Some Basic Concepts and Definitions

The influence of chemical equilibrium and/or kinetics on the progress of chemical reactions often determines the abundance, distribution, and fate of substances in the environment. An understanding of the basic concepts of chemical equilibrium and chemical kinetics, therefore, may help us to explain and predict the environmental concentrations of inorganic and organic species in aqueous systems, whether these species are present naturally or have been introduced by humans. In this chapter we will examine chemical equilibrium. The following chapter considers chemical kinetics or the study of rates of chemical reactions. 

Given sufficient time, chemical substances in contact with each other tend to come to chemical equilibrium. Chemical equilibrium is the time-invariant, most stable state of a closed system (the. state of minimum Gibbs free energy). We study chemical equilibrium concepts so as to learn the direction of spontaneous change of chemical reactions in any system, especially for conditions of constant temperature and pressure. We want to be able to compute the hypothetical equilibrium stale of a system. We would like to predict the conditions for equilibrium in different systems and at different temperatures and pressures without having to measure them. 

The statements above have introduced several concepts that need to be defined and expanded upon. A system is a grouping of atoms, minerals, rocks, and/or gases and waters under consideration within a single volume of space, the boundaries of which can be defined as is convenient. A system could be one mineral grain, a drop of rain, a water-logged soil, a well-mixed lake, or a regional groundwater/rock system tens of kilometers in diameter. 

Whether a system can be considered open or closed depends not only on the specific substances under study, but also on both the rates of flux of matter in and out of the system and the time scale of interest. For slow rates of flux and/or short time scales, systems tend to be closed with respect to many substances. Given fast rates of flux and/or long time scales, systems will behave as if they were open with respect to many substances. Also, if reaction rates are much faster than flux rates of related components in and out of the system, we can assume the system is closed (and vice versa). 

A phase is a restricted part of a system with distinct physical and chemical properties (Wood and Fraser 1976). A phase can also be defined as a physically and chemically homogeneous portion of a system with definite boundaries (Brownlow 1979). These attributes mean that a phase should be mechanically separable from a system. Example phases are minerals and well-mixed gases and liquids. Not true phases, because they are comprised of more than one mineral, are rocks such as granite or minerals such as the feldspars when they are chemically zoned and have spatially variable compositions. 

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In this Chapter, we quickly review some basic definitions and concepts from thermodynamics. We then provide a brief description of the first and second laws of thermodynamics. Next, we discuss the mathematical consequences of these laws and cover some relevant theorems in multivariate calculus. Finally, free energies and their importance are introduced. 

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