General Concepts of Chemical Equilibrium

The need to abstract from the considerable complexity of real natural water systems and substitute an idealized situation is met perhaps most simply by the concept of chemical equilibrium in a closed model system. Figure 2 outlines the main features of a generalized model for the thermodynamic description of a natural water system. The model is a closed system at constant temperature and pressure, the system consisting of a gas phase, aqueous solution phase, and some specified number of solid phases of defined compositions. For a thermodynamic description, information about activities is required therefore, the model indicates, along with concentrations and pressures, activity coefficients, fiy for the various composition variables of the system. There are a number of approaches to the problem of relating activity and concentrations, but these need not be examined here (see, e.g., Ref. 11). 

A system that undergoes no net chemical changes over time is said to be in chemical equilibrium. Generally, this condition occurs when the forward and reverse rates of the chemical reactions taking place in a system are equal. Common phenomena, including many phenomena associated with living organisms, can be analyzed in terms of this concept of chemical equilibrium. Several examples are presented below. 

This last point is central to the concept of chemical equilibrium. It makes no difference whether we start with two moles of HI or one mole each of H2 and I2 once the reaction has run to completion, the quantities of these two components will be the same. In general, then, we can say that the composition of a chemical reaction system will tend to change in a direction that brings it closer to its equilibrium composition. Once this equilibrium composition... 

In this chapter we apply the general criteria for equilibrium developed in Chap. 6 to systems in which chemical reactions may occur. In Sec. 8-1, we present a general discussion of chemical equilibrium in homogeneous and heterogeneous systems. The concept of a progress variable is introduced, and the conditions for chemical equilibrium are derived. The equilibrium constant is defined, and some of its properties are developed. A discussion of the Le Chatelier-Braun principle applied to chemical reactions is presented. In Sec. 8-2, the results of Sec. 8-1 are applied to chemical reactions in mixtures of real gases. 

Specifically, the major topics covered by this review are (1) a brief discussion of models of element release rates based on diffusion and on TST, and (2) glass-water reactions that dominate near equilibrium. We will discuss these themes with the assumption of some general understanding of chemical kinetics, but the concepts should be comprehensible to readers from outside this field as well. 

Science is fundamentally empirical—it is based on experiment. The development of the equilibrium concept is typical. From observations of many chemical reactions, two Norwegian chemists, Cato Maximilian Guldberg (1836-1902) and Peter Waage (1833-1900), proposed the law of mass action in 1864 as a general description of the equilibrium condition. For a reaction of the type... 

Because of its simplicity, the concept of Donnan equilibrium is an attractive approach for the modelling of fibre-ion interactions. However, despite its general success in many cases, the Donnan theory alone is not sufficient to describe the experimentally observed distributions. In those cases, it is necessary to use ion-specific complexation equilibria with the acidic groups as a part of the model to describe the observed phenomena satisfactorily. Such behaviour has been shown to exist, for example, with chemically modified highly charged fibres. An apparent systematic deviation in the ionic distribution from the predictions of the Donnan theory has also been noted with common oxygen-delignified kraft pulps, partly, but... 

Irreversible reactions can go one way only. Equilibrium is not possible in general, or for existing conditions of temperature and pressure. The status of such reactions can often be described using the concepts of chemical kinetics. Radioactive decay is generally such an irreversible reaction. For example... 

Understand the general concept of equilibrium, which is very important in the application of thermodynamics in chemical engineering Understand the difference between intensive and extensive variables Understand that total mass and total energy are conserved in any process... 

A conception of abridged description of non-equilibrium systems is not principally a new theory of chemical kinetics, since the Bodenshtein-Semenov s principle has been used as a principle of quasi-stationary reactions for a long time. When applicable to a state, the Bodenshtein-Semenov s principle confirms that the quasi-stationary state of a chemical system is described essentially more simply than these processes, realization of which leads to the achievement of this state. Exactly from this point of view, Bodenshtein-Semenov s principle by itself represents a partial formulation of general conception of abridged description of non-equilibrium systems, according to which every state of the evolution can be and should be described by a different set of independent values. This, in turn, means that characteristic numbers of a chemical system need to be considered via an evolution waiting for their successive reduction when the chemical system approaches the equilibrium state. 

What conceptions do Philippine general chemistry students have before instruction about chemical reactions, regarding reversible reactions and the attainment of chemical equilibrium ... 

The concept of chemical potential is very general, applicable to almost any transformation of matter as long as there is a well-defined temperature. We have already seen how the condition for thermodynamic equilibrium for chemical reactions leads to the law of mass action. We shall now see how diffusion, electrochemical reactions and relaxation of polar molecules in the presence of an electric field, can all be viewed as chemical transformations with associated chemical potential and affinity. 

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