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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