Biochemical reactions equilibrium criterion

When the pH is specified, the criterion for spontaneous change and equilibrium is provided by the transformed Gibbs energy and the reactants can be taken to be HgvM, HgvMOa, and O2 that are involved in the following biochemical reaction. 

Much rarer are used redox-couples of anions like NO /NH, HCO / CH, SO "/HS , etc. Their equilibrium ratio is reached much slower. The reason is that these reactions have high activation energy associated with the destruction of strong covalent bonds. For this reason, many of them are implemented only on the accoimt of biochemical catalysis (for instance, NO " NOj ). Rates of such processes are very low and imstable as they depend on the environment, nature and abundance of microbial populations and sufficiency of substrate for them. Many redox reactions of these couples are essentially irreversible. Their half-life may reach several years. The only relatively rapid process is oxidation of sulphide sulphur in alkaline medium. Besides, concentrations of anions often depend on pH value. For this reason. Eh value of individual anion redox-couples rarely may be a criterion of the solutions oxidation potential as a whole. 

From the second law of thermodynamics, we know that all natural processes tend to approach equilibrium spontaneously, energy being required to drive the system away from equilibrium. The second law provides us with a criterion of spontaneity, i.e., whether a given process (e.g., biochemical or biophysical reactions) is feasible under a specified set of conditions. It points the direction for the evolution of physicochemical processes since it capsulizes the observation that, in all processes, some of the energy becomes unavailable to perform work, owing to the increased random motion of some of the component molecules of the system. In other words, the second law distinguishes between irreversible processes, which by their very nature are unidirectional, and reversible processes, which are bidirectional. 

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