Standard-state Free Energies, Equilibrium Constants, and Concentrations

5 Standard-state Free Energies, Equilibrium Constants, and Concentrations 

While our focus has been primarily on thermodynamic quantities, like free energy, it should be borne in mind that the ultimate motivation for computing free energy differences is usually to pennit calculation of chemical concentrations in actual systems. To accomplish this for a generic equilibrium is straightforward. For example, consider the following reaction (chosen in a completely arbitrary fashion) 

From the relationship between the equilibrium constant and the free energies of the reactants and the products we may write 

One variation on this theme that should be borne in mind when analyzing actual chemical situations is tliat certain species in the real system may be buffered . That is, their concentrations may be held constant by external means. A good example of this occurs in condensed phases, where solvent molecules may play explicit roles in chemical equilibria but tlie concentration of the free solvent is so much larger than that for any other species that it may be considered to be effectively constant. Modeling solvation phenomena in general is covered in detail in the next two chapters, but it is instructive to consider here a particular case as it relates to computing equilibria. Consider such a reaction as 

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