Gibbs energy exergonic reactions

The biological stcuidcud reaction Gibbs energy is (-147) —2(—31) kJ moT = —85 kl moT. The reaction is exergonic and therefore spontaneous under biological standard conditions the oxidation of glucose is used to recharge the ATP. 

The reaction Gibbs energy, AjG, with electron transfer becoming more efficient as the reaction becomes more exergonic. For example, efficient photo-oxidation of S requires that the reduction potential of S be lower them the reduction potential of Q. 

By convention, thermodynamic functions of state refer to the system and not the environment, so - AG (exergonic) represents the Gibbs energy potentially available for expenditure and potentially dissipated to the environment. Under suitable conditions, this energy could be made to perform work. An endergonic reaction (+ AG) cannot proceed spontaneously and requires an input of Gibbs energy to proceed from its initial to its final state. 

In electron transfer reactions with relatively low exothermicity, — AG12 < A, the excess of the Gibbs energy leads only to vibrational and orientational excitation of the surrounding solvent shell. If the reaction is more exergonic, vibrational and/or electronic excitation of reaction products is generally possible. The overall reaction rate is then the superposition of many channels. 

Table 2 Selected reaction enthalpies and free energies of dissociation reactions of S (n=4, 6, 8, 10) in the gas phase and in SO2 solution at 298 K [in kj mor j. Some equilibria were also assessed using HF (DC=83) or oleum (DC=110) as solvent. Exergonic Gibbs energies of reaction in solution are printed in bold... 

Figure 1 Gibbs energy profile illustrating the relationship between the decrease in energy of activation and the exergonicity of a reaction. Reaction II has a larger Gibbs energy of activation and is less exergonic than reaction I.

A reaction in which the Gibbs free energy of the products is lower than that of the reactants. The position of equilibrium for an exergonic reaction favors products. 

Gibbs free energy is also a part of energy that is converted to work, but not converted to heat. We call the process where AG < 0 the exergonic reaction, whereas AG > 0 is the endergonic reaction ( ergon implies work). 

One of the major uses of the redox potential scale is to determine the exergon-icity of redox reactions in solution. In water, for example, the oxidized form of the redox couple having the highest redox potential is the oxidant, and the reduced form of the couple having the lowest redox potential is the rednctant. At ITIES, the standard Gibbs energy AG of the interfacial electron reaction discussed earlier (Equation 1.74) is then sinqtly given by... 

There is often a confusion between enthalpy, A,//, and Gibbs energy, A G defined in Equation (1). Negative values of A G correspond to exergonic reactions that proceed spontaneously in the direct sense, and conversely, positive values of A G correspond to endergonic reaction that will not proceed unless they are... 

AGr Gibbs free-cnergy change The energy difference between reactants and products. When AG° is negative, the reaction is exergonic, has a favorable equilibrium constant, and can occur spontaneously. When AGC is positive, the reaction is endergonic, has an unfavorable equilibrium constant, and cannot occur spontaneously. 

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