Ionic strength rate effects

If a reaction is reversible and if one has assumed a rate function that does not take the reverse reaction into account, one observes a downward curvature. As equilibrium is approached, the slope of this curve approaches zero. Another cause of curvature is a change in temperature during the course of the experiment. An increase in temperature causes an increase in the reaction rate, leading to an upward curvature. Bunnett (3) has discussed a number of other sources of curvature, including changes in pH and ionic strength, impurity effects, autocatalysis, and side reactions. 

Ionic Strength The effect of ionic strength on rates of elementary reactions readily follows. Using equation 127, we can let t>e the value of the second-order rate constant in the reference state, such as an infinitely dilute solution (where all the activity coefficients are unity) k is the rate constant at any specified ionic strength ... 

Figure 18 illustrates the pronounced effect of the presence of counterions in HPAA. In pure water as a solvent, the flow resistance enhancement occurs at a very low flow rate. As the ionic strength is increased, the strain rate at which the dramatic enhancement in flow resistance occurs is markedly increased until beyond about 0.5 M, where no further effect occurs. The magnitude of the effect, however, remains approximately constant. The shear viscosity is greatly reduced by the increase in ionic strength. This effect is attributed to a progressive collapse of the HPAA coils because of counterion screening and results in a reduction in the conformational relaxation time and a consequent increase in the strain rate required to produce a coil-stretch transition. 

In addition to the effects of temperature and ionic strength, rates are also affected by changing pressure. By analogy to the effect of pressure on equilibrium constants, the expected effect of pressure on rate constants (Asano andleNobel, 1978 Drljaca etal, 1988 Eckert, 1972) is related to the change in volume that results from the conversion of the reactants to the activated complex. This activation volume (A F ) is the difference between the partial molal volumes of the reactants and the transition state. 

For example, van den Tempel [35] reports the results shown in Fig. XIV-9 on the effect of electrolyte concentration on flocculation rates of an O/W emulsion. Note that d ln)ldt (equal to k in the simple theory) increases rapidly with ionic strength, presumably due to the decrease in double-layer half-thickness and perhaps also due to some Stem layer adsorption of positive ions. The preexponential factor in Eq. XIV-7, ko = (8kr/3 ), should have the value of about 10 " cm, but at low electrolyte concentration, the values in the figure are smaller by tenfold or a hundredfold. This reduction may be qualitatively ascribed to charged repulsion. 

FIGURE 4.17 Effect of ionic strength on the elution of anionic polymers. Column TSK-GEL GMPW, two 17 fjLirt, 7.5 mm X 60 cm columns in series. Sample 0.5 ml of 0.05-0.1% of the sodium salt of polyacrylic acid, an anionic polymer. Elution Water 0.01, 0.025, 0.05, or 0.1 M NaNOs in water. Flow rate 0.5 ml/min. Detection Rl. 

In the process of establishing the kinetic scheme, the rate studies determine the effects of several possible variables, which may include the temperature, pressure, reactant concentrations, ionic strength, solvent, and surface effects. This part of the kinetic investigation constitutes the phenomenological description of the system. 

An effective experimental design is to measure the pseudo-first-order rate constant k at constant pH and ionic strength as a function of total buffer concentration 6,. Very often the buffer substance is the catalyst. Let B represent the conjugate base form of the buffer. Because pH is constant, the ratio (B]/[BH ] is constant, and the concentrations of both species increase directly with 6 where B, = [B] -t-[BH"]. 

There is a third experimental design often used for studies in electrolyte solutions, particularly aqueous solutions. In this design the reaction rate is studied as a function of ionic strength, and a rate variation is called a salt effect. In Chapter 5 we derived this relationship between the observed rate constant k and the activity coefficients of reactants l YA, yB) and transition state (y ) ... 

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. 

Examination of the effect of pH on the rates of protodeboronation of the 2,6-dimethoxy compound at 90 °C in malonic acid-sodium malonate buffer solutions of ionic strength 0.14 gave the data in Table 199. A plot of these data revealed the curve shown in Fig. 3 (one of the points was misplotted on the original) and the linear portions of the plot were attributed to acid and base catalysis as shown on Fig. 3, and since the rates in the region of pH 4-5 are higher than would be... 

In this reaction the effect of changing the ionic strength of the medium was also studied by addition of sodium perchlorate which caused the rate coefficient to decrease. However, this was shown to arise from the resultant change in pH of the solution, for when this was allowed for, the normal positive salt effect was observed. 

The kinetics of decarboxylation of 4-aminosalicylic acid in some buffer solutions at 50 °C were studied. The first-order rate coefficients increased with increasing buffer concentration, though the pH and ionic strength were held constant (Table 217). This was not a salt effect since the rate change produced by substituting potassium chloride for the buffer salt was shown to be much smaller. It follows from the change in the first-order rate coefficients (kx) with... 

These large increases in rate might be attributed to the operation of a neutral salt effect, and, in fact, a plot of log k versus the square root of the ionic strength, fi, is linear. However, the reactants, in this case, are neutral molecules, not ions in the low dielectric constant solvent, chloroform, ionic species would be largely associated, and the Bronsted-Bjerrum theory of salt effects51 52, which is valid only for dilute-solution reactions between ions at small n (below 0.01 M for 1 1 electrolytes), does not properly apply. 

Calculated electrostatic component" of the ionic strength effects on second-order rate constants for ions of different charges... 

If one of the partners in a second-order reaction is not an ion, then in ideal solutions there will be little effect of added salts on the rate. The activity coefficient of a nonelectrolyte does not depend strongly on ionic strength the way that the activity coefficients of ions do. In a reaction with only one participating ion, it and the transition... 

It then also follows that the rate constant for a first-order reaction, whether or not the solvent is involved, is also independent of ionic strength. This statement is true at ionic strengths low enough for the Debye-Huckel equation to hold. At higher ionic strengths, predictions cannot be made about reactions of any order because all of the kinetic effects can be expected to show chemical specificity. 

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. 

Salt effects. The rate of reaction of vanadium(V) with iodide ions is independent of ionic strength.28 The rate law is v = fc[V(V)][I ][H+]2. What is the charge on the predominant V(V) species in these solutions ... 

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. 

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