Thermodynamic redox sequence

The processes occurring in the hypolimnion and in the epilimnion have to be considered separately. The appearance of As(III) in the anoxic hypolimnion is in qualitative agreement with the thermodynamic redox sequence, because it appears together with Fe(II) and S(-II). The pe calculated from the As(III)/As(V) couple at 30-31 m on October 19, 1989, is in agreement with pe calculated from Fe(II)/Fe(III) (pe 0), but is higher than indicated by the presence of S(-II). 

The oxidation state of redox-sensitive trace elements such as As(III)/ As(V) and Cr(III)/Cr(VI) is thus affected by the redox conditions, as indicated by the occurrence of major reduced species. Kinetic control of the redox reactions plays an important role. As(III) appears in the anoxic hypolimnion in agreement with the thermodynamic redox sequence together with Fe(II) and sulfide, although the reduction of As(V) is incomplete under these conditions. Whereas the reduced As(III) species can clearly be observed in the... 

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... 

The above observations are consistent with a thermodynamically controlled process shown in Figure 9.17. Thus, when two strands, A and B, with complementary H bonding sequences and termini that can be reversible cross-linked, are present in the same solution under redox conditions, products A-A (or self-cyclized A ), B-B (or self-cyclized B ), and A-B, may be generated. Among these products, A-B gains the most stabilization from the newly generated, complementary intramolecular H bonds because of the formation of the two disulfide bonds. Thus,... 

Reversible redox reactions can initiate radical chemistry without a follow-up reduction or oxidation reaction. In successful reactions of this type, the redox step that produces the radical is thermodynamically disfavored. For example, Cu(I) complexes react reversibly with alkyl hahdes to give Cu(II) hahde complexes and an alkyl radical. The alkyl radical can react in, for example, an addition reaction, and the product radical will react with the Cu(II) hahde to give a new alkyl halide. This type of reaction sequence, which has been apphed in living radical polymerizations, is in the general family of nonchain radical reactions discussed earlier. ... 

While all of these reactions are favored thermodynamically, they are almost always enzymatically catalyzed by bacteria. It has been observed from the study of pore waters in deepsea sediments (e.g., Froelich et al., 1979) and anoxic basins (e.g., Reeburgh, 1980) that there is an ordered sequence of redox reactions in which the most energetically favorable reactions occur first and the active electron acceptors do not overlap significantly. Bacteria are energy opportunists. Using estimates of the stoichiometry of the diagenesis reactions (Table 2) one can sketch the order and shape of reactant profiles actually observed in sediment pore-water chemistry... 

Most reviewers5 8 now argue that photosynthetic oxygen evolution results from a sequential four-step electron-transfer process in which oxidizing equivalents from chi fl+- are accumulated in a "charge-storing" complex to accomplish the concerted four-electron oxidation of two H2O molecules to one O2 molecule. The photooxidant (chi + ) and reductant (pheo O are one-electron transfer agents, and the matrix is the lipoprotein thylakoid membrane. Hence, evaluation and consideration of the one-electron redox potentials for PS 11 components within a lipoprotein matrix are necessary in order to assess the thermodynamic feasibility of proposed mechanistic sequences. 

The improvement in the first two areas has been possible because of the progress made in understanding the mechanism and kinetics of radical polymerization and the stracture of radical intermediates. Further advancement requires detailed structure-reactivity correlation for radicals and also for dormant species. Both experimental measurements of rate and equilibrium constants as well as computational evaluation of thermodynamic (bond dissociation, redox properties) and kinetic properties of the involved species is needed. This will help to establish order of reactivities for various species and will assist selection of the efficient initiators or sequence of monomer addition for block copolymerization. 

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