Thermodynamic sequences potentials

S.3.3 Electrocatalytic Modified Electrodes Often the desired redox reaction at the bare electrode involves slow electron-transfer kinetics and therefore occurs at an appreciable rate only at potentials substantially higher than its thermodynamic redox potential. Such reactions can be catalyzed by attaching to the surface a suitable electron transfer mediator (45,46). Knowledge of homogeneous solution kinetics is often used to select the surface-bound catalyst. The function of the mediator is to facilitate the charge transfer between the analyte and the electrode. In most cases the mediated reaction sequence (e.g., for a reduction process) can be described by... 

Figure 16. Three thermodynamic sequences for calculating the reduced stan-dardrstate chemical potential difference (a) method I, (b) method II, and (c)...

With the standard state for each component chosen as the pure component in the phase of interest and at the temperature of interest, Chang et al. (4-) have discussed three thermodynamic sequences for the calculation of the reduced standard state chemical potentials. The pathways for each sequence are shown in Figure 2. 

Figure 2. Three thermodynamic sequences for evaluating the reduced standard state chemical potential change.

The oxidizing power of the halate ions in aqueous solution, as measured by their standard reduction potentials (p. 854), decreases in the sequence bromate > chlorate > iodate but the rates of reaction follow the sequence iodate > bromate > chlorate. In addition, both the thermodynamic oxidizing power and the rate of reaction depend markedly on the hydrogen-ion concentration of the solution, being substantially greater in acid than in alkaline conditions (p, 855). 

Electrode potentials are determined by the affinities of the electrode reactions. As the affinities are changes in thermodynamic functions of state, they are additive. The affinity of a given reaction can be obtained by linear combination of the affinities for a sequence of reactions proceeding from the same initial to the same final state as the direct reaction. Thus, the principle of linear combination must also be valid for electrode potentials. The electrode oxidation of metal Me to a higher oxidation state z+>2 can be separated into oxidation to a lower oxidation state z+>1 and subsequent oxidation to the oxidation state z+>2. The affinities of the particular oxidation processes are equivalent to the electrode potentials 2 0, i-o> and E2-. ... 

For the sake of completeness, Figure 4-5 illustrates the more general situation of isothermal, isobaric matter transport in a multiphase system (e.g., Fe/Fe0/Fe304 / 02). A sequence of phases a, (3, y,... is bounded by two reservoirs which contain both neutral components (i) and electronic carriers (el). The boundary conditions imply that the buffered chemical potentials (u,(R)) and the electrochemical potentials (//el(R)) are predetermined in R] and Rr. Depending on the concentrations and mobilities (c/, b), c, 6 ) in the various phases v, metallic conduction, semiconduction, or ionic conduction will prevail. As long as the various phases are thermodynamically stable and no decomposition occurs, the transport equations (including the boundary conditions) are well defined and there is normally a unique solution to the transport problem. 

This does not mean, however, that the rules based on those assumptions must necessarily be incorrect. Though, for example, the original derivation of Evans equation is definitely incorrect, the final equation itself is quite correct (see Chapter 1). Further work is required to check the applicability of the proposed rules to other binary systems of different chemical nature. Also, much efforts are to be undertaken to find out other relationships between the thermodynamic properties of chemical compounds and the sequence of occurrence of their layers at the A-B interface. This sequence seems to be more dependent on the partial, rather than on the integral values of thermodynamic potentials. 

Thus one can quickly generate alternative solvay clusters. They also describe a convenient method to represent pictorially the free energy of the reactions versus reaction conditions (T and F). Using this representation one can quickly select reactions sequences which form a thermodynamically viable solvay cluster. Included in the article are some clever ways to extend the list of potential reactions. 

---