Chemically irreversible systems

An example of a chemically irreversible system is also provided (Figure 2.3d), where an electrochemically fast oxidation process is followed by a fast and irreversible chemical transformation of species Ox to a new compound, X. Although there are differences in the profiles of the voltammograms presented in Figure 2.3c and d, which would be detected by an experienced electrochemist, it is easy to confuse electrochemical and chemical ... 

This chapter is restricted to homogeneous, single-phase reactions, but the restriction can sometimes be relaxed. The formation of a second phase as a consequence of an irreversible reaction will not affect the kinetics, except for a possible density change. If the second phase is solid or liquid, the density change will be moderate. If the new phase is a gas, its formation can have a major effect. Specialized models are needed. Two-phase ffows of air-water and steam-water have been extensively studied, but few data are available for chemically reactive systems. 

The theory of the thermodynamics of irreversible systems (Prigogine, 1979 Prigogine and Stengers, 1986) shows that the differential quotient of entropy with time (the change of entropy with time) can be expressed as the sum of products, the terms of which contain a force factor and a flow factor. In chemical systems, the... 

To answer this question, information obtained from studies of irreversible systems needs to be examined. Irreversible protein processes may occur as a result of intermolecular interactions (i.e., aggregation, chemical modification, intermolecular cross-linking). Although an attempt is generally made to search for conditions that provide maximal reversibility, perhaps by altering the solution conditions (i.e., pH, salt content, lowering the protein concentration) that minimize contact and electrostatic interactions, many systems can still exhibit little or no reversibility. This would be the case for the core protein obtained by limited... 

Although the one-electron reduction of nitrobenzene to its radical anion in dipolar aprotic solvents is a classical example of a chemically reversible redox couple, the reductions of many organic compounds are chemically irreversible. The redox behavior of /7-chlorobenzonitrile is typical of those systems in which the initial electrode product undergoes rapid, irreversible chemical reaction to give another reducible species. 

From the discussions above, the following general principle for selection of target systems for IS application can be concluded the gas-continuous impinging streams method is especially applicable to gas-liquid reaction or chemical absorption systems involving fast-irreversible reaction(s) in liquid. 

For irreversible systems the peak potential of a reduction process is shifted toward more negative potentials by about 0.030 V for a decade increase in the scan rate [Eq. (3.43)]. By analogy, a peak of an anodic process is shifted toward more positive potentials. The most characteristic feature of a cyclic voltammogram of a totally irreversible system is the absence of a reverse peak. However, it does not necessarily imply an irreversible electron transfer but could be due to a fast following chemical reaction. 

Nernstian boundary conditions, or those for quasireversible or irreversible systems. All of these cases have been analytically solved. As well, there are two systems involving homogeneous chemical reactions, from flash photolysis experiments, for which there exist solutions to the potential step experiment, and these are also given they are valuable tests of any simulation method, especially the second-order kinetics case. 

In an open system, the entropy may change due to either increases caused by spontaneous thermodynamically irreversible internal processes in the system, djS, or exchanges between the system and the surrounding, dgS. In chemically reactive systems, djS may change as a result, for example, of spontaneous reactions inside the system, while dgS may change as a result of supply or extraction of heat and/or some reactants. 

The problem of spontaneous evolution of chemically reactive systems has a close relation to the topics of chemical kinetics. Hence, thermody namics of irreversible processes allows, among others, some important interrelations to be estabHshed between kinetics of particular chemical pro cesses and thermodynamic parameters of the reactants involved. 

The fact that US influences the mechanism of chemical reactions via the action of highly reactive radicals such as OH- and H- formed during solvent sonolysis is well known (see Chapter 7). Solvents sensitive to thermolysis or sonolysis (e.g. dimethylformamide [158], dimethylsulphoxide [159]) decompose slowly in the presence of intense US. Thus, radical species formed by cavitation are highly reactive and may participate as activators or enhancers in the electrode process [160]. In fast, qt/asr-reversible or irreversible systems, however, the only effect of US is to enhance mass transport without any direct effect on the rate of simple electron transfer processes. 

The chromium carbonyl complexes with one monodentate and one bidentate ligand in System 8 show systematic changes in F ,i and F ,2 (but in opposite directions), when the ligands are changed from a P—P to a P—As and an As—As type (for -dpadpe the coordination to Cr is through P ). The first, chemically reversible oxidation is made easier in going from dppe to dpae, while the second, chemically irreversible oxidation becomes increasingly more difficult. The one-electron oxidation products slowly decompose to trons-[Cr(CO)2( -L-L)2]+ - . 

In the converse situation, three limiting cases may be observed. In the first case the electron transfer is intrinsically slow. The RDS of the overall process is then the forward electron transfer, and the redox system is said to be slow and chemically irreversible. The electrochemical wave is then observed at potentials sufficiently different from E° for n(E — E°) 0 (Sec. III.C.3). The two other situations are encountered when is large... 

Yet because of the chemical reaction, the activity of P at the electrode surface is considerably decreased. From Eq. (122) it is seen that the electrode potential is then positive (for a reduction negative for an oxidation) to that observed for the same current density, but in the absence of the follow-up reaction. As a result the current-potential characteristic is observed in a potential range positive to E° for a reduction and negative to E for an oxidation. The system is then said to be nernstian and chemically irreversible. 

In a chemically reversible redox system the rate of the following reaction(s) is insufficient to perturb the concentration of within the electrochemical reaction layer near the electrode surface. When the following reaction is rapid so that Y is depleted during the experiment, the couple is chemically irreversible. The term quasireversible should not be used in conjunction with chemical reversibility. That terminology is restricted to the electron-transfer step itself. This error is made frequently. Redox systems that are less than totally chemically reversible should be described as having limited chemical reversibility. 

M LiOH, but below this value the Np(VI)/Np(V) couple tends toward a system with reduced electrochemical reversibility. Voltammetric behavior in NaOH solutions is very similar to the voltammograms in LiOH, with a shift in potential for the Np(VI)/Np(V) couple to = 0.106(6) V versus SHE in 3 M NaOH. In the mixed hydroxo-carbonate solutions (0.8 M NaOH/0.4M Na2CO3 and 1.8 M NaOH/0.1 M Na2CO3) the Np(VII)/Np(VI) becomes chemically irreversible and the Np(VI)/Np(V) couple is quasi-reversible. This behavior is... 

Now just what is the importance of micromixing in chemical reaction systems The question is perhaps best answered using a famous example from the paper of Danckwerts cited above. Consider the isothermal, second-order irreversible reaction A + B C, carried out homogeneously and at constant volume. The initial concentrations of A and B, Cao and Cbo, are set in the ratio... 

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