neous (product-favored). We see from the equation that Gbecomes more negative as
(1) Hbecomes more negative (the process gives off more heat) and (2) Sbecomes more
positive (the process results in greater disorder). If there is a net increase in free energy
of the system during a process, Gis positive and the process is nonspontaneous
(reactant-favored). This means that the reverse process is spontaneous under the given
conditions. When G0, there is no net transfer of free energy; both the forward and
reverse processes are equally favorable. Thus, G0 describes a system at equilibrium.
The relationship between Gand spontaneity may be summarized as follows.
In other words, are the reactantsor
the productsmore stable at standard
conditions?15-15 Free Energy Change, G, and Spontaneity 629The free energy content of a system depends on temperature and pressure (and, for
mixtures, on concentrations). The value of Gfor a process depends on the states and
the concentrations of the various substances involved. It also depends strongly on temper-
ature, because the equation GHTSincludes temperature. Just as for other
thermodynamic variables, we choose some set of conditions as a standard state reference.
The standard state for G^0 is the same as for H^0 — 1 atm and the specified tempera-
ture, usually 25°C (298 K). Values of standard molar free energy of formation, Gf^0 ,
for many substances are tabulated in Appendix K. For elementsin their standard states,
Gf^0 0. The values of Gf^0 may be used to calculate the standard free energy change of
a reaction at 298 Kby using the following relationship.
G^0 rxnnG^0 f productsnG^0 f reactants (1 atm and 298 K only)The value of G^0 rxnallows us to predict the spontaneity of a very special hypothetical
reaction that we call the standard reaction.
In the standard reaction,the numbers of moles of reactants shown in the balanced
equation, all at standard conditions, are completelyconverted to the numbers of moles
of products shown in the balanced equation, all at standard conditions.We must remember that it is G,and not G^0 , that is the general criterion for spon-
taneity. Gdepends on concentrations of reactants and products in the mixture. For most
reactions, there is an equilibrium mixtureof reactants and products that is more stable than
either all reactants or all products. In Chapter 17 we will study the concept of equilib-
rium and see how to find Gfor mixtures.
Spontaneity of Reaction
G (constant Tand P)Gis positive Reaction is nonspontaneous (reactant-favored)
Gis zero System is at equilibrium
Gis negative Reaction is spontaneous (product-favored)0
G 0 G 0
Reaction is spontaneous Reaction is not spontaneous
Product-favored reaction Reactant-favored reaction
Forward reaction is favored Reverse reaction is favored