Rates, reaction ionic strength effects

The reader can show that a third scheme also gives the same answer. In it the two cations first associate (however unlikely), and this dinuclear complex reacts with Cl-. To summarize any reaction scheme consistent with the rate law is characterized by the same ionic strength effects. In other words, it is useless to study salt effects in the hopes of resolving one kinetically indistinguishable mechanism from another. 

Ferrocyanide reduces persulphate, the reaction being second-order in a fairly saline medium (0.5 M K2S04) with /c2 = 3.2x 10 exp(—11.9 x lO /Hr) l.mole . sec. The rate is strongly influenced by the presence of potassium ions and this has been shown not to be merely an ionic strength effect" . Consideration of all possible modes of ion-pairing led to the conclusion that the two reactants are [K(Fe(CN)6] and [KS20g] . At zero ionic strength, E = 9.6 kcal.mole and AS = —34.7 eu. Kershaw and Prue have measured the specific effects of many other cations on the rate of this reaction. 

The reaction scheme of Schwarz, with the specific rates, is shown in Table 7.1. Comparison with later compilations (Anbar et al., 1973, 1975 Farhataziz and Ross, 1977) indicates that most of these rates are reasonable within the bounds of experimental error. Some of the rates are pH-dependent, and when both reactants are charged there is a pronounced ionic strength effect these have been corrected for by Schwarz. He further notes that the second-order rates are not accurate for times less than 1 ns if the reaction radius... 

The high ionic concentration at the micellar surface may result in an ionic strength effect on reaction rate. Salt effects in water, however, are generally smaller for ion-molecule reactions than for reactions which involve an increase or decrease of charge, and they should be approximately zero in the... 

Effect of Ionic Strength. Both yE systems were examined for ionic strength effects. Microemulsion compositions were prepared at 70% water, with a cyanide concentration of 0.032 M with respect to the water content. Potassium bromide was used to vary the ionic strength of the reaction mixtures. Ionic strength in the CTAB yE was varied from 0.04 to 0.34. Since the Brij yE tolerated a much higher salt concentration without phase separation, ionic strength in that system was varied between 0.04 and 1.80. As will be seen, the Brij system exhibits a salt effect, while the CTAB yE does not. Rate constants obtained for reaction (1) in the Brij yE were therefore corrected to take into account the effect of ionic strength in that system (vide infra). 

An additional factor was found to influence the rate of reaction in the experiments involving tetrakis ( -mercaptoethylamine) trinickel (II) ion. The addition of nickel chloride retarded the process. Methanol was used as the solvent to demonstrate that the dependence was actually due to the presence of nickel ion and not an ionic strength effect. Magnesium chloride accelerates the rate slightly, while nickel ion greatly retards the rate of reaction. This effect was studied in greater detail, but solubility requirements necessitated the use of a water-methanol mixed solvent. A solution of 5.5M water in methanol was found to be satisfactory to obtain the necessary solubilities of complex and nickel chloride. 

The presence of H3 0+ or HO may alter drastically the observed reaction rate either because they catalyse the reaction (acid or base catalysis, see Section 3.2.3 for the Aldol reaction, and Chapter 11) or because of ionic strength effects. Proper pH control in an aqueous solution will require a buffer system which is described by the appropriate version of the Henderson-Hasselbach equation, according to whether the acid or base is the charged species ... 

As ice crystals grow in the freezing system, the solutes are concentrated. In addition to increased ionic strength effects, the rates of some chemical reactions—particularly second order reactions—may be accelerated by freezing through this freeze-concentration effect. Examples include reduction of potassium ferricyanide by potassium cyanide (2), oxidation of ascorbic acid (3), and polypeptide synthesis (4). Kinetics of reactions in frozen systems has been reviewed by Pincock and Kiovsky (5). 

Over the entire range of pH and temperature studied there was little or no dependence of the rate on ionic strength, that is, no salt effect. This result is consistent with the simple mechanism in that the slow step does not involve two ions. With at least one neutral molecule in the slow step, transition state theory predicts that the rate of the reactions will be independent of ionic strength. 

It is also important to point out that the direction of the ionic strength effect on rate constants does not always correlate with the protein net charge. Thus, in reactions with FMN [47] and flavodoxin [48], three species of cytochrome C2 having net charges of —7, 0 and -1-2 all show attractive electrostatic interactions during ET with these negatively charged species. The reason for this behavior lies in the fact... 

The effect of added salt on the decay rate (ionic strength effect) was determined at pH 3.3 and 9.0. Added salt should increase the rate of reaction between two ions of the same sign to a predictable degree (Reaction 19), and should have little or no effect on the rate of the first-order Reactions 18 and 20. At pH 9, where Reaction 19 does not occur appreciably and the salt effect is governed by Reaction 17, salt increased the decay rate (22) in quantitative agreement with expectation. At pH 3.3, where Reactions 17 and 19 affect the decay rate, no appreciable salt effect was found, again as predicted (29). 

If a simple electrostatic model (neglecting ionic-strength effects) is considered, the effect of solvent on reaction rates of polar molecules can be assessed by calculating the free energy of solvation of a spherical molecule of radius r containing a point dipole of magnitude fii at its center. The value obtained by Kirkwood [4] with a reference state of c = 1 (all other state variables being held constant) is... 

Fig. 1. The primary salt effect-variations of reaction rates with ionic strength. The circles are observed values the straight lines are theoretical (Equation (2)).

If counter ions have an effect on the electron-transfer rates, then Marcus theory would have a problem because these ions are not included in the theory. For most cationic reactants, such as those in Table 6.2, anions affect the rate through normal ionic strength effects and ion pairing. The latter has been observed generally to inhibit reaction in nonaqueous solvents. The Co(phen)3 " system is somewhat unusual in that N03 seems to have some catalytic effect. For anionic reactants the situation is quite different and cations often provide significant catalysis. One of the most widely studied of these is the Fe(CN)g system for which Wahl and... 

Dithionite is widely employed in biochemistry for the study of electron-transfer proteins, both as a means of preparing the reduced form of the protein and as a kinetic probe into the reactive behavior of the protein. For example, ionic strength effects are used to infer the charge of the reactive site. Indeed, most of the 140 reactions for which 802 rate constants have been reported are for biological molecules [4]. Essentially all of these have relied upon the equilibrium... 

A. Reaction rate and ionic strength, also known as the primary salt effect. For reactions between ions in solution, the presence of other ions influences the reaction rate. The Debye-Hiickel theory provides an explanation for both the direction and the magnitude of the effect. It stems from the stabilization of an ion by a cloud of oppositely charged ions. The stabilization of an ion of charge z scales as where /u. is the ionic strength, /u = (1 /2) c z due to the other... 

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