Reversibility/irreversibility

Since these were preliminary conclusions, further explanations of the.se disadvantages are given using the second law of thermodynamics in this chapter. The ideas of reversibility, irreversibility, and the thermodynamic properties steady-flow availability and exergy are also developed. 

Removal by sorption. It is generally assumed that adsorption is reversible. Irreversible removal directly from groundwater would be included in the term for precipitation, although slow irreversible incorporation of sorbed atoms is not considered in Equation (1). 

Reaction features (exothermic, endothermic, reversible, irreversible, number of species, parallel, consecutive, chain, selectivity)... 

Note that in the mechanistic schemes presented, the dissolution steps of reactant and products have been omitted for the sake of brevity. These include, for example CO (g) <-> CO (1), C02 (1) <-> C02 (g), and H2 (1) <-> H2 (g). From the standpoint of thermodynamics, when the equilibrium lies far to the right, reactions are deemed to be irreversible and may be denoted with a forward arrow - symbol. In cases where the reaction is considered to be reversible (i.e., equilibrium lies somewhere in the middle), the forward and backward arrows (e.g., <-> ) are employed. In some cases, however, we do not specify reversible/irreversible steps, and therefore arrows (e.g., or <-> ) might be used in a general sense. From a kinetic standpoint, in some cases a step will be defined that is considerably slower than the others (i.e., the rate determining step) in those cases, the remaining steps may be considered to be pseudo-equilibrated. The reader must therefore use discretion in interpreting the mechanistic schemes. The context of the discussion should clue the reader into how to interpret the arrows. 

The effect of any chemical at a biological target depends on its ability to attain a target site concentration that exceeds the threshold required to ehcit the response. The intensity and duration of the response depends on the toxicokinetic properties of the compound (absorption, distribution, metabolism, and excretion) and the nature of the target site interaction (reversible, irreversible). If recovery is complete between successive exposures, no cumulative toxicity is to be expected. However, a short-term acute exposure could potentially add to the long-term burden of a persistent chemical and be relevant for the magnitude of the chronic effect. 

Fischer NO, Verma A, Goodman CM, Simard JM, Rotello VM. Reversible irreversible inhibition of chymotrypsin using nanoparticle receptors. J Am Chem Soc 2003 125 13387-13391. 

Adsorbent area An2g-1 diameter /nm Total Reversible Irreversible... 

Thrombin inhibition Irreversible Reversible Irreversible Reversible... 

Enzyme inhibitors are divided into two classes, irreversible and reversible. Irreversible inhibition implies destruction or permanent modification of chemical groups in the enzyme. In contrast, reversible inhibitors form a complex with the enzyme that can dissociate and release the active enzyme. An enzyme E can bind either to substrate S, to form an ES complex (which can go on to products) or to inhibitor I, to form the complex EL... 

It is often convenient to report an empirical and conveniently measured half-peak potential Ep/2, at which the current has reached ip/2j one-half of its maximum value. This Ep/2 depends on experimental conditions (the potential scan speed v, and whether the electrode reaction is reversible, irreversible,... 

R. S. Nicholson and I. Shain, Single scan and cyclic methods applied to reversible, irreversible and kinetic systems, Anal. Chem. 36 706-723 (1964). 

The methods, described in this chapter are for the determination of adherent protein binding, which is reversible. Irreversible covalent binding (e.g. caused by reactive intermediates) has not been considered. 

The -> polarization curves for irreversible and quasireversible systems are shown in Figure (a). The respective -> Tafel plots are presented in Figure (b). Tafel plots can be constructed only for electrochemically irreversible systems, and kinetic parameters can be determined only when irreversible kinetics prevails. A switching from reversible to irreversible behavior and vice versa may occur. It depends on the relative values of ks and the -> mass transport coefficient, km. If km ks irreversible behavior can be observed. An illustration of the reversibility-irreversibility problem can be found in the entry -> reversibility. 

For the reactions considered in the previous chapter the adjustment of the thermodynamic equilibrium, as the most stable time-independent form of a closed system, was always assumed. To what extent or in which time this equilibrium is reached can not be described by thermodynamic laws. Thus, slow reversible, irreversible or heterogeneous reactions actually require the consideration of kinetics, i.e. of the rate at which a reaction occurs or the equilibrium adjusts. 

It can be seen that the relative rates of electron transfer and the potential scan rate may crucially determine whether voltammograms are observed to be reversible, irreversible or quasi-reversible. Matsuda and Agabe (1955) proposed the (ko, v) regimes in (38), (39) and (40) in order to define whether an electron-transfer process will be observed to be reversible, quasi-reversible or irreversible ... 

A 0.5 wt. % Pt on silica catalyst gave the data listed below for the sorption of H2. Upon completion of Run 1, the system was evacuated and then Run 2 was performed. Find the dispersion and average particle size of the Pt particles. Hint. Run 1 measures the total sorption of hydrogen (reversible + irreversible) while Run 2 gives only the reversible hydrogen uptake. Calculate the dispersion based on the chemisorbed (irreversible) hydrogen. 

For the LSV and CV techniques, the concept of reversibility/irreversibility is therefore very important. Electrochemists are responsible for some confusion about the term irreversible, since a reaction may be electrochemically irreversible, yet chemically reversible. In electrochemistry, the term irreversible is used in a double sense, to describe effects from both homogeneous and heterogeneous reactions. In both cases, the irreversible situation arises when deviations from the Nernst equation can be seen as fast changes in the electrode potential, E, are attempted and the apparent heterogeneous rate constants, /capp, for the O/R redox couple is relatively small. The heterogeneous rate constant can be split into two parts a constant factor in terms of the standard rate constant, k°, and an exponential function of the overpotential E - Eq), as expressed in Eq. 59, where only the reductive process is considered (see also Eq. 5). 

The shape of the polarographic wave is further influenced by the nature of the electrode process occurring at the drop surface. Polarographic waves may be reversible, irreversible, or quasireversible. The overall electrode process comprises the diffusion, electron transfer, and electrochemical reaction steps. 

Figure 3. The results from the modeling reversible, irreversible and total coverage of coke. The experimental results are indicated with dashed lines and the model with solid lines. The ratios between the coverage of reversible coke and the surface not covered with irreversible coke are also shown. The predicted versus the experimental surface coverages are shown in the last graph.

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