Thermodynamic Equilibrium and Kinetics

In general, chemical reactions are not necessarily explained in terms of thermodynamic equilibrium. In fact, many reactions do not proceed, in a practical sense, even when the products are energetically much more stable than the starting materials. Such reactions should be thermodynamically very favorable. Why do such reactions not proceed spontaneously It is because the reactions are too slow and do not proceed at an appreciable rate. In such cases we say the reactants are kinetically stable. If the reactions leading to the products have a very slow rate, the reactants do not change appreciably for a long time. 

Most organic substances are kinetically stable in air otherwise they could not exist. Major forms of life, including our bodies, consist of organic substances such as DNA, proteins, sugars, lipids, and so on. The 

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In deciding process conditions, the two principles of thermodynamic equilibrium and kinetics need to be considered together indeed, any complete rate equation for... 

These findings suggest that it may be possible to estimate lower bounds for the amount of C02 attainable from organic sources during pyrolysis, based on a knowledge of the carboxyl group content of the coal. Combination of this information with a determination of the carbonate content and appropriate thermodynamic equilibrium and kinetic calculations would allow estimates of the total amount of C02 evolution expected from pyrolysis. 

Chemistry can be divided (somewhat arbitrarily) into the study of structures, equilibria, and rates. Chemical structure is ultimately described by the methods of quantum mechanics equilibrium phenomena are studied by statistical mechanics and thermodynamics and the study of rates constitutes the subject of kinetics. Kinetics can be subdivided into physical kinetics, dealing with physical phenomena such as diffusion and viscosity, and chemical kinetics, which deals with the rates of chemical reactions (including both covalent and noncovalent bond changes). Students of thermodynamics learn that quantities such as changes in enthalpy and entropy depend only upon the initial and hnal states of a system consequently thermodynamics cannot yield any information about intervening states of the system. It is precisely these intermediate states that constitute the subject matter of chemical kinetics. A thorough study of any chemical reaction must therefore include structural, equilibrium, and kinetic investigations. 

There was therefore a clear need to assess the assumptions inherent in the classical kinetic approach for determining surface-catalysed reaction mechanisms where no account is taken of the individual behaviour of adsorbed reactants, substrate atoms, intermediates and their respective surface mobilities, all of which can contribute to the rate at which reactants reach active sites. The more usual classical approach is to assume thermodynamic equilibrium and that surface diffusion of reactants is fast and not rate determining. 

A central theme of our work in Calgary has been to seek connections between equilibrium and kinetic properties—that is, between thermodynamic and extrathermodynamic quantities (3). An equilibrium constant K is governed by... 

This review will introduce basic techniques for calculating equilibrium and kinetic stable isotope fractionations in molecules, aqueous complexes, and solid phases, with a particular focus on the thermodynamic approach that has been most commonly applied to studies of equilibrium fractionations of well-studied elements (H, C, N, O, and S) (Urey 1947). Less direct methods for calculating equilibrium fractionations will be discussed briefly, including techniques based on Mossbauer spectroscopy (Polyakov 1997 Polyakov and Mineev 2000). 

The equilibrium and kinetic behavior of chemical reactions are governed by the chemical potential, the temperature, and the pressure. The dependence on the former two thermodynamic variables has been studied quite extensively, both theoretically and experimentally, and it can be safely assumed that it is well understood in all details. The same cannot be said so far for the pressure variable for a number of reasons. As it has been discussed in previous sections, technical... 

Near-equilibrium and non-equilibrium reactions This steady-state condition is usually achieved by the presence, in the pathway, of two classes of reactions those that are very close to equilibrium (near-equilibrium) and those that are far removed from equilibrium (non-equilibrium). The difference between these two types of reactions can be explained both thermodynamically (above) and kinetically (in Chapter 3). 

The formation of an oxide layer is thermodynamically favourable and kinetically rapid at room temperature, but as the temperature rises, the free energy of oxide formation (originally negative) increases to the point where the metal, oxide and oxygen are in equilibrium. At temperatures above this equilibrium value, and if the oxygen partial pressure is low enough, the oxide can decompose. 

We emphasize several cautions about the relationships between kinetics and thermodynamic equilibrium. First, the relations given apply only for a reaction that is close to equilibrium, and what is close is not always easy to specify. A second caution is that kinetics describes the rate with which a reaction approaches thermodynamic equilibrium, and this rate cannot be predicted from its deviation from the equilibrium compositiorr... 

An alternative way of studying a system is to examine its kinetics. Using kinetics, we can study the pathways along which a system may evolve between states of thermodynamic equilibrium and determine the rates of change of system properties along those pathways. Cosmochemical systems can evolve along a variety of pathways, some of which are more efficient than others. In a kinetic study, the task is often to determine which of the competing pathways is dominant. 

With the discussion of the free-energy function G in this chapter, all of the thermodynamic functions needed for chemical equilibrium and kinetic calculations have been introduced. Chapter 8 discussed methods for estimating the internal energy E, entropy S, heat capacity Cv, and enthalpy H. These techniques are very useful when the needed information is not available from experiment. 

If the (equilibrium) system (upper index °) is disturbed by an externally applied field E, we then assume that the (first order) changes of the system s thermodynamic (p) and kinetic (co) parameters are given by... 

Despite the fact that both normal and monomethyl-substituted paraffins readily enter the pores of ZSM-5 and ZSM-11, preferential sorption of the normal isomer is observed under thermodynamic equilibrium, non-kinetically controlled conditions. Whereas small-pore zeolites, such as 5A and erionite, totally exclude branched hydrocarbons, and large-pore zeolites exhibit little preference, the intermediate pore-size zeolites ZSM-5 and ZSM-11 show a marked preference for sorption of the linear paraffin, even under equilibrium conditions. Competitive liquid phase sorption studies at room temperature indicated selectivity factors greater than ten in favor of n-hexane relative to... 

All the work just mentioned is rather empirical and there is no general theory of chemical reactions under plasma conditions. The reason for this is, quite obviously, that the ordinary theoretical tools of the chemist, — chemical thermodynamics and Arrhenius-type kinetics - are only applicable to systems near thermodynamic and thermal equilibrium respectively. However, the plasma is far away from thermodynamic equilibrium, and the energy distribution is quite different from the Boltzmann distribution. As a consequence, the chemical reactions can be theoretically considered only as a multichannel transport process between various energy levels of educts and products with a nonequilibrium population20,21. Such a treatment is extremely complicated and - because of the lack of data on the rate constants of elementary processes — is only very rarely feasible at all. Recent calculations of discharge parameters of molecular gas lasers may be recalled as an illustration of the theoretical and the experimental labor required in such a treatment22,23. ... 

TABLE 5.6 Comparison of Equilibrium and Kinetic Approaches for Determining Thermodynamics of Potassium Exchange in Soils... 

The thermodynamics and kinetics of the thermal equilibrium between previtamin D3 and vitamin D3 have been studied (34,35). The isomerization of previtamin D3 to vitamin 63 is an exothermic first order reaction. The vitamin D3/previtamin D3 equilibrium ratio depends on the temperature and can be calculated from the appropriate equilibrium and kinetic constants reported by Hanewald et al. (36). The rate constants for the equilibrium have been shown to be independent of the nature of the solvent, of acidic or basic catalysis and of factors known to affect free radical process (37,38). The percentages of vitamin D3 in equilibrium with previtamin D3 ranges from 98% at -20° to 78% at 80°. Thus, when vitamin D3 is stored in the cold, the equilibrium constant hinders the conversion to previtamin D3. 

Recently, workers (2) have been examining the equilibrium and kinetic factors that are important at the oxic-anoxic interface. The kinetic behavior is difficult to characterize completely due to varying rates of oxidation and absomtion above the interface and varying rates of reduction, precipitation and dissolution below the interface (2.51. Bacterial catalysis may also complicate the system (1). Although one can question the importance of abiotic thermodynamic and kinetic processes at this interface, we feel it is useful to use simple inorganic models to approximate the real system. Recently, the thermodynamics and kinetics of the H2S system in natural waters has been reviewed (0. From this review it became apparent that large discrepancies existed in rates of oxidation of H2S and the thermodynamic data was limited to dilute solution. In the last few years we have made a number of thermodynamic (7.81 and kinetic (9 101 measurements on the H2S system in natural waters. In the present paper we will review these recent studies. The results will be summarized by equations valid for most natural waters. 

Pytkowicz M. and Cole M. R. (1980) Equilibrium and kinetic problems in mixed electrolyte solutions. In Thermodynamics of Aqueous Systems with Industrial Applications (ed. S.A. Newman), ACS Symp. Series, 133, Washington, D.C., 644-652. 

Chemical reactivity is influenced by solvation in different ways. As noted before, the solvent modulates the intrinsic characteristics of the reactants, which are related to polarization of its charge distribution. In addition, the interaction between solute and solvent molecules gives rise to a differential stabilization of reactants, products and transition states. The interaction of solvent molecules can affect both the equilibrium and kinetics of a chemical reaction, especially when there are large differences in the polarities of the reactants, transition state, or products. Classical examples that illustrate this solvent effect are the SN2 reaction, in which water molecules induce large changes in the kinetic and thermodynamic characteristics of the reaction, and the nucleophilic attack of an R-CT group on a carbonyl centre, which is very exothermic and occurs without an activation barrier in the gas phase but is clearly endothermic with a notable activation barrier in aqueous solution [76-79]. 

In a closed system and at a fixed temperature, the thermodynamic equilibrium constant of any reaction has a fixed value. A catalyst has an impact on the reaction rate by lowering the activation energy, reducing the required time to achieve the thermodynamic equilibrium and it can have an impact on the reaction channel by favoring one of several possible transition states, but a catalyst does not influence the thermodynamic equilibrium itself. That means the maximal achievable yield cannot be higher than that predicted by thermodynamics. An important consequence of this limitation is that for a comparison of different catalysts the experimental conditions must not allow that the thermodynamic equilibrium is reached. Typical reaction conditions for this case would be low reactant flow (setup then resembling a closed system) and/or a temperature allowing a very fast reaction. If the experimental conditions allow the reaction to approach the thermodynamic equilibrium, a comparison of different catalysts is usually impossible. For any kinetic studies the... 

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