Redox reactions free energy changes

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. 

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.)...

The free energy change of the redox reaction 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 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... 

The absolute redox potentials for these reactions are defined by a compensation of the free energy change dGredox by the free energy of the reacting electrons. Therefore, at equilibrium conditions, the electrons have not to be at the vacuum level but at the energy level °Fredox... 

The equilibrium situation can be achieved with the reacting electrons coming from the Fermi level Ep of a metal electrode in contact with the solution of the redox couple. The free energy change in the respective redox reaction is then zero. 

In terms of these redox potentials the free energy change for the net two electron transfer reaction is given as... 

The standard free energy change per electron transferred, ArG°(W )/n, of reaction Eq. 14-29 can now be simply derived from Table 14.2 by adding the ArG°(W) value of reaction (12) (+41.0 kJ mor1) and reversed reaction (1) (+78.3 kJ-mor1) ArG°(W)/ n = +119.3 kJ-mol-1. Thus, on a per-electron basis, under standard conditions (pH 7), we have to invest 119.3 kJ-mol-1 to (photo)synthesize glucose from C02 and H20. In our standard redox potential picture using h(W) values, this is equivalent... 

It is thus possible to calculate the free-energy change for any biological redox reaction at any concentrations of the redox pairs. 

Wait you protest. How can there be a redox potential for a reaction that is not a redox reaction Box 14-3 shows that the redox potential is just another way of expressing the free-energy change of the reaction. The more energetically favorable the reaction (the more negative AG°), the more positive is E°. 

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