Potential energy reduction half-reactions

Since reactions (i) and (iii) are both reduction half reactions, we cannot simply subtract the potential for (i) from the potential for (iii). Instead, we are forced to obtain the voltage for (ii) via the free energy changes for the three half reactions. Thus,... 

Since tables of standard apparent reduction potentials and standard transformed Gibbs energies of formation contain the same basic information, there is a question as to whether this chapter is really needed. However, the consideration of standard apparent reduction potentials provides a more global view of the driving forces in redox reactions. There are two contributions to the apparent equilibrium constant for a biochemical redox reaction, namely the standard apparent reduction potentials of the two half-reactions. Therefore it is of interest to compare the standard apparent reduction potentials of various half reactions. 

Module[ energy),( Calculates the standard apparent reduction potential of a half reaction at specified pHs and ionic strengths for a biochemical half reaction typed in the form nadox+de==nadred. The names of the reactants call the corresponding functions of pH and ionic strength, nu is the number of electrons involved. pHlist and islist can be lists. ) energy = Solve[eq, de] ... 

In theory, the standard reduction potential for a metal ion can be calculated using a Born-Haber-type thermochemical cycle. The reduction half-reaction is the sum of the negative of the atomization energy, the negative of the ionization energy, and the negative of the hydration enthalpy, as shown in Equations (14.26)-( 14.28) ... 

Reconsider the voltaic cell shown in Figure 2.1. There are two electrodes, Zn and Cu. These two metals each have different tendencies for accepting electrons. This tendency for the haif-reaction of either copper or zinc to occur as a reduction half-reaction in an electrochemical cell can be quantified as a reduction potential. There are two half-cells in Figure 2.1 a strip of zinc placed in a solution of ZnSO and a strip of copper placed in a solution of CuSO. The difference in potential between an electrode and its solution is known as electrode potential. When these two half-cells are coimected and the reaction begins, a difference in potential is observed between the electrodes. This potential difference, or voltage, is proportional to the energy required to move a certain electric charge between the electrodes. A voltmeter connected across the Zn Cu voltaic cell measures a potential difference of about 1.10 V when the solution concentrations of Zn + and Cu2+ ions are each 1 M. 

Values of AjG,aq) and A,G oin) for several radical species have been given in Table 8. Generally the AfG(aq were obtained from the reduction potentials in Tables 6 and 7 and A, G,aq of the products of reduction half reactions. The A, G,aq) of sulfur species were calculated from A, G(g/ s in Wagman et al. [5] and Stein et al. [10], and solution free energies [87]. In cases where ions were generated as well as the sulfur parent species, for example reaction (41) ... 

Figure 1.3 Energy level diagrams illustrating reduction potentials of relevant half reactions. Ground state and excited state potentials of porphyrins 1 (red), 2 (green) and 3 (blue) were determined by cyclic voltammetry measurements together with absorption and emission spectra. Approximate potentials for the Ti02 and Sn02 conduction bands and the I3 /I" and BrJ/Br couples are also shown. The O2/H2O couple is 0.82 V at pH = 7. All potentials are reported in V vr. the normal hydrogen electrode (NHE).

It is also possible to obtain the value of oxidation reactions from Table 9.1. The halfcell potential of an oxidation reaction is simply the negative of the reported reduction half-reaction. The half-reaction potential and the hydrogen reduction reaction reference are analogous to our use of Gibbs energy of formation and the elemental form of molecules in the other parts of this chapter. 

The half-reactions and reduction potentials in Table 21.1 can be used to analyze energy changes in redox reactions. The oxidation of NADH to NAD can be coupled with the reduction of a-ketoglutarate to isocitrate ... 

Illustrative Example 14.4 Calculating Free Energies of Reaction from Half Reaction Reduction Potentials... 

Finally, we should note that the E value of a multielectron transfer half reaction is given by the average of the respective standard one-electron reduction potentials. This is easy to rationalize when recalling that the overall standard free energy of reaction of a sequence of reaction steps is given by the sum of the ArG° values of each step. Hence, we may write ... 

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