Rate constants potential-determining ions

In early work on the effect of potential on ET reactions [76], Solomon and Bard showed that an ET reaction between Fe(CN)g in an aqueous phase and 7,7,8,8-tetra-cyanoquinodimethane (TCNQ) in 1,2-dichloroethane (DCE) could be promoted by judiciously adjusting the potential drop across the ITIES, using tetraphenylarsonium cation as a potential determining ion. In a similar period, Selzer and Mandler [77] reported a study of the ET reaction between aqueous IrClg and Fc in a NB phase, without any potential determining ion in either phase. A first-order rate constant of 0.013 cm s was obtained... 

The ET reaction between aqueous oxidants and decamethylferrocene (DMFc), in both DCE and NB, has been studied over a wide range of conditions and shown to be a complex process [86]. The apparent potential-dependence of the ET rate constant was contrary to Butler-Volmer theory, when the interfacial potential drop at the ITIES was adjusted via the CIO4 concentration in the aqueous phase. The highest reaction rate was observed with the smallest concentration of CIO4 in the aqueous phase, which corresponded to the lowest driving force for the oxidation process. In contrast, the ET rate increased with driving force when this was adjusted via the redox potential of the aqueous oxidant. Moreover, a Butler-Volmer trend was found when TBA was used as the potential-determining ion, with an a value of 0.38 [86]. 

The equilibrium at an electrode is dynamic, the potential determining ions moving in opposite directions at equal rates. If the process is reversible, the ions move smoothly from one phase to the other via the phase boundary and perform no work. The electrode potential remains constant and independent of current. 

Interfacial Gating Limit For a high barrier in the electrochemical potential of the ion, the same conditions as those discussed by Marcus prevail [123], and the rate constant is determined by the time needed for the creation of the favorable protrusion that reduces the barrier for the ionic transition. 

The initial kinetic energy of 0 ions produced by dissociative attachment in 02 at an electron energy of 6.9 e.v. may be determined from Equation 4 to be 1.64 e.v. using values of 1.465 e.v. (1) for A(0) and 5.09 e.v. (7) for D(O—O). The residence time for 0 ions calculated from Equation 1 is 6.0 X 10 7 sec. at 10 volts repeller potential. Rate constants for Reaction 6 determined from data at varying Vr are shown in Table I and are seen to increase sharply with increasing repeller potential, as expected for an endothermic process. 

Pulsed source techniques have been used to study thermal energy ion-molecule reactions. For most of the proton and H atom transfer reactions studied k thermal) /k 10.5 volts /cm.) is approximately unity in apparent agreement with predictions from the simple ion-induced dipole model. However, the rate constants calculated on this basis are considerably higher than the experimental rate constants indicating reaction channels other than the atom transfer process. Thus, in some cases at least, the relationship of k thermal) to k 10.5 volts/cm.) may be determined by the variation of the relative importance of the atom transfer process with ion energy rather than by the interaction potential between the ion and the neutral. For most of the condensation ion-molecule reactions studied k thermal) is considerably greater than k 10.5 volts/cm.). 

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. 

To get support for the postulation of ion-radical nature of the rate-determining step, one can plot the reaction rate constant values against the oxidation and reduction potentials of the reaction partners. When both plots occur to be linear, it will support the postulation of the ion-radical route. 

This question was addressed by use of classical trajectory techniques with an ion-quadrupole plus anisotropic polarizability potential to determine the collision rate constant (k ). Over one million trajectories with initial conditions covering a range of translational temperature, neutral rotor state, and isotopic composition were calculated. The results for the thermally average 300 K values for are listed in the last column of Table 3 and indicate that reaction (11) for H2/H2, D2/D2, and HD /HD proceeds at essentially the classical collision rate, whereas the reported experimental rates for H2/D2 and D2/H2 reactions seem to be in error as they are significantly larger than k. This conclusion raises two questions (1) If the symmetry restrictions outlined in Table 2 apply, how are they essentially completely overcome at 300 K (2) Do conditions exist where the restriction would give rise to observable kinetic effects ... 

A number of rate constants for reactions of transients derived from the reduction of metal ions and metal complexes were determined by pulse radiolysis [58]. Because of the shortlived character of atoms and oligomers, the determination of their redox potential is possible only by kinetic methods using pulse radiolysis. In the couple Mj/M , the reducing properties of M as electron donor as well as oxidizing properties of as electron acceptor are deduced from the occurrence of an electron transfer reaction with a reference reactant of known potential. These reactions obviously occur in competition with the cascade of coalescence processes. The unknown potential °(M /M ) is derived by comparing the action of several reference systems of different potentials. 

Sluyters and coworkers [34] have studied the mechanism of Zn(II) reduction on DM E in NaCl04 solutions at different water activity (uw) using faradaic impedance method. Dqx and E p were determined from dc polarographic curves. Hydration numbers of Zn(Il) ion were estimated from the dependence of E[p on In Uw The obtained standard rate constant was changing with a NaCl04 concentration and the slope of the dependence of In k on potential was changing with potential (see Fig. 1). Therefore, the following mechanisms were proposed ... 

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