Reaction orders from half-wave potentials

Chemical Kinetics from Half-Wave Potentials Determination of Rate Constants and Reaction Orders... 

Dihydrogen evolves from vanadium(II)-cysteine at pH6.0-9.5. This reduction is first order in V11 and independent of pH in the range 7.5-8.5. If cysteamine or cysteine methyl ester is used, dihydrogen is still evolved. The reaction with serine is 1000 times slower than with cysteine even though the half wave potentials are comparable.156 This reaction may be explained by a hydride pathway similar to that proposed for catechol complexes or alternatively Scheme 8. 

Figure 11.19. Correlation between the second-order rate constants for reactions of substituted phenoxide anions with chlorine dioxide and estimated values of AG°. The circles are experimental data and the curves represent fits of the data to the Marcus equation. The solid curve corresponds to equation 46 (X = 30.1 kcal moP ). AG° (and log K) values were calculated from electrode half-wave potentials. (Adapted from Tratnyek and Hoigne, 1994.)...

Hagisawa [41 HAG] estimated log p (V.I2) = - 11 from the pH of a dilute solution of NaHSe. This procedure may introduce an uncertainty of several orders of magnitude depending on the purity of the reagents employed. Lingane and Niedrach [48LI14/NIE] studied the half-wave potential of the reaction ... 

With reference to reversible electrode reactions, we have tackled the question of how slow the rate of change in potential has to be for the resulting voltammogram to be indistinguishable from that under true steady-state conditions [67]. It was concluded that, in order to obtain both the correct half-wave potential (to within 1 mV) and Tafel slope, the scan rate, va, must be such that... 

FAST REACTIONS ACCOMPANYING THE ELECTRODE PROCESS AND RATES OF ELECTRODE PROCESS PROPER From the measurements of polarographic limiting kinetic currents (and sometimes of their half-wave potentials), and their dependence on certain parameters (mainly pH, buffer composition, drop-time etc.), it is possible to compute rate constants for the fast chemical reactions, antecedent, parallel or consecutive to the electrode process proper. Rate constants of the second order reactions of the order 10 to 10 1. mol. sec have been determined in this way. The mathematical basis and the method of computation of the rate constants is beyond the scope of this text, and the reader is referred to other texts. 

The kinetics of the electron-transfer reactions between oxalatocobalt(m) complexes and iron(n) have been described. For cationic complexes, the rates decrease in the order (ox = oxalate) [Co(ox)(phen)J+>[Co(ox)(bipy)2] > [Co(ox)(NH3)4]+> [Co(ox)(en)2]+> [Co(oxXtrien)]+ the variation in reactivity is attributed to changes in the enthalpy of activation, a linear relationship being observed between and log k (k = observed rate constant). The much faster reactions of the phenanthroline and bipyridyl complexes are also considered to derive from the eflSciency of these ligands as electron mediators in reactions of this type. A relationship has also been observed between log k and the half-wave potential of the polarographic reduction of the cobalt(iii)... 

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