Rate constant ionic strength dependence

In determining the values of Ka use is made of the pronounced shift of the UV-vis absorption spectrum of 2.4 upon coordination to the catalytically active ions as is illustrated in Figure 2.4 ". The occurrence of an isosbestic point can be regarded as an indication that there are only two species in solution that contribute to the absorption spectrum free and coordinated dienophile. The exact method of determination of the equilibrium constants is described extensively in reference 75 and is summarised in the experimental section. Since equilibrium constants and rate constants depend on the ionic strength, from this point onward, all measurements have been performed at constant ionic strength of 2.00 M usir potassium nitrate as background electrolyte . 

If the rate equation contains the concentration of a species involved in a preequilibrium step (often an acid-base species), then this concentration may be a function of ionic strength via the ionic strength dependence of the equilibrium constant controlling the concentration. Therefore, the rate constant may vary with ionic strength through this dependence this is called a secondary salt effect. This effect is an artifact in a sense, because its source is independent of the rate process, and it can be completely accounted for by evaluating the rate constant on the basis of the actual species concentration, calculated by means of the equilibrium constant appropriate to the ionic strength in the rate study. 

Berliner et a/.284-8 have examined kinetics of bromination in aqueous acetic acid in an attempt to find the acid concentration at which the change in kinetic order principally occurs, though it follows from the earlier work that this will depend upon the aromatic reactivity. In 50 % acid the bromination of naphthalene was second-order overall284, and at constant ionic strength the rate coefficient showed a dependence on [Br-] according to equation (140)... 

Rate law flooding. The second-order rate constant for the reaction between the hydrated ions of vanadium(3+) and chromium(2+) depends on [H+ ]. From the data given, which refer to T = 25.0 °C and a constant ionic strength of 0.500 M, formulate a two-parameter equation that describes the functional dependence. Evaluate the two constants. Compare your result to the one derived in to Problem 1 -2. 

This composite rate constant is predicted to have a different ionic strength dependence for the two schemes. According to the Br0nsted-Debye-Hiickel equation, the composite rate constant for Eq. (9-76) will be independent of ionic strength if Scheme ... 

The rate of a reaction that shows specific acid (or base, or acid-base) catalysis does not depend on the buffer chosen to adjust the pH. Of course, an inert salt must be used to maintain constant ionic strength so that kinetic salt effects do not distort the pH profile. 

Hydration of compounds 2, 3, 4, 5 was found to be first order both in substrate and in hydronium ion (4-10). Furthermore, a careful kinetic study of compounds 2c-g and the sulfur analog 4 revealed that the hydration rate at constant ionic strength was dependent on the buffer concentration and hence was general acid catalyzed. 

The rate depends on the square of the acidity function Aq at constant ionic strength... 

The existence of such anion effects also implies that, if one wishes to do temperature studies, one cannot simply sit at a constant ionic strength and obtain meaningful activation parameters, because the equilibrium constant involving the association with the anion will also change, of course, as one varies the temperature. Thus, it is necessary to resolve out each rate constant at each temperature and then do the temperature dependencies on individual rate constants. 

The rate constants for the reaction of a pyridinium Ion with cyanide have been measured in both a cationic and nonlonic oil in water microemulsion as a function of water content. There is no effect of added salt on the reaction rate in the cationic system, but a substantial effect of ionic strength on the rate as observed in the nonionic system. Estimates of the ionic strength in the "Stern layer" of the cationic microemulsion have been employed to correct the rate constants in the nonlonic system and calculate effective surface potentials. The ion-exchange (IE) model, which assumes that reaction occurs in the Stern layer and that the nucleophile concentration is determined by an ion-exchange equilibrium with the surfactant counterion, has been applied to the data. The results, although not definitive because of the ionic strength dependence, indicate that the IE model may not provide the best description of this reaction system. 

Nolle, H. J. Rosenberry, T. L. Neumann, E. Effective charge on acetylcholinesterase active sites determined from the ionic strength dependence of association rate constants with cationic ligands. Biochemistry 1980, 19, 3705-3711. 

LiBr and in the presence of cyclopentene as a scavenger olefin. The kinetics, determined by monitoring the formation of strong acids (TfOH or HBr), show that the rate of solvolysis of 65 is dependent on [Br-] (at a constant ionic strength). In the presence of Br-, the products are trans- 1,2-dibromides and bromo-solvates of both cyclohexene and cyclopentene. The cyclopentenyl products have been shown to arise from the electrophilic addition of Br2/Br3 to cyclopentene, while trans-l, 2-dibromocyciohexane 67 is formed by Br- capture of the bromonium ion 66 on carbon. The Br2 required for bromination of cyclopentene results from attack by Br- on the bromonium ion 66 on Br+. On the basis of the ratio of the cyclopentyl products to 67, Br- capture of the solvolytically produced bromonium ion 66 (by attack on Br+) is 4-5 times more prevalent than attack on carbon in AcOH, and ca 25 times more preferred in MeOH123. 

The base hydrolysis of an ester in constant ionic strength aqueous solutions of T1N03 shows a dependence of the observed rate constant on [Tl+],... 

The ionic strength dependencies for both are different, but this does not mean that the two mechanisms can be distinguished on this basis. The differences are merely a reflection of the different ways of expressing the overall observed rate constant. These are kinetically equivalent, and so the ionic strength dependencies are merely a reflection of the two ways of expressing the catalytic contribution to the overall rate. 

Predicted values of the rate for all four speculations in Table I are much more sensitive to ionic strength than observed values. At high ionic strengths, the energy barrier disappears and the rate becomes equal to the maximum value (6110 particles/cm2 sec) predicted from Levich s equation (Eq, 3J). One possible explanation for the discrepancy in ionic strength dependence is that the Hamaker constant has a different value in each of the five electrolyte solutions tested. The Hamaker constant can be.affected by adsorbed layers of surfactant (18). Since the concentration of surfactant used in solutions of different ionic strengths varied between 1 X 10 4 and 4X 10-i 37/liter, the Hamaker constant could be affected differently. However, to obtain agreement between predicted and observed rates under speculation 1, the Hamaker constant would have to vary from 0.98 X10 13 erg... 

The above discussion has focused attention on electron transfer through the exposed edge of the heme group. This model is suggested from experience with small metal complexes and is substantiated by the rate constants and ionic strength dependence of the cytochrome c crossreactions. 

In comparisons of rate coefficients measured at different pH values, it is necessary to keep a constant ionic strength by addition of a neutral salt such as NaCl, KC1, NaNOs, or NaC104, as the pH of a buffer system may be altered by a change of ionic strength (secondary salt effect [1]). Furthermore, the catalytic rate coefficient of a hydrogen ion or hydroxide ion catalyzed reaction is dependent on the ionic strength also (primary salt effect [1], see also Vol. 2, p. 337). 

Equation (47) was suggested for the first time by Bredig and Ripley [202]. In order to establish it unambiguously, it is necessary to carry out experiments at a constant ionic strength since feH and kHX are influenced by salt effects. Studies in the presence of halides at a constant ionic strength have never been done. Other approaches have been used instead. Albery and Bell [200] measured hydrolysis rates of ethyl diazoacetate in moderately concentrated perchloric acid and hydrochloric acid solutions. Rates in hydrochloric acid were faster than those in perchloric acid at the same stoichiometric concentration. In order to verify the dependence on the chloride ion concentration, it was assumed that rates of the reaction without participation of chloride (first term in eqn. (47)) are the same in perchloric acid and hydrochloric acid if the H0 values are equal. Activity coefficients were introduced in eqn. (47) as follows ... 

Dependence of the rate of hydrolysis on the first power of [H30 +] at a constant ionic strength has been established for diazoacetone [206], diazoacetophenone [207], and a few other compounds [207]. For 4 primary diazoketones [206] and 4 primary diazosulfones [208, 209], hydrolysis rates have been measured in moderately concentrated perchloric acid in water or in dioxane—water. In all examples, slopes of plots of log ft versus H0 are between —0.95 and —1.17. 

The pH dependence of the decarboxylation rate of some aromatic amino acids has been studied in dilute solutions of strong acids and in acetate buffers, at a constant ionic strength of ju = 0.1 N. The results can be fitted into a rate equation with two terms, viz. 

The ionic strength dependence of the rate constant is completely contained within the X, Y and Z terms. Eq. 8 allows a non-linear least-squares fitting of experimental data to be carried out, yielding values for /tx and V. In practice, the monopole and dipole terms cannot be uniquely characterized by this procedure, and using only the Fii term in the fitting procedure yields a satisfactory value for V [. 

Figure 1. Ionic strength dependence of observed rate constants for ET from two reduced cytochromes (c555 and C551) to oxidized plastocyanin. Solid lines are theoretical fits to the data points using the parallel plate model (only the V term is included).

Figure 4. Ionic strength dependence of the observed rate constant for electron transfer from reduced Fd to oxidized FNR. The dashed line corresponds to a theoretical fit to the higher ionic strength data using the parallel-plate model. The solid line is a smooth curve drawn through the data points.

Fig. 3-59. Dependence of the retention of various inorganic anions on the pH value of the buffer mixture at constant ionic strength. - Separator column IonPac AS4 eluent 0.0028 mol/L NaHC03 + 0.0022 mol/L Na2C03, pH adjusted with H3BC>3/NaOH flow rate 2 mL/min detection suppressed conductivity injection 50 pL anion standard.

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