9.13 Gibbs free energy of formation

[Paula Sing 1J]

In the last paragraph on page 353, it says " A thermodynamically stable compound is a compound with a negative standard Gibbs free energy of formation. A thermodynamically unstable compound is a compound with a positive standard Gibbs free energy of formation." Why is this so if when delta G is negative, a reaction is considered spontaneous and vice versa? Wouldn't a spontaneously decomposing compound be less stable and therefore have a negative delta G value?

[Ridhi Ravichandran 1E]

The reaction they are referring to has the compound as the product. For example, the reaction H2 + (1/2)O2 ----> H2O would have a certain Delta G, which would also equal the delta G of formation for H2O. If delta G were negative, the reaction would move in the forward direction and H2O would spontaneously form, not decompose. That is what they mean by H2O being stable. If delta G were positive, the reverse process would be spontaneous, so the decomposition of H2O would be spontaneous. Thus, H2O would spontaneously decompose and would therefore be unstable.