Graphs of Gibbs free energy

 Whether a reaction is product-favored, that is, whether the reactants are converted to products under standard-state conditions, is reflected in the sign of its ΔrG°. This equation

ΔrG° = ΔrH° − TΔrS°

shows that the sign of ΔrG° depends on the signs of ΔrH° and ΔrS°, and, in some cases, the absolute temperature (which can only have positive values). Four possibilities exist:

1. Both ΔrH° and ΔrS° are positive—an endothermic process with an increase in system entropy. ΔrG° is negative if TΔrS° > ΔrH°, and positive if TΔrS° < ΔrH°. Such a process is product-favored at high temperatures and reactant-favored at low temperatures.
2. Both ΔrH° and ΔrS° are negative—an exothermic process with a decrease in system entropy. ΔrG° is negative if |TΔrS°| < |ΔrH°| and positive if |TΔrS°| > |ΔrH°|. Such a process is product-favored at low temperatures and reactant-favored at high temperatures. (Remember that |TΔrS°| represents the magnitude of TΔrS°, ignoring the mathematical sign.)
3. ΔrH° is positive and ΔrS° is negative—an endothermic process with a decrease in system entropy. ΔrG° is positive regardless of the temperature. Such a process is reactant-favored at all temperatures.
4. ΔrH° is negative and ΔrS° is positive—an exothermic process with an increase in system entropy. ΔrG° is negative regardless of the temperature. Such a process is product-favored at all temperatures.

Gibbs free energy vs Extent of reaction

Thermochemistry Notes