17.1 b) GIBBS FREE ENERGY AND EQUILIBRIUM
*Review:
GIBBS FREE ENERGY (G)  Gibbs free energy describes the spontaneity and temperature dependence of a reaction. It
also provides us with information regarding the amount of energy available to do work.
*For a reaction to be spontaneous, the sign of G has to be ______________________.
*Ways to calculate Gibbs free energy:
1. ΔG˚ = ΣnG˚products - ΣnG˚reactants
2. G˚ =
Where: G˚ = change in free energy;
S˚ = entropy change
H˚ = enthalpy change;
T = temperature in _________________ ;
*PREDICTING SPONTANEITY  SUMMARY:
H
S
G
SPONTANEITY
-
+
Always ________________________
__________________ spontaneous
+
-
Always _______________________
___________________ spontaneous
-
-
Depends on ____________________
+
+
Depends on ____________________
Spontaneous ____________ a specific, calculated,
threshold temperature; spontaneous when TS<H
Spontaneous ____________ a specific, calculated
threshold temperatures; when TS>H
*With respects to EQUILIBRIUM:
FORWARD RXN
G
REVERSE RXN
EQUILIBRIUM
CONSTANT
DESCRIPTION
(at Equilibrium)
_________________
______________ favoured
*reaction proceeds in the
_____________ direction
+
__________________
______________ favoured
*reaction proceeds in the
_____________ direction
0
_______________
reactants nor products
favoured
*The position of equilibrium corresponds to a maximum value of entropy and a minimum value of the Gibbs free energy.
NEW EQUATION (provided in data booklet):
**3. G˚ = -RT (ln K)
*Therefore: ln K =
SAMPLE PROBLEM:
1. Calculate the equilibrium constant at 300 K for the oxidation of iron:
*Be sure to be consistent with UNITS!
2Fe(s) +
3
O2(g)
2
Fe2O3(s)
*Where: H˚ = -824.2 kJ mol-1 ; S˚ = -270.5 J K-1 mol-1 ; R = 8.31 J K-1 mol-1