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.
Hence the temperature, being a multiplying parameter for entropy factor (T∆S), plays an important role in determining the spontaneity of the reaction. Two cases are their which are discussed below:
Case1: At high temperature:
At the high temperature, the entropy factor predominates. Hence the ∆H is positive i.e. reaction is endothermic. Thus energy factor does not favor the reaction. But if the reaction is accompanied by even a small increase of entropy, the high temperature will increase the entropy factor T∆S to a large extent. Hence, ∆G = ∆H – T∆S may become negative at a sufficient high temperature. This explains why endothermic reactions become feasible at increasing temperature.
Case2: At low temperature:
At the low temperature, the energy factor predominates. Hence the ∆H is always negative i.e. reaction is exothermic. Thus energy factor always favor the reaction. But if the reaction is accompanied by even a small decrease of entropy, the opposing factor T∆S may remain very small and therefore, the value of ∆G = ∆H – T∆S may still be negative at a sufficient low temperature. This explains why exothermic reactions will remain feasible at decreasing temperature.
