Reactions, irreversible/reversible

But if there is no volume variation sa = 0), we obtain the same expression for space time (Equation 14.38), meaning that the mean residence time is equal to the space time. When there is volume variation ba 0), the mean residence time and space are different. The rate expression depends on the reaction order and the factor Sa for any kind of reaction (irreversible, reversible multiple, etc.). In short ... 

Furthermore, for each reaction the reaction center was specified, information was given on whether the reaction is reversible or irreversible, and catabolic or anabolic. Finally, it was specified whether a reaction is part of a general pathway or occurs only in unicellular organisms, in higher plants, or in animals (Figure 10.3-21). 

All these facts—the observation of second order kinetics nucleophilic attack at the carbonyl group and the involvement of a tetrahedral intermediate—are accommodated by the reaction mechanism shown m Figure 20 5 Like the acid catalyzed mechanism it has two distinct stages namely formation of the tetrahedral intermediate and its subsequent dissociation All the steps are reversible except the last one The equilibrium constant for proton abstraction from the carboxylic acid by hydroxide is so large that step 4 is for all intents and purposes irreversible and this makes the overall reaction irreversible... 

Hydrolysis in neutral aqueous solutions proceeds slowly at room temperature and more rapidly at acidic conditions and elevated temperatures. The hydrolysis—esterification reaction is reversible. Under alkaline conditions hydrolysis is rapid and irreversible. Heating the alkaline hydrolysis product at 200—250°C gives 4,4 -oxydibutyric acid [7423-25-8] after acidification (148). 

Galvanic cells in which stored chemicals can be reacted on demand to produce an electric current are termed primaiy cells. The discharging reac tion is irreversible and the contents, once exhausted, must be replaced or the cell discarded. Examples are the dry cells that activate small appliances. In some galvanic cells (called secondaiy cells), however, the reaction is reversible that is, application of an elec trical potential across the electrodes in the opposite direc tion will restore the reactants to their high-enthalpy state. Examples are rechargeable batteries for household appliances, automobiles, and many industrial applications. Electrolytic cells are the reactors upon which the electrochemical process, elec troplating, and electrowinning industries are based. 

Some chemical reactions are reversible and, no matter how fast a reaction takes place, it cannot proceed beyond the point of chemical equilibrium in the reaction mixture at the specified temperature and pressure. Thus, for any given conditions, the principle of chemical equilibrium expressed as the equilibrium constant, K, determines how far the reaction can proceed if adequate time is allowed for equilibrium to be attained. Alternatively, the principle of chemical kinetics determines at what rate the reaction will proceed towards attaining the maximum. If the equilibrium constant K is very large, for all practical purposes the reaction is irreversible. In the case where a reaction is irreversible, it is unnecessary to calculate the equilibrium constant and check the position of equilibrium when high conversions are needed. 

In the case of thermodynamics, the designer can investigate the nature of the reaction heat and whether the reaction is reversible. If these exothermic reactions are irreversible, attention may be focused on the influence of reactor design on conversion and with heat transfer control. An objective of reactor design is to determine the size and type of reactor and mode of operation for the required job. The choice... 

The reaction mechanisms for reversible reactions are slightly different. In the above section, the second part of the reaction that leads to product was irreversible. However, if all the steps in enzyme reactions were reversible, the resulting rates may be affected. 

Polymerization equilibria frequently observed in the polymerization of cyclic monomers may become important in copolymerization systems. The four propagation reactions assumed to be irreversible in the derivation of the Mayo-Lewis equation must be modified to include reversible processes. Lowry114,11S first derived a copolymer composition equation for the case in which some of the propagation reactions are reversible and it was applied to ring-opening copalymerization systems1 16, m. In the case of equilibrium copolymerization with complete reversibility, the following reactions must be considered. 

Reaction rates almost always increase with temperature. Thus, the best temperature for a single, irreversible reaction, whether elementary or complex, is the highest possible temperature. Practical reactor designs must consider limitations of materials of construction and economic tradeoffs between heating costs and yield, but there is no optimal temperature from a strictly kinetic viewpoint. Of course, at sufficiently high temperatures, a competitive reaction or reversibility will emerge. 

Theoretically, all chemical reactions are reversible. There are, however, many reactions in which the extent of the reverse reaction (i.e., combination of the products to produce the reactants) is very small as to be considered negligible. Such reactions which are ordinarily found to proceed to completion in one direction are said to be irreversible reactions. The decomposition of potassium chlorate... 

Finally, a remark should be made on the effect of the scan rate an increase in the scan rate, e.g., from 50 through 100 to 200mV s 1, causes a sharper and apprecially higher peak, as expected. If the electrode reaction is reversible, the half-wave potential, Up/2, remains nearly unaltered, otherwise there is a shift to the right (more negative in reductive LSV). It should be borne in mind that in a follow-up reaction such as the system EC (see p. 124) an increase in scan rate may cause a transition from irreversibility to apparent reversibility if the charge-transfer reaction E becomes predominant. 

While the first two reactions are reversible in chloroform, the third is irreversible, shifting the equilibrium to the side of the acid chloride.. 

The reversibility of reactions is another important characteristic in assessing the fate of deep-well-injected wastes. Depending on environmental conditions, reversible reactions readily proceed in either or both directions. Most acid-base reactions exemplify reversible processes. In aqueous solutions, relatively minor changes in such factors as pH or concentration can change the direction of these reactions. Irreversible reactions, typified by hydrolysis, have a strong tendency to go in one direction only. 

Determine the effectiveness factor for the ion exchange resin at 85 °C, assuming that the reaction is reversible even though the authors presumed the reaction to be irreversible in reporting their data. They note that at 100 °C the equilibrium for the reaction corresponds to a conversion greater than 94%. If the equilibrium constant for the reaction is expressed as the ratio of the t-butanol concentration to the isobutylene concentration and corrected for the temperature change in going from 100 °C to 85 °C, a value of 16.6 may be considered appropriate for use. 

Type II, when the majority of the reactions are reversible, but the final product is formed irreversibly. 

In order to clear up whether the sorption intermediates of CO and 1 0 participating in the shift reaction are reversibly or irreversibly adsorbed a similar experiment has been accomplished with an additional stripping period of 30 min with purified N, enclosed between the testing periods. As it could be shown, this stripping phase was sufficient to remove the adsorbed 1 0 but not the CO (see (9)), therefore we like to regard the 1 0 sorption as reversible and the CO sorption as irreversible. 

Other reaction schemes more complicated than the irreversible reaction between the catalyst and the substrate may be analyzed according to the same principles. For example, treatments of cases where the catalytic reaction is reversible may be found in reference 19. Another of these more complicated reaction schemes is treated in the next section. 

Our knowledge of enzymes also tells us that under usual physiological conditions (i.e. at typical cellular concentrations of substrate) most metabolic reactions are reversible. Energetically irreversible reactions, i.e. those with a large positive free energy change, effectively act as one-way valves allowing substrate flow in the forward direction only. 

Here, there are only two species, designated reactant and product , and two reaction steps. In principle, all simple reactions are reversible, but if the concentration of A or B at equilibrium is very low the reaction is considered irreversible. If we considered a reaction to be complex only when species other than the reactants and desired products were present, then reversible reactions would not be included. Obviously, problems with product distribution do not arise. However, the appearance of a reversible step in a more complicated reaction scheme can affect relative yields of reaction products. 

We have used the first-order irreversible reaction as an example, but this is easy to generalize for any reaction, irreversible or reversible, with any kinetics. In a PFTR the mass-balance equation for an arbitrary reaction becomes... 

The ability of fluorinated substituents to prevent the development of a positive charge on the a position has been used to slow down or even inhibit enzymatic processes involving positively charged transition states. According to the case, the observed result can be (1) slowing down the reaction, (2) reversible inhibition, or (3) irreversible inhibition. Various examples are now given. 

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