If the free energy change i.e. ∆G of a chemical reaction is positive, the reaction would not proceed. If the free energy change i.e. ∆G of a chemical reaction is negative, the reaction would be spontaneous i.e. feasible. If the free energy change i.e. ∆G of a chemical reaction is zero, the reaction would be in a state of equilibrium.
Mathematically,
G = H – TS
At constant temperature,
∆G = ∆H – T∆S
Hence two factors contribute to the value of free energy ∆G. One is energy factor ∆H and the other is entropy factor T∆S.
Both the factors are necessary for calculating the spontaneity of the reaction. The spontaneity is decided by overall value of both the factors i.e. by the value of ∆G. For this change in enthalpy ∆H will be negative at constant temperature and T∆S. should be negative. Both these factors tend to increase the feasibility of a reaction.
Case 1. When both factors are favorable:
When both the factors are favorable i.e. ∆H is negative and T∆S is positive, then it will result in a large value of ∆G. hence the reaction will be highly feasible.
Case2. When the energy factor is favorable:
When the energy factor is favorable i.e. ∆H is negative, but the entropy factor is not favorable i.e. T∆S is not positive, It is negative. In this case the feasibility of the reaction depends on the factor which predominates. If the energy factor predominates i.e. the numerical value of ∆H is more than T∆S, the reaction would be feasible because the value of ∆G will be negative. But if the entropy factor predominates i.e. the numerical value of T∆S is more than ∆H then the reaction would not be feasible because the value of ∆G will be positive.
Case3. When the entropy factor is favorable:
When the entropy factor is favorable i.e. T∆S is positive, but the energy factor is not favorable i.e. ∆H is positive. In this case the feasibility of the reaction depends on the factor which predominates. If the entropy factor predominates i.e. numerical value of T∆S is more than ∆H, the reaction would be feasible because the value of ∆G will be negative. But if the energy factor predominates i.e. the numerical value of ∆H is more than T∆S, then the reaction would not be feasible because the value of ∆G will be positive.
Case4. Neither factor predominates:
When neither factor predominates then both the factors are equal to each other. Hence ∆G is equal to zero. In this case, the reaction will be in a state of equilibrium.
Very knowledgeable page.