Free energy changes from oxidation/reduction

Free Energy Changes from Oxidation/Reduction... 

See also Free Energy Changes from Oxidation/Reduction, Oxidations and Energy Generation, Electron Transport, Cytochromes... 

If the free energy changes for the reduction of the sulfate to make H2S rather than SO2 had been considered in this analysis, the oxides of several more elements would have been included in the boxed-off area of appropriate materials. From past practice, however, it is known that oxides of calcium, strontium and lithium, for instance, are not as effective. An appropriate future area of research would be to investigate those materials more thoroughly. 

To calculate the free energy change of an oxidation-reduction reaction, the reduction potential of the electron donor (NADH) is added to that of the acceptor (Oj). The AE° for the net reaction is calculated from the sum of the half reactions. For NADH donation of electrons, it is = +0.320 volts, opposite of that shown in Table 4 (remember. Table 4 shows the E° for accepting electrons), and for Oj acceptance, it is +0.816. The number of electrons being transferred is 2 (so, n = 2). The direct relationship between the energy changes in oxidation-reduction reactions and AG° is expressed by the equation... 

Calculating the free-energy change from electrode potentials Given standard electrode potentials, calculate the standard free-energy change for an oxidation-reduction reaction. (EXAMPLE 20.9)... 

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. 

In a reduction process the flow of electrons is from left to right (i.e., from oxidizing to reducing media). A Gibbs free energy change is associated... 

The thermodynamic criterion for spontaneity (feasibility) of a chemical and electrochemical reaction is that the change in free energy, AG have a negative value. Free-energy change in an oxidation-reduction reaction can be calculated from knowledge of the cell voltage ... 

Here n represents the number of electrons transferred in the reaction. With this equation we can calculate the free-energy change for any oxidation-reduction reaction from the values of E" in a table of reduction potentials (Table 13-7) and the concentrations of the species participating in the reaction. 

Thus, the electron flows from the reductant to the oxidant through photoexcited semiconductor particles. Therefore, the particle suspended in a solution of reductant and oxidant can be regarded as a very small electrochemical cell in which the irradiated light energy is used as free energy change and/or activation energy of the redox reaction of the reductant and the oxidant (Fig. 11.2). 

The standard electrode potential E° of a redox reaction is a measure of the potential that would be developed if both reductants and oxidants were in their standard states at equal concentrations and with unit activities. The units of E° are volts and ° can be calculated from the Gibbs free energy change (AG ) of the redox reaction from the relationships... 

The oxidation-reduction potential, E, (or redox potential) of a substance is a measure of its affinity for electrons. The standard redox potential (E0 ) is measured under standard conditions, at pH 7, and is expressed in volts. The standard free energy change of a reaction at pH 7, AG0, can be calculated from the change in redox potential AE0 of the substrates and products. A reaction with a positive AE0 has a negative AG0 (i.e. is exergonic). 

The free-energy change of an oxidation-reduction reaction can be readily calculated from the reduction potentials of the reactants. For example, consider the reduction of pyruvate by NADH, catalyzed by lactate dehydrogenase. 

A compound with a more positive potential wiU oxidize the reduced form of a substance of lower potential with a standard free energy change AG° = -nP AE° = -n A ° x 96.49 kj moH where n is the number of electrons transferred from reductant to oxidant. The temperature is 25°C unless otherwise indicated. E° refers to a standard state in which the hydrogen ion activity = 1 E° refers to a standard state of pH 7, but in which all other activites are unity. 

---