Primary salt effects

In the process of identifying the quasi-thermodynamic activation parameters in Section 9.9 we assumed that activity could be equated with concentration. While reasonable for the examples discussed in Sections 9.10 and 9.11 it is obviously inappropriate if we wish to consider ionic reactions. From (9.41) the rate constant is related to the ratio wIb/wa b which can no longer be set equal to an equilibrium constant. It is the activities, not the concentrations, which are related to the equilibrium constant 

The quantity / o(T) is the rate constant in very dilute solution where all the activity coefficients are unity. The formula (9.46) is obviously applicable to any system but the activity corrections are most dramatic for ionic reactions. 

In ionic solution the ion activity coefficients are strongly charge dependent from Debye-Hiickel theory we have 

Thus increasing the ionic strength speeds up reactions between similarly charged ions and slows down those between ions of opposite charge, a point already mentioned in our discussion of diffusion-controlled reactions 

Some reactions have not been balanced. [Adapted from S. W. Benson, The Foundations of Chemical Kinetics (New York McGraw-Hill, I960), p. 525, where original references are given.] 

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An effect of ionic strength on as a consequence of effects on the activity coefficient ratio is called a primary salt effect. We will, in Section 8.3, consider this effect quantitatively. 

It seemed to us that the concept of primary salt effect was worth consideration for the polyelectrolyte catalysis156 . According to Bronsted157 and Bjerrum1 s8 the rate constant of the reaction is accounted for in terms of the activated complex theory A + B X -> C + D, X is the activated complex, C and D denote the product. The second-order rate constant, k2, is given by... 

In many drug solutions, it is necessary to use buffer salts in order to maintain the formulation at the optimum pH. These buffer salts can affect the rate of drug degradation in a number of ways. First, a primary salt effect results because of the effect salts have on the activity coefficient of the reactants. At relatively low ionic strengths, the rate constant, k, is related to the ionic strength, p, according to... 

Equation 7.1.2 characterizes what is known as the primary salt effect (i.e. the influence of ionic strength on the reaction rate through the activity coefficients of the reactants and the activated complex). Much early work on ionic reactions is relatively useless because this effect was not understood. Now it is common practice in studies of ionic reactions to add a considerable... 

It is generally observed that the rate of reaction can be altered by the presence of non-reacting or inert ionic species in the solution. This effect is especially great for reactions between ions, where rate of reaction is effected even at low concentrations. The influence of a charged species on the rate of reaction is known as salt effect. The effects are classified as primary and secondary salt effects. The primary salt effect is the influence of electrolyte concentration on the activity coefficient and rate of reaction, whereas the secondary salt effect is the actual change in the concentration of the reacting ions resulting from the addition of electrolytes. Both effects are important in the study of ionic reactions in solutions. The primary salt effect is involved in non-catalytic reactions and has been considered here. The deviation from ideal behaviour can be expressed in terms of Bronsted-Bjerrum equation. 

In primary salt effect, addition of an electrolyte (salt) or variation of ionic strength affects the activity coefficients and hence the rate of reaction. However, in a reaction where H+ or OH ions produced from a weak acid or weak base act as catalyting agent, the addition of salt influences the concentration of H+ or OH ions. Since the rate of reaction depends upon the concentration of H+ or OH, it will be affected by the salt concentration. This phenomenon is known as secondary salt effect. 

What is primary salt effect What will be the effect of ionic strength on the rate of reaction if... 

Salt effects in kinetics are usually classified as primary or secondary, but there is much more to the subject than these special effects. The theoretical treatment of the primary salt effect leans heavily upon the transition state theory and the Debye-Hii ckel limiting law for activity coefficients. For a thermodynamic equilibrium constant one should strictly use activities a instead of concentrations (indicated by brackets). 

Although these effects are often collectively referred to as salt effects, lUPAC regards that term as too restrictive. If the effect observed is due solely to the influence of ionic strength on the activity coefficients of reactants and transition states, then the effect is referred to as a primary kinetic electrolyte effect or a primary salt effect. If the observed effect arises from the influence of ionic strength on pre-equilibrium concentrations of ionic species prior to any rate-determining step, then the effect is termed a secondary kinetic electrolyte effect or a secondary salt effect. An example of such a phenomenon would be the influence of ionic strength on the dissociation of weak acids and bases. See Ionic Strength... 

One should mention here that in reactions involving ions, the effects of electrolytes can be pul into two principal categories (a) primary salt effect and lb) secondary salt effects. 

Throughout this chapter we have formulated rate laws in terms of concentrations and ignored activity corrections, as is almost always done in environmental chemistry. However, where ionic strength, I, varies and both reactants are charged, a substantial "primary salt effect" can be expected (167). The effect is described by... 

Figure 7.1 The primary salt effect plot of logio k versus /l...

Question. Use the following data relating to a reaction of the type PtLX+(aq) + Y-(aq) to test the primary salt effect. 

The theoretical treatment of the primary salt effect can be extended to A S and A H, and magnitudes of the slope in plots of the observed values against /7 or /7/( 1 + /7) can be compared with the theoretical values. 

Kinetic results which apparently do not fit the above treatment of the primary salt effect do so when the observed rates are correlated with the actual ionic strengths rather than the stoichiometric values. The actual concentrations in the reaction solution are calculated using the known value of the equilibrium constant describing the ion pair. This is discussed in Problem 7.5. 

This confirms the essential correctness of the theoretical treatment of the primary salt effect. 

Reaction involves an ion and a molecule and could show a primary salt effect. The equilibrium constant describing the formation of the ion pairs also depends on ionic strength. 

C2H5)3N+CH2COOC2H5(aq) + OH (aq) - kinetic analysis on experimental data demonstrating the primary salt effect, 285-289... 

Bronsted-Bjerrum equation — The rate constant k of a chemical reaction involving ionic species A and B may be influenced by other ionic species in solution not directly participating in this reaction (i.e., a dissolved salt, thus the associated observation is called primary salt effect). The change of the rate as a function of the ionic charge of the involved species and the - ionic strength of the solution is given by the Bronsted-Bjerrum equation... 

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). 

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