Redox free energy change

The free energy changes of the outer shell upon reduction, AG° , are important, because the Nernst equation relates the redox potential to AG. Eree energy simulation methods are discussed in Chapter 9. Here, the free energy change of interest is for the reaction... 

Reduction Potentials—An Accounting Device for Free Energy Changes in Redox Reactions... 

We have already noted that the standard free energy change for a reaction, AG°, does not reflect the actual conditions in a ceil, where reactants and products are not at standard-state concentrations (1 M). Equation 3.12 was introduced to permit calculations of actual free energy changes under non-standard-state conditions. Similarly, standard reduction potentials for redox couples must be modified to account for the actual concentrations of the oxidized and reduced species. For any redox couple. 

FREE ENERGY CHANGES CAN BE EXPRESSED IN TERMS OF REDOX POTENTIAL... 

The reactivities of potassium and silver with water represent extremes in the spontaneity of electron-transfer reactions. The redox reaction between two other metals illustrates less drastic differences in reactivity. Figure 19-5 shows the reaction that occurs between zinc metal and an aqueous solution of copper(II) sulfate zinc slowly dissolves, and copper metal precipitates. This spontaneous reaction has a negative standard free energy change, as does the reaction of potassium with water ... 

Fig. 10. Relationship between (AC i,2 —0.5 ACJi.i) and the standard free energy change (AG ij) of the redox reactions at 25 °C. Open circles, Ce(IV) + Fe(phen)3 reactions in 0.50 Af HjS04. Closed circles, Fe + +Fe(phen)3 reactions in 0.50 M HCIO4. Numbers refer to complexes in Table 32. (From Dulz and Satin, by courtesy of The American Chemical Society.)...

Cations in general should be reactive toward eh, but considerations of redox potential and free energy change are important. Thus, the alkali metal cations, having higher redox potentials, are unreactive toward eh. Another example is... 

In the photoelectrosynthetic cells, there are two effective redox couples in the electrolyte so that there is a net chemical change. When the net free energy change is positive, it is termed photoelectrolysis. However, when the net free energy change is negative, it is termed... 

AG° corresponds to the standard free energy change of the redox process D + A D+ + A , and the work terms wp and wr represent the energy required to bring together the products and reactants,... 

Eqs. 9 and 10 make clear predictions about the dependence of quenching rate constants on the free energy change in the quenching step. One way of testing the theory is to observe the quenching of the excited state by a series of related quenchers where the parameters kq(0), K, and k j) should remain sensibly constant and yet where the potentials of the quenchers as oxidants or re-ductants can be varied systematically. Such experiments have been carried out, most notably with the MLCT excited state, Ru(bpy)3 + (1). The experiments have utilized both a series of oxidative nitroaromatic and alkyl pyridinium quenchers, and a series of reductive quenchers based on aniline derivatives. From the data and known redox potentials for the quenchers, plots of RTlnk q vs. 

The free energy change of the redox reaction is given by,... 

In addition, the standard free-energy change AG° is related to the change in standard redox potential E by... 

The free-energy change, AG, per electron transferred related to the potential of the combined redox couples, E, is given by ... 

Which redox couple in a redox reaction has the oxidizing role and which the redncing role depends on the relative abilities of the two couples to accept or donate electrons. For example O2 has a greater affinity for electrons than other potential oxidants in natnral systems, and is therefore reduced preferentially. The means of quantifying the relative abilities of redox conples to accept or donate electrons and the corresponding free energy changes is as follows. 

Figure 4.3 Free energy changes in redox reactions mediated by microbes, (a) Oxidation of reduced inorganic compounds linked to reduction of O2. (b) Oxidation of organic matter CH2O linked to reduction of various organic and inorganic oxidants. pH = 7 and unit oxidant and reductant activities except (Mn +) = 0.2mM and (Fe +) = ImM...

The free energy change for a particular redox reaction varies with pe, pH, and the concentrations of reductants and oxidants according to Equation (4.26) ... 

The partial equilibria of equations 8.165 8.168 reveal the usefulness of standard potentials the Gibbs free energy change AG of the redox equilibrium is always given by applying Faraday s equation to the algebraic sum of the standard potentials of the redox couples in question. For equation 8.163, the bulk potential is thus... 

The rates of electron-transfer reactions can be well predicted provided that the electron transfer is a type of adiabatic outer-sphere reaction and the free-energy change of electron transfer and the reorganization energy (X) associated with the electron transfer are known [1-7]. This means that electron-transfer reactions can be designed quantitatively based on the redox potentials and the reorganization energies of molecules involved in the electron-transfer reactions. 

Biochemical reactions are basically the same as other chemical organic reactions with their thermodynamic and mechanistic characteristics, but they have the enzyme stage. Laws of thermodynamics, standard energy status and standard free energy change, reduction-oxidation (redox) and electrochemical potential equations are applicable to these reactions. Enzymes catalyse reactions and induce them to be much faster . Enzymes are classified by international... 

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