Rate constants, also

The question arises as to whether comparisons with protein enzymes are justified. In other words, what can ribozymes really do An important parameter for measuring the efficiency of enzymes is the value of kc-JK. This quotient is derived from the values of two important kinetic parameters kc-Al is a rate constant, also called turnover number, and measures the number of substrate molecules which are converted by one enzyme molecule per unit time (at substrate saturation of the enzyme). Km is the Michaelis-Menten constant it corresponds to the substrate concentration at which the rate of reaction is half its maximum. 

Pressure effects The diffusion through liquids is governed by the number of defects or atomic-sized holes in the liquid. A high external pressme can reduce the concentration of holes and slow diffusion. Therefore, in a liquid, a diffusion-controlled rate constant also depends on the pressure. 

A second-order reaction in which both the reactants were at same initial concentration was 50% completed in 500 sec. How long will it take to complete 75% of the reaction Determine the rate constant also. 

Literature values of the active-site concentrations range from tenths or hundredths of a percent to tens of percents of the transition metal concentration [Lieberman and Barbe, 1988 Tait and Watkins, 1989]. Much of this is indicative of the range of activity of different initiators, especially when comparing older initiators to the recent high-mileage initiators. However, some of the variation is due to the problems inherent in measurement of [C ]. Literature values of kp and other rate constants also show a considerable range. 

In addition to temperature, the rate constant also depends on pressure, but this dependence... 

For example, with a Henry s law constant for HONO of 49 M atm-1, a gas-phase concentration of 1 ppb would result in a solution-phase concentration of only 4.9 X 10-8 mol L-1, compared to an anticipated H202 solution-phase concentration of 10-4 mol L-1 at the same gas-phase concentration. The rate constants also favor the H202 reaction at a pH of 3.0, that for oxidation of H202 is approximately a factor of 104 larger than that for reaction with HONO. Thus, the combination of concentrations and rate constants makes HONO unlikely to be a significant S(IV) oxidant in solution unless other oxidants such as O-, or H202 are absent. 

Written in this form, k is called the rate constant. Rate constants also appear in chemical kinetics. For instance, in Chapter 4 we will show that the rate of a unimolecu-lar reaction (such as an internal rearrangement of some atoms within a single molecule) changes with temperature according to the equation... 

The kinetics of the recombination of Chl+ and MV+ radical ions formed during the quenching of 3Chl, consists of the fast and slow stages. The corresponding rate constants also occurred different at the inner and outer surfaces of the vesicles. The observed difference in the recombination rates was explained by different structural and dynamic characteristics of the inner and outer monolayers of the membrane. 

An increase in reactivity also can be observed in Diels-Alder reactions with normal electron demand if a given dienophile is reacted with a series of more and more electron-rich dienes. The reaction rates of the Diels-Alder reactions of Figure 15.22 show that the substituents MeO > Ph > alkyl are such reactivity-enhancing donors. The tabulated rate constants also show that a given donor substituent accelerates the Diels-Alder reaction more if located in position 1 of the diene than if located in position 2. 

The high-pressure limiting rate constants also correlate with the ionization potential of the alkyl radical the lower the ionization potential of the alkyl radicals, the higher is their rate constant with molecular oxygen [55, 56]. 

The maximum amount of coke deposited depends only upon the catalyst used. Hence, this value should not need to be obtained separately for every additive used in accelerated coking tests. The initiation rate constant depends upon the aromatic content of the feed and also upon the nature of the aromatic compound. However, for a given additive, the relationship appears to be linear, at least for one- and two-ring compounds. Hence, experiments need only be performed with reactant-additive mixtures at no more than two levels. The propagation rate constant also depends upon the aromatic nature of the feed, but appears to be independent of the nature of the aromatic compound. The present results allow extrapolation to different additive levels they will also allow the reverse — extrapolation from experiments using additives to those involving only the pure reactant. 

Interest in the kinetics of alkaline hydrolysis of esters in DMSO + water mixtures was stimulated by the observation that the rate constant often increased gradually as x2 increased. This is observed, for example, in the alkaline hydrolysis of ethyl acetate. For higher esters, e.g. ethyl p-nitrobenzoate, the rate constant drops slightly at low x2 but then rises again until k/k x2 = 0) > 1 (Tommila, 1964). The rate of alkaline hydrolysis of esters of benzoic acid is accelerated when DMSO is added (Tommila and Palenius, 1963), as also is the rate of alkaline hydrolysis of 2,4-dinitrofluorobenzene. In the latter case the effect is less dramatic because the rate constant for spontaneous hydrolysis also increases (Murto and Hiiro, 1964). The rate constants also increase when DMSO is added to aqueous solution for reactions between hydroxide ions and benzyl chloride (Tommila... 

The rate constants also hold a useful clue to concentration and time ranges for new investigations of intermolecular ET processes of cyt c4 in solution, from which the intramolecular rate constants may be substantiated. Voltammetry, therefore, stands forward as a powerful tool towards otherwise elusive ET dynamics in composite metalloproteins. 

It may be noted that the denominator of the heterogeneous term resembles the format expected from the Langmuir adsorption equation (8) even though Scheme (XLVI) has not stipulated equilibrium between bulk and adsorbed concentrations. As eqn. (82) predicts, kobs was indeed found to increase linearly with the mass of carbon (Spheron 6) added. The rate constant also fell with rising binaphthyl concentration (cf. Fig. 14) but the equation fitted these results only qualitatively. However, the fall in rate with added inhibitor could be fitted better, a plot of 1 l(kohs — kohB unc) rising linearly with increasing concentration of inhibitor. The relative inhibitory properties in acetone increased with aromatic size in the order benzene, nil naphthalene,... 

At low temperatures, where quantum effects such as resonances can play an important role, the classical and quantum results for the thermal rate constant also diverge, as shown in Figure 3.11 of Section 5.5. 

Reversibility is reached asymptotically, but for > 10 tn/s the system can be considered to be reversible. To generalize this curve, we have shown the values of the rate constant also in dimensionless form as... 

The rate constant for the first-step reaction increases with increasing concentration of copper(II) or zinc(II) and reaches a plateau. The rate constant also decreases with increasing mercury(II) concentration (Fig. 3a). These observations lead to the following rate law ... 

The rate constants also increase with alkyl substitution at the C=C double bond (see Table 1), again evidence for the polar nature of the transition state of the interaction of S04 " with... 

Standing of the structural features of the systems, the search for the method provides an excellent framework for the structural discussion. It must be remembered, however, that the insight obtained from the general analysis is much more broadly useful than merely providing a method for the extraction of the rate constants from experimental data. In the new method, quantities that correspond to the constants c,- and X, in Eq. (6) are determined but in addition, their relation to the rate constants also appears. 

Like equilibrium constants, rate constants also depend on environmental factors such as pressure and, especially, temperature. An increase in temperature usually gives rise to an increase in the chemical reaction rate, because molecules are moving faster and colliding more frequently with greater energy. If rate constants are known for two different temperatures, the rate constant for any other temperature can be calculated using the Arrhenius rate law,... 

In this equation, kd is the apparent first-order degradation rate constant (also called out). This constant can be obtained experimentally from the slope of a ln(P) versus time plot, after administration of a synthesis-blocking dose of coumarin anticoagulant (Nagashima et al., 1969 Pitsui et al., 1993). P0 is the baseline value of the prothrombin time, Cw(S) is the concentration of ( -warfarin and IC50s is the concentration of warfarin at 50% of maximal blocking effect. It was also possible to estimate the half-life of the apparent first-order degradation. 

Rate constant (also called specific rate constant) An experimentally determined proportionality constant that is different for different reactions and that, for a given reaction, changes only with temperature or the presence of a catalyst k in the rate-law expression. Rate = [A] [B]J. ... 

The explanation for this effect (known variously as the gel effect, Tromsdorff effect or auto-acceleration effect) is that the chain termination reaction slows down during conversion and, as can be seen by reference to equations (2.5) and (2.6), a decrease in the termination rate constant leads to an increase in both overall rate and molecular weight. The reason for the drop in termination rate is that as the reaction mixture becomes more viscous the radical ends of the polymer chains find increased difficulty in diffusing towards each other, leading to the important mutual termination reaction. Small monomer molecules on the other hand find little difficulty in diffusion at moderate conversion so that propagation reactions are relatively little affected, until the material becomes semi-soUd, when the propagation rate constant also decreases. It is of interest to note that the gel effect may be induced by the addition of already formed poly(methyl methacrylate) or even another polymer such as cellulose tripropionate because such additions increase the viscosity of the system. 

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