Oxidation-reduction reactions free energy change

Answer Enz-FAD, having a more positive standard reduction potential, is a better electron acceptor than NAD+, and the reaction is driven in the direction of fatty acyl-CoA oxidation (a negative free-energy change). This more favorable free-energy change is obtained at the expense of 1 ATP only 1.5 ATP molecules are formed per FADH2 oxidized in the respiratory chain, compared with 2.5 ATP per NADH. 

Let us now consider the reduction of a metal oxide by carbon which is itself oxidised to carbon monoxide. The reaction will become energetically feasible when the free energy change for the combined process is negative (see also Figure i.i). Free energies. 

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. 

One of the important differences between calciothermic and aluminothermic reduction of oxides concerns the interaction between the reduced metal and the reductant. Calcium does not form stable solid solutions or alloys with the reduced metals calcium contamination in the metal is, therefore, relatively small. Aluminum, on the other hand, readily forms solid solutions with the reduced metals, and the product generally contains appreciable quantities of residual aluminum. This is not a serious problem because in many cases either a certain aluminum content is desired in the reduced metal or the residual aluminum can be effectively removed in post-reduction purification operations. The extent of the contamination of a reduced metal with the reductant can be related to factors such as the reaction temperature, the standard free energy change associated with the reaction, and the slag composition. Let the following generalized reaction be considered ... 

Fig. 22.7. Thermodynamic driving forces for various anaerobic (top) and aerobic (bottom) microbial metabolisms during mixing of a subsea hydrothermal fluid with seawater, as a function of temperature. Since the driving force is the negative free energy change of reaction, metabolisms with positive drives are favored thermodynamically those with negative drives cannot proceed. The drive for sulfide oxidation is the mirror image of that for hydrogentrophic sulfate reduction, since in the calculation 02(aq) and H2(aq) are in equilibrium.

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 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 ... 

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... 

Practically in every general chemistry textbook, one can find a table presenting the Standard (Reduction) Potentials in aqueous solution at 25 °C, sometimes in two parts, indicating the reaction condition acidic solution and basic solution. In most cases, there is another table titled Standard Chemical Thermodynamic Properties (or Selected Thermodynamic Values). The former table is referred to in a chapter devoted to Electrochemistry (or Oxidation - Reduction Reactions), while a reference to the latter one can be found in a chapter dealing with Chemical Thermodynamics (or Chemical Equilibria). It is seldom indicated that the two types of tables contain redundant information since the standard potential values of a cell reaction ( n) can be calculated from the standard molar free (Gibbs) energy change (AG" for the same reaction with a simple relationship... 

Notice, however, that if the reaction at the Zn/Zn2+ interface is reversed and written as an electronation (reduction) rather than a deelectronation, then this electronation does not proceed spontaneously and its free-energy change is positive. This positive value of AG° = -nFE0 implies that E° must be negative. The standard reduction potentials for the zinc and copper systems are, therefore, -0.76 and +0.34 V, in contrast to the standard oxidation potentials, which are +0.76 and -0.34 V, respectively. 

The energy made available by this spontaneous electron flow (the free-energy change for the oxidation-reduction reaction) is proportional to AE ... 

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. 

The emf of a reversible cell can be regarded either as a function of the free energy change associated with the overall cell reaction or as a sum of the Galvani potential differences between phases within the cell. It was noted above, however, that individual Galvani potential differences between nonidentical phases could not be measured and it is therefore impossible to resolve a cell emf into its interphasial components. On the other hand, every cell reaction consists of an oxidation and a reduction process and thus can be considered as the sum of two notional half-reactions occurring in notional half-cells . For example, the Daniell cell can be visualized as consisting of the half-cells... 

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). 

To illustrate, let s calculate the standard free-energy change for the reaction in Worked Example 17.6—the reduction of iron(III) oxide with carbon monoxide ... 

Standard half-cell potentials can be used to compute standard cell potentials, standard Gibbs free energy changes, and equilibrium constants for oxidation-reduction reactions. 

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). 

Fig. 11. Schematic representation of the relationship between the free energy change (difference between the product and reactant minima) and free energy of activation (height of the crossing point with respect to the reactant minimum) for the corss reactions between a single reductant (A ) and a homogeneous series of oxidants (A2a, A2b> A2c) having variable oxidation potential. The curve corresponding to the formation of Ain an excited state is also shown...

For biochemical reactions undergoing oxidation-reduction (redox reaction), the free energy change is related to the electromotive force (emf) or redox potential (AEQ of the reaction ... 

There are two fermentative processes that at first appear to be quite similar to oxygen and nitrate-dependent respirations the reduction of C02 to methane and of sulfate to sulfide. However, on closer examination, it is clear that they bear little resemblance to the process of denitrification. In the first place, the reduction of C02 and of sulfate is carried out by strict anaerobes, whereas nitrate reduction is carried out by aerobes only if oxygen is unavailable. Equally important, nitrate respirers contain a true respiratory chain sulfate and C02 reducers do not. Furthermore, the energetics of these processes are very different. Whereas the free energy changes of 02 and nitrate reduction are about the same, the values are much lower for C02 and sulfate reduction. In fact, the values are so low that the formation of one ATP per H2 or NADH oxidized cannot be expected. Consequently, not all the reduction steps in methane and sulfide formation can be coupled to ATP synthesis. Only the reduction of one or two intermediates may yield ATP by electron transport phosphorylation, and the ATP gain is therefore small, as is typical of fermentative reactions. 

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