Change in Free Energy or Spontaneity of a Reaction The Second Law of Thermodynamics: The entropy of the universe increases for any spontaneous reaction. Applied to a chemical system this means that the entropy of a system can increase or decrease but if it does decrease, then the entropy of the surroundings must increase to a greater extent so that the overall entropy change in the universe is positive. This criteria can be calculated out and is referred to as the G or Gibb's Free Energy. G is related to the enthalpy change, the entropy change and the temperature of the system. ΔG = ΔH - T ΔS G can be used to predict the spontaneity of a reaction at constant temperature and pressure. the criteria are listed below: 1. If G is negative, then the reaction is spontaneous (probable) as written. 2. If G is positive, the reaction is improbable as written, but the reverse reaction is probable. Reversing the equation would reverse the sign of G. 3. If G is 0, the system is at equilibrium and there is no net reaction. When G = 0 then H = tS. thus at equilibrium, the entropy factor is balanced by the enthalpy factor. -Exothermic reactions which are accompanied by an increase in entropy of the system are probable. -Endothermic reactions accompanied by a decrease in entropy are improbable. -At very high temperatures, the sign and magnitude of G and the spontaneity of the reaction are determined primarily by the change in entropy. -At very low temperatures, the sign and magnitude of G and the spontaneity of the reaction are determined primarily by the enthalpy change, ΔH. Thermal Spontaneity of Effect               H    S    Reaction as Written Comment Exothermic       -     +       probable no exceptions Exothermic       -     -       probable at low temps Endothermic     +    +       probable at high temps Endothermic     +    -        improbable no exceptions Predict the probability of H2O(l) ------> H2 (g) + ½O2(g) at  25oC and 1500oC.
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