Accounting for Electrons in Redox

To interpret redox reactions in terms of electron exchange, one must account for electrons in the various reacting species. Various textbooks (e.g., 4, 5) provide simple rules, such as the following, for assigning oxidation states for inorganic redox couples ... 

We first examine briefly the binding domains of the primary (Qa) and secondary (Qb) quinone-acceptor molecules in the bacterial reaction centers, and see how one can rationalize their functional relationship. In Rp. viridis, the two quinones are different, Qa being menaquinone-7 and Qb ubiquinone-10. In this case, electron transfer from Qa to Qb could be accounted for by the difference in redox potential of the two types of quinones. In Rb. sphaeroides, however, both Qa and Qb are ubiquinone-10 and the condition for an exothermic electron transfer from Qa to Qb might be satisfied if the environments of the two quinone molecules were sufficiently different. Such differences in local structural features might also account for differences in other properties of the two quinone molecules. 

Another situation is observed when salts or transition metal complexes are added to an alcohol (primary or secondary) or alkylamine subjected to oxidation in this case, a prolonged retardation of the initiated oxidation occurs, owing to repeated chain termination. This was discovered for the first time in the study of cyclohexanol oxidation in the presence of copper salt [49]. Copper and manganese ions also exert an inhibiting effect on the initiated oxidation of 1,2-cyclohexadiene [12], aliphatic amines [19], and 1,2-disubstituted ethenes [13]. This is accounted for, first, by the dual redox nature of the peroxyl radicals H02, >C(0H)02 and >C(NHR)02 , and, second, for the ability of ions and complexes of transition metals to accept and release an electron when they are in an higher- and lower-valence state. 

We have seen how analytical calculations in titrimetric analysis involve stoichiometry (Sections 4.5 and 4.6). We know that a balanced chemical equation is needed for basic stoichiometry. With redox reactions, balancing equations by inspection can be quite challenging, if not impossible. Thus, several special schemes have been derived for balancing redox equations. The ion-electron method for balancing redox equations takes into account the electrons that are transferred, since these must also be balanced. That is, the electrons given up must be equal to the electrons taken on. A review of the ion-electron method of balancing equations will therefore present a simple means of balancing redox equations. 

More simple is to account for the difference between the relatively high redox potentials of Rieske ferredoxins and the low potential values of plant 2Fe ferredoxins. In fact, the dianionic form of plant ferredoxins [(Cys)2Fem(/i2-S)2Feni(Cys)2]2- certainly electrostatically disfavours electron addition with respect to the neutral charge of Rieske ferredoxins [(Cys)2FeIIWS)2FeII,(His)2]°... 

From the expressions given for example in Refs. [4,9,29], it can be seen that the nuclear factor, and consequently the electron transfer rate, becomes temperature independent when the temperature is low enough for only the ground level of each oscillator to be populated (nuclear tunneling effect). In the opposite limit where IcgT is greater than all the vibrational quanta hco , the nuclear factor takes an activated form similar to that of Eq. 1 with AG replaced by AU [4,9,29]. The model has been refined to take into account the frequency shifts that may accompany the change of redox state [22]. 

Effects of Isomerism Geometrical isomers of coordination compounds can exhibit different values of redox potential, as accounted for, in various cases, by simple djr orbital level splitting diagrams or by MO calculations. The dependence of the relative stability and redox behavior of the geometrical isomers on the electronic configuration of the metal is also... 

However, the extent of the activity enhancement cannot be related to the higher surface area of this material. Two possible explanations were proposed to account for the effect of mirror plane composition on combustion activity one is related to the different oxidation state of the cation in the mirror plane the other is associated with the crystal structure of layered-alumina materials (i.e., magne-toplumbite and (3-Al203) which have different population and co-ordination of the ions in the mirror planes. Both these electronic and structural factors can, in principle, affect the redox properties. 

Another key feature of redox thermodynamic cycles is that the free energy change in solution is still defined to involve a gas-phase electron, that is, the solvation free energy of the electron is happily not an issue. And, once again, redox potentials in soludon typically assume 1 M standard states for ad species (but not always in this chapter s case study, for instance, all redox potentials were measured and computed for chloride ion concentrations buffered to 0.001 M). So, free energy changes associated with concentration adjustments must also be properly taken into account. 

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