 

Depending on the nature of the class, the instructor may wish to spend more time with the basics, such as the mass balance concept, chemical equilibria, and simple transport scenarios more advanced material, such as transient well dynamics, superposition, temperature dependencies, activity coefficients, the thermodynamics of redox reactions, and Monod kinetics, may be omitted. Similarly, by excluding Chapter 4, an instructor can use the text for a course focused only on the water environment. In the case of a more advanced class, the instructor is encomaged to expand on the material suggested additions include more rigorous derivation of the transport equations, discussions of chemical reaction mechanisms, introduction of quantitative models for atmospheric chemical transformations, use of computer software for more complex chemical equilibrium problems and groundwater transport simulations, and inclusion of case studies. References are provided with each chapter to assist the more advanced student in seeking additional material. 

A defining characteristic of a solid is the ability to resist shear. Therefore, stress is an additional feature which has to be taken into account when the physical chemistry of solids is at issue. Gibbs treated the thermodynamics of stressed solids a century ago in his classic work Equilibrium of Heterogeneous Substances under the title The Conditions of Internal and External Equilibrium for Solids in Contact with Fluids with Regard to all Possible States of Strain of the Solid . We have already mentioned in the introduction that stress is an unavoidable result of chemical processes in solids. Let us therefore briefly discuss the basic concepts of the thermodynamics of stressed solids. 

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