Chemically irreversible processes

This criterion is good for establishish whether a process is under thermodynamic control. Care should be taken however to understand the term reversibility in this case. The folding of a protein is generally per se a chemically irreversible process, in the sense that the chemical equilibrium is overwhelmingly shifted towards the folded form - there is not a low activation energy barrier between the native folded and the unfolded form and a corresponding chemical equilibrium in the native state between the two forms. Thus, in the case of the thermodynamic hypothesis of... 

At slow scan rates, the E step remains reversible and the C step is irreversible. This would be termed as an electrochemically reversible and chemically irreversible process. 

With only few exceptions, all of the electrochemical oxidation studies have been carried out in MeCN containing varying amounts of water. The compounds of the structure R R R M, R, R, R = aryl, alkyl, M = As, Sb (Bi), arc in most cases (for exceptions see below) oxidized in a chemically irreversible process in MeCN. Half-wave potentials for the chemically irreversible oxidations of R R R M are given in Table 14. 

Second, it can be shown that the conventional relationships [Eq. (i), 12.3.7.2] between the rates of electrochemical and homogeneous exchange reactions can be extended to chemically irreversible processes. Therefore, if the assumptions contained in this equation are valid, the electrochemical rate constant, k, at the intersection of the cathodic and anodic log rate-potential plots will be related approximately to the rate constant, k. ., for the homogeneous cross reaction between the same pair of redox couples - ... 

The thermochemical cycle in Scheme 7 has been used to estimate the effect of a one-electron oxidation on the BDEs of metal hydrides. " Similar methods have been applied to the study of G-H bonds.Accurate determination of the BDE data through Equation (15), Scheme 7 requires electrode potential data for the oxidation of the parent metal hydride and of the metal fragment that is formed by its homolytic M-H cleavage. Table 7 gives a list of compounds that have been investigated. For entries 1-20, at least one of the underlying electrode potentials was for a chemically irreversible process entries 21-24 are based entirely on reversible voltammograms. 

Nitrobenzene was studied in [C4dmim][N(Tf)2] on a 25 pm diameter gold microdisk electrode at a scan rate of 0.1 V s (Fig. 15.6) [49]. The voltammetric wave at —1.2 V vs. Ag wire QRE (peak 1) corresponds to the reversible one-electron reduction of nitrobenzene to the radical anion, which is commonly known in aprotic solvents [8]. A second peak was observed at —1.85 V vs. Ag wire (peak 2), which is related with a two-electron chemically irreversible process. This result is in contrast with conventional aprotic solvents [8], where a four-electron reduction with the consequent formation of phenylhydroxylamine is achieved. Compton et al. suggested that the dianion, formed by further reduction of the radical anion, is unstable in this environment and most likely undergoes rapid... 

In an irreversible process the temperature and pressure of the system (and other properties such as the chemical potentials to be defined later) are not necessarily definable at some intemiediate time between the equilibrium initial state and the equilibrium final state they may vary greatly from one point to another. One can usually define T and p for each small volume element. (These volume elements must not be too small e.g. for gases, it is impossible to define T, p, S, etc for volume elements smaller than the cube of the mean free... 

Irreversible processes are mainly appHed for the separation of heavy stable isotopes, where the separation factors of the more reversible methods, eg, distillation, absorption, or chemical exchange, are so low that the diffusion separation methods become economically more attractive. Although appHcation of these processes is presented in terms of isotope separation, the results are equally vaUd for the description of separation processes for any ideal mixture of very similar constituents such as close-cut petroleum fractions, members of a homologous series of organic compounds, isomeric chemical compounds, or biological materials. 

Real irreversible processes can be subjected to thermodynamic analysis. The goal is to calciilate the efficiency of energy use or production and to show how energy loss is apportioned among the steps of a process. The treatment here is limited to steady-state, steady-flow processes, because of their predominance in chemical technology. 

The main problem of elementary chemical reaction dynamics is to find the rate constant of the transition in the reaction complex interacting with its environment. This problem, in principle, is close to the general problem of statistical mechanics of irreversible processes (see, e.g., Blum [1981], Kubo et al. [1985]) about the relaxation of initially nonequilibrium state of a particle in the presence of a reservoir (heat bath). If the particle is coupled to the reservoir weakly enough, then the properties of the latter are fully determined by the spectral characteristics of its susceptibility coefficients. 

It should be kept in mind that all transport processes in electrolytes and electrodes have to be described in general by irreversible thermodynamics. The equations given above hold only in the case that asymmetric Onsager coefficients are negligible and the fluxes of different species are independent of each other. This should not be confused with chemical diffusion processes in which the interaction is caused by the formation of internal electric fields. Enhancements of the diffusion of ions in electrode materials by a factor of up to 70000 were observed in the case of LiiSb [15]. 

Vulcanization A process in which rubber or TS plastic (elastomer) undergoes a change in its chemical structure brought about by the irreversible process of reacting the materials with sulfur and/or other suitable agents. These cross-linking action results in property changes such as decreased plastic flow, re-... 

Tethering may be a reversible or an irreversible process. Irreversible grafting is typically accomplished by chemical bonding. The number of grafted chains is controlled by the number of grafting sites and their functionality, and then ultimately by the extent of the chemical reaction. The reaction kinetics may reflect the potential barrier confronting reactive chains which try to penetrate the tethered layer. Reversible grafting is accomplished via the self-assembly of polymeric surfactants and end-functionalized polymers [59]. In this case, the surface density and all other characteristic dimensions of the structure are controlled by thermodynamic equilibrium, albeit with possible kinetic effects. In this instance, the equilibrium condition involves the penalties due to the deformation of tethered chains. 

Figure 2. Cyclic voltammograms of a poly(2,2 -bithiophene)-coated electrode in acetonitrile containing 0.1 M Bu4NC 04.34 (Reprinted from G. Zotti, C. Schiavon, and S. Zecchin, Irreversible processes in the electrochemical reduction of polythiophenes. Chemical modifications of the polymer and charge-trapping phenomena, Synth. Met. 72 (3) 275-281, 1995, with kind permission from Elsevier Sciences S.A.)...

Chemical reactions in the system are irreversible processes, affecting transport processes, as they result in the formation and disappearance of components of the system and in the release or consumption of thermal energy. 

The irreversible processes described must not occur even on open circuit. In a reversible cell, a definite equilibrium must be established and this may be defined in terms of the intensive variables in a similar way to the description of phase and chemical equilibria of electroneutral components. 

Table 20.3 lists the reversible and irreversible processes that may be significant in the deep-well environment.3 The characteristics of the specific wastes and the environmental factors present in a well strongly influence which processes will occur and whether they will be irreversible. Irreversible reactions are particularly important. Waste rendered nontoxic through irreversible reactions may be considered permanently transformed into a nonhazardous state. A systematic discussion of mathematical modeling of groundwater chemical transport by reaction type is provided by Rubin.30... 

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. 

The Chemical Meaning of an Electrochemically Irreversible Process. As a chemical consideration, the occurrence of an electrochemically irreversible process implies so large an activation barrier to the electron transfer that it is likely that (as discussed in the introductory section, Figure. 1.2) it causes breakage of the original molecular frame with formation of new species (see Chapter 7, Section 5). 

To continue the present topic, we would like to underline that often irreversible redox processes do not cause the appearance of new, well-defined redox waves such as to induce an accurate examination of the underlying chemical complications. Nevertheless, not always irreversible redox changes lead to complete destruction of the metal complex under examination. Analysis of the products arising from irreversible processes can sometimes reveal interesting chemical pathways. 

For a scientist, the primary interest in thermodynamics is in predicting the spontaneous direction of natural processes, chemical or physical, in which by spontaneous we mean those changes that occur irreversibly in the absence of restraining forces—for example, the free expansion of a gas or the vaporization of a hquid above its boiling point. The first law of thermodynamics, which is useful in keeping account of heat and energy balances, makes no distinction between reversible and irreversible processes and makes no statement about the natural direction of a chemical or physical transformation. 

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