Ionic reactions Salt effects

The majority of reactions between ions in solution, particularly between simple ions of opposite charge, occur so rapidly that until recently it was impossible to measure the rates of these reactions. Relaxation techniques such as those described in Section 32.19 are now used to determine the rate of reactions such as HjO + OH 2 H2O. The rate constant 

A plot of logio k against the square root of the ionic strength should yield, in dilute solution, a straight line with a slope equal to ZaZb- 

1 What is the lowest possible value of the activation energy for an endothermic reaction  

2 The reaction 2CH3 A C2Hg proceeds with negligible activation energy. Estimate k via the collision theory. 

3 Long range electrical forces operate between the reactants in ion-polarizable molecule reactions. What are they Would Eq. (33.9) apply to such reactions  

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The first step on the cobalt wave ("prewave ") corresponds to discharge of Co (II) from its complex with cysteine. The height of this wave is limited by the formation rate of complexes between adsorbed cysteine anions and Co (II) ions in the layer adjacent to the electrode (henceforth written as [CoJs). Under these conditions cysteine is clearly present in excess with respect to [Co]. Reduction of the complex Co (ID ions (compared with the free ions) evidently arises from increase in their concentration near the electrode owing to adsorption of the complexes. The decrease in the height of this surface kinetic wave with increase in ionic strength is due both to a reduction in the rate constant of the ionic reaction (salt effect) and to a decrease in [Co]5 resulting from a decrease in the absolute value... 

Effect of Electrolytes in the Ionic Reactions These effects are generally known as salt effects. They are of two types—... 

Primary Salt Effect It is defined as the effect of ionic strength on the velocity of the ionic reaction. This effect is involved in non-catalytic reactions. 

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 ) ... 

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. 

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. 

SALT EFFECTS ON SECOND-ORDER IONIC REACTIONS... 

Ionic reactions are usually studied in the presence of an inert electrolyte so as to avoid salt effects. The investigator decides on one ionic strength and then adjusts the concentration of the electrolyte from one experiment to the next as the reactant... 

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. 

One factor that complicates the kinetic picture is the salt effect. An increase in ionic strength of the solution usually increases the rate of an SnI reaction (p. 451). But when the reaction is of charge type II, where both Y and RX are neutral, so that X is negatively charged (and most solvolyses are of this charge type), the ionic strength increases as the reaction proceeds and this increases the rate. This effect must be taken into account in studying the kinetics. Incidentally, the fact that the addition of outside ions increases the rate of most SnI reactions makes especially impressive the decrease in rate caused by the common ion. 

We have seen how the polarity of the solvent influences the rates of Sn 1 and Sn2 reactions. The ionic strength of the medium has similar effects. In general, the addition of an external salt affects the rates of SnI and Sn2 reactions in the same way as an increase in solvent polarity, though this is not quantitative different salts have different effects. However, there are exceptions though the rates of SnI reactions are usually increased by the addition of salts (this is called the salt effect), addition of the leaving-group ion often decreases the rate (the common-ion effect, p. 395). 

Electrostatic effects other than ionization are also important. Interactions between reacting ions depend on the local electrical environment of the ions and thus reflect the influence of the dielectric constant of the solvent and the presence of other ions and various solutes that may be present. In dilute solutions the influence of ionic strength on reaction rates is felt in the primary and secondary salt effects (see below). 

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... 

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... 

An unusual solvent effect was observed in cycloadditions of aromatic nitrile N-oxides with alkyl-substituted p-benzoquinones in ethanol-water (60 40) the reaction rates were 14-fold greater than those in chloroform (148). The use of ion pairs to control nitrile oxide cycloadditions was demonstrated. A chiral auxiliary bearing an ionic group and an associated counterion provides enhanced selectivity in the cycloaddition the intramolecular salt effect controls the orientation of the... 

Another arena for the application of stochastic frictional approaches is the influence of ionic atmosphere relaxation on the rates of reactions in electrolyte solutions [19], To gain perspective on this, we first recall the early and often quoted triumph of TST for the prediction of salt effects, in connection with Debye-Hiickel theory, for reaction rates In kTST varies linearly with the square root of the solution ionic strength I, with a sign depending on whether the charge distribution of the transition state is stabilized or destabilized by the ionic atmosphere compared to the reactants. 

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. 

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). 

We have made no systematic studies of ionic strength effects. Ionic strength does not seem important in the catalytic hydrolysis of the acylnitrophenolates, which carry no charge. There are substantial salt effects in the decarboxylation reaction with nitrobenzisoxazole, which is an anionic substrate. These may reflect general ion cloud effects or anionic competition for binding to the polymer. 

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