Electrochemical reaction orders reactions

Electrochemical reaction orders in electrode polymerization, 317 Electrochemical relaxation, as a function of cathode potential, 388 Electrochemical responses during polymer formation, 400... 

For many electrochemical reactions the reaction rate is proportional to the concentration of the reacting species (first-order reaction) ... 

In the past two chapters we have already encountered examples of reactions involving several steps, and introduced the notion of rate-determining step. Here we will elaborate on the subject of complex reactions, introduce another concept the electrochemical reaction order, and consider a few other examples. 

An example will be presented in the next section. The coefficients Xi>a and Xijj are called electrochemical reaction orders. Usually the species A and B react only with a few of the substances Si, so that the reaction orders for the other species vanish. We assume that the reactions (11.16) and (11.17) are in equilibrium. The total reaction is ... 

Figure 11.2 Tafel plot for various concentrations of reactant 5 in equilibrium with A and electrochemical reaction order xa = 1 the dashed curve refers to a concentration of 5 lowered by a factor of two, the dotted curve to a concentration lowered by a factor of four with respect to the full curve.

If one of the concentrations Cj is varied, the Tafel lines are shifted, and the electrochemical reaction orders xai and x b can be determined from ... 

The quantities a, z, n can be determined separately, so that Eq. (11.32) offers an alternative way of obtaining the electrochemical reaction orders. A good discussion of the coupling of electrochemical with chemical reactions has been given by Parsons [1]. 

As an example of the determination of electrochemical reaction orders, we consider the deposition of silver from an aqueous solution containing cyanide. The latter forms various complexes with silver ions, such as AgCN, Ag(CN) ", Ag(CN)3. Therefore, in the bulk of the solution reactions of the type ... 

At potentials sufficiently cathodic to neglect the anodic reaction, the electrochemical-reaction orders d and b are defined as... 

Note that in all these equations for electrochemical-reaction orders, Aty has been stipulated as a constant. 

K. J. Vetter, Electrochemical Kinetics Ch. 3, on electrochemical reaction orders, Academic... 

Notice that, because of the strong dependence of the kinetics on electrode potential, the determination of the electrochemical reaction order requires that the partial cathodic or anodic current densities are measured at constant potential in addition to the activities of the other species remaining constant. 

Vetter has extensively used the electrochemical reaction order to establish the mechanism of complex electrode reactions [7]. For instance, for the overall electrode reaction... 

Also, in complex electrode reactions involving multistep proton and electron transfer steps, the electrochemical reaction order with respect to the H+ or HO may also vary with pH, indicating a change of mechanism with pH. In this respect, the use of schemes of squares outlined in Sect. 2.2 is very useful in the analysis of these complex kinetics [13]. 

Using concentrations instead of activities and eqn. (89), the respective electrochemical reaction orders are... 

The same equation had previously been derived by Fronaeus and Ostman [249]. These workers made two further important contributions to the theory. Taking it for granted that the catalysed exchange proceeded by an electron transfer mechanism, they related ucat to the concentrations of Ox and Red by the theory of electrode kinetics. For simple redox couples whose electrochemical reaction orders are unity, this leads to... 

The electrochemical reaction order of one leads to the formation of the cation complex with one anion. This complex has less positive charge and thus requires less activation energy for its transfer from its potential well within the matrix of... 

Millicoulometry is employed in Clarks and Lubs buffer of pH 2.00 to find out number of electrons in the electrode process. The results show that the number of electrons involved in the electrode process is four. The product of controlled potential electrolysis (carried out at -l.OV vs. SCE) in Clarks and Lubs buffer of pH 2.00 is identified as hydroxylamine. The number of protons involved in the rate determining step is evaluated as one from E1/2 vs. pH and Ep vs. pH plots. The electrochemical reaction order is shown to be one from log i vs. log C plots at constant E. 

Determination of electrochemical reaction orders from Tafel lines... 

Measurement of the Tafel lines for different concentrations, the partial electrochemical reaction orders, can be detennined from Eqs. (6.34) and (6.35). This implies that the concentration at the electrode surface remains approximately equal to the bulk concentration (elimination of the diffusion overpotential). It should be pointed out that for this evaluation the logarithm of current must be plotted versus the electrode potential and not versus the overpotential. 

The electrochemical reaction orders for the anodic process are determined from the concentration dependence of Ig (Figure 6.9) the electrochemical reaction orders for the cathodic process are determined from the concentration dependence of Ig (Figure 6.10). 

Figure 6.9 Plot of Ig 4 versus Ig and determination of the electrochemical reaction order...

With the charge transfer coefficient determined from the Tafel plots (Figure 6. 8) and the concentration dependence of the exchange current densities, the following partial electrochemical reaction orders were determined... 

The nitrous acid-nitiic acid redox system is an example of a controversy in the hterature. The above mechanism was developed by Vetter from one of the first applications of the concept of electrochemical reaction orders on a redox system. G. Schmid - intuitively suggested another mechanism based on a charge transfer reaction... 

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