Reactions and AG

It has already been stated that the criterion for a spontaneous chemical reaction is that AG must be negative. Let us consider a reaction that can be represented as follows  

The reaction when A and B are initially mixed takes place toward the right but eventually the rate of the reaction to the left becomes equal to the rate of the reaction to the right. 

At that point, equilibrium is established. The equilibrium constant for the reaction shown in Eq. (4.7) can be written as 

Three possible free energy changes that may accompany a reaction. 

Because large compilations of AGf° values are available, it is often possible to determine easily whether a given reaction will work. For example, consider the following reaction where the free energy of formation of each substance is written below its formula  

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Under most conditions, the process is spontaneous/ A chemical potential difference drives the reaction and AG < 0. When the reactants are separated as shown in Figure 9.3, the chemical potential difference can be converted to an electrical potential E. When the electrodes are connected through an external circuit, the electrical potential causes an electric current to flow. Because the electrical potential is the driving force for electrons to flow, it is sometimes... 

Figure 4.6 The value of the Gibbs function AG decreases as the extent of reaction until, at (eq), there is no longer any energy available for reaction, and AG = 0. =0 represents no reaction and = 1 mol represents complete reaction...

This reaction has the advantage that it is sensitized by visible light out to 6 35 nm and the quantiam yield is high. Unfortunately, it is difficult to prevent the back reaction and Ag° for the reaction is only 20.5 kJ mol"l. Nevertheless, it is theoretically possible to store up to 4% of the input solar energy so more work on this reaction would seem to be warranted. 

If the intersecting curves are parabolic in form, then the reactivity pattern expected is described by the Marcus equation (Marcus, 1964, 1977). The magnitude of a may then be shown to be that in (82). Here AGjisthe intrinsic barrier for reaction and AG° the free energy of reaction. 

The difference in pKa values between the proton donor and the proton acceptor in Eq. 9-97 can be expressed as the Gibbs energy change which at 25°C is equal to 5.71 x ApKa. This is often referred to as the thermodynamic barrier AG 0 to a reaction and AG can be expressed as the sum of the thermodynamic barrier AG0 plus an intrinsic barrier AG in. . For the proton transfer of Eq. 9-97 the intrinsic barrier (for step b) is thought to be near zero so that AG 5.71 ApfCa. 

The thermodynamic term of widest use in soil chemistry is the free energy, or more explicidy, the Gibbs free energy. This is the energy of a substance or a reaction that, at constant temperature and pressure, is available for subsequent use. Energy drives chemical reactions and AG is the most widely useful. It is directiy related to (1) the activity or chemical potential, (2) the energy of formation of compounds, (3) the equilibrium constant of a reaction, and (4) the electrode potential. The first three are discussed here the electrode potential is discussed in Chapter 4. 

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... 

Table 2. Reactions and AG° values of ammonium and nitrite oxidizing processes in...

The free eneig> is a measure of the affinity and direction of the reaction. For a cell working in galvanic (fuel cell) mode, energy is released from the electrochemical reaction and AG is negative. The reaction enthalpy AH) and entropy (AS) must also be eonsidered in order to describe the efficiency of an electrolytic or galvanic... 

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