Half-reaction, general

An organic molecule can be used as the sacrificial donor in a reduction half reaction. Generally there is no net energy storage, but (depending on the reaction) hydrogen may be evolved at the same time as a surplus reaction product. For example, the reaction... 

Standard-state potentials are generally not tabulated for chemical reactions, but are calculated using the standard-state potentials for the oxidation, E°o, and reduction half-reactions, fi°red- By convention, standard-state potentials are only listed for reduction half-reactions, and E° for a reaction is calculated as... 

Pourbaix has classified the various equilibria that occur in aqueous solution into homogeneous and heterogeneous, and has subdivided them according to whether the equilibria involve electrons and/or hydrogen ions. The general equation for a half reaction is... 

The general procedure for balancing the chemical equation for a redox reaction is first to balance the half-reactions separately. 

In the ion-electron method of balancing redox equations, an equation for the oxidation half-reaction and one for the reduction half-reaction are written and balanced separately. Only when each of these is complete and balanced are the two combined into one complete equation for the reaction as a whole. It is worthwhile to balance the half-reactions separately since the two half-reactions can be carried out in separate vessels if they are suitably connected electrically. (See Chap. 14.) In general, net ionic equations are used in this process certainly some ions are required in each half-reaction. In the equations for the two half-reactions, electrons appear explicitly in the equation for the complete reaction—the combination of the two half-reactions—no electrons are included. 

Though accelerating effect of redox mediators is proved, differences in electrochemical factors between mediator and azo dye is a limiting factor for this application. It was reported that redox mediator applied for biological azo dye reduction must have redox potential between the half reactions of the azo dye and the primary electron donor [37], The standard redox potentials for different azo dyes are screened generally between -430 and -180 mV [47],... 

In general, the number of electrons in each half must be the same when ionic half-equations are combined. To do this, one or both half-reactions may have to be multiplied by an integer. 

Since the enzymatic reactions are generally rapid, it may be assumed that the steady-state approximation applies. Note, however, that although true is most systems, this is not always the case, as exemplified in Section 5.2.5. Each half-reaction is characterized by three rate constants, defined in Scheme 5.1. They may alternatively be characterized by the following... 

The transfer coefficient a is generally an index indicative of the symmetry of the energy barrier for a redox half-reaction. The significance of this definition is the following. 

You could balance the chemical equation for the reaction of magnesium with aluminum nitrate by inspection, instead of writing half-reactions. However, many redox equations are difficult to balance by the inspection method. In general, you can balance the net ionic equation for a redox reaction by a process known as the half-reaction method. The preceding example of the reaction of magnesium with aluminum nitrate illustrates this method. Specific steps for following the half-reaction method are given below. 

For example, in oxic natural waters the principal oxidant is O2 and in agreement with expectations the pe of such waters is generally poised in the range expected for the O2-H2O couple (Morel and Herring, 1993). Thns for water at pH = 7 in eqnilibrinm with atmospheric Pq2 (= 10 atm), the half reaction is... 

Methanol is one of the few alcohols that can be fed directly into a fuel cell and can be converted electrochemically at the anode. The DMFC can be fed with a gaseous or liquid fuel feed. The liquid DMFC generally uses a diluted methanol in water mixture (ty pically 1-2 molar) and only a fraction of the methanol is used at the anode (Collins, 2001). The DMFC, like an ordinary battery, provides DC electricity according to the following half reactions. 

Dilute aqueous solutions of strong acids (e.g. HCl or H2SO4) contain sufficient concentrations of hydrated protons to oxidize many metals, to produce their most stable states in solution. The only thermodynamic condition for metal oxidation is that the reduction potential of the metal ion produced should be negative. In general, for the metal ion M + undergoing reduction to the metal, if the standard reduction potential for the half-reaction ... 

The general form of a problem involving the relation between E° values for half-reactions and K for a net reaction is... 

We can make the following general statement. To balance any electron-transfer equation, you must subtract the reducing half-reaction equation from the oxidizing half-reaction equation after the two equations have first been written to show the same number of electrons. Simplify the final equation, if needed. 

The general method of balancing electron-transfer equations requires that halfreaction equations be available. Short lists of common half-reactions, similar to Table 17-1, are given in most textbooks, and chemistry handbooks have extensive lists. However, no list can provide all possible half-reactions, and it is not practical to carry lists in your pocket for instant reference. The practical alternative is to learn to make your own half-reaction equations. There is only one prerequisite for this approach you must know the oxidation states of the oxidized and reduced forms of the substances involved in the electron-transfer reaction. In Chapter 8 you learned the charges on the ions of the most common elements now we review the method of determining the charge (the oxidation state) of an element when it is combined in a radical. 

In some cases, we find that the tables of data available do not contain precisely the standard potential we need but do contain closely related values. In such cases, we calculate the standard potential of the half-reaction we need by combining the values for related half-reactions. However, the potential of a half-reaction cannot in general be determined by adding or subtracting the standard potentials directly. Instead, we must combine the values of AG° for each half-reaction, then convert the final AG° for the desired half-reaction into the corresponding standard potential by using Eq. 3. 

All organisms use the same pair of pyridine nucleotides as carrier molecules for hydrogen and electrons. Both of these molecules accept hydrogen and electrons in the redox reactions of catabolism and become reduced. The oxidative half-reactions of catabolism generally produce two H+ and two electrons. The nicotinamide ring can accept two electrons and one H+ and, since the second H+ is released into the solution, most redox reactions in biological systems take the form ... 

Once the relevant oxidation and reduction half-reactions have been identified (e.g., from Tables 16.1-5), they can be combined and balanced to determine the overall reaction for any redox transformation. In generalized form, this can be written... 

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