|Change in Free Energy or Spontaneity of a Reaction|
|The Second Law of Thermodynamics:
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
surroundings must increase to a greater extent so that the overall
change in the universe is positive.
|This criteria can be calculated out and is referred to as
or Gibb's Free Energy. G is related to the enthalpy change, the entropy
change and the temperature of the system.
|ΔG = ΔH
|G can be used to predict the spontaneity of a reaction at
temperature and pressure. the criteria are listed below:
|1. If G is negative, then the reaction is spontaneous
|2. If G is positive, the reaction is improbable as
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
When G = 0 then H = tS. thus at equilibrium, the entropy factor is
by the enthalpy factor.
|-Exothermic reactions which are accompanied by an increase
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
of the reaction are determined primarily by the enthalpy change, ΔH.
|Thermal Spontaneity of
H S Reaction as Written Comment
|Predict the probability of H2O(l) ------> H2 (g) + ½O2(g) at 25oC and 1500oC.|