Physical Foundations-: Keq and ΔG Are Measures of a Reaction’s Tendency to Proceed Spontaneously

The tendency of a chemical reaction to go to completion can be expressed as an equilibrium constant. For the reaction aA+bB→ cC+dD the equilibrium constant, Keq, is given by

where [Aeq] is the concentration of A, [Beq] the concentration of B, and so on, when the system has reached equilibrium. A large value of Keq means the reaction tends to proceed until the reactants have been almost completely converted into the products. Gibbs showed that trations of reactants and products: G for any chemical reaction is a function of the standard free-energy change, ΔG— a constant that is characteristic of each specific reaction—and a term that expresses the initial concentrations of reactants and products:

where [Ai] is the initial concentration of A, and so forth; Ris the gas constant; and T is the absolute temperature. When a reaction has reached equilibrium, no driving force remains and it can do no work: ΔG=0. For this special case, [Ai]= [Aeq], and so on, for all reactants and products, and

Substituting 0 for ΔG and Keq for [Ci]^c[Di]^d/[Ai]^a[Bi]^b in Equation 1–1, we obtain the relationship , ΔG= RT ln Keq , from which we see that ΔG is simply a second way (be sides Keq) of expressing the driving force on a reaction. Because Keq is experimentally measurable, we have a way of determining ΔG, the thermodynamic constant characteristic of each reaction.

The units of ΔG and ΔG are joules per mole (or calories per mole). When Keq>>1 ΔG is large and, negative; when Keq<<1, ΔG is large and G is large and positive. From a table of experimentally determined values of ei ther Keq or ΔG, we can see at a glance which reactions tend to go to completion and which do not. One caution about the interpretation of ΔG: ther modynamic constants such as this show where the final equilibrium for a reaction lies but tell us nothing about how fast that equilibrium will be achieved. The rates of reactions are governed by the parameters of kinetics.

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