1. Chemical Thermodynamics – Gibbs Free Energy and Spontaneity
Chapter 19
Chemical Thermodynamics –
Gibbs Free Energy and Spontaneity

2. Free Energy Changes
Gibbs Free Energy, ΔG, A measure of the maximum useful work that can be done by a chemical reaction.
ΔGo = ΔHo - TΔSo
Note that ΔG, like ΔS, is dependent on pressure and concentration. Unlike ΔH and ΔS, ΔG is strongly temperature dependent because of T in the equation.

3. If ΔG < 0 the reaction is spontaneous at standard conditions.
If ΔG > 0 the reaction is not spontaneous at standard conditions.
If ΔG = 0 the reaction is at equilibrium.

4. The effect of ΔH and ΔS on spontaneity:
ΔG = ΔH - TΔS
The effect of ΔH and ΔS on spontaneity:
If ΔH > 0, ΔS < 0, then ΔG > 0 at all temperatures
not spontaneous
If ΔH < 0, ΔS > 0, then ΔG < 0 at all temperatures
spontaneous
If ΔH > 0, ΔS > 0, then ΔG > 0 at low T
ΔG < 0 at high T (spontaneous)
If ΔH < 0, ΔS < 0, then ΔG < 0 at low T
ΔG > 0 at high T(nonspontaneous)

5. Calculate ΔGo for the following reaction.
Fe2O3(s) + 3 H2(g) → 2 Fe(s) + 3 H2O(g)
At what temperature does the reaction become spontaneous?

7. You can also calculate ΔGo from ΔGfo values at the back of the book.
Ex - Calculate the standard Gibbs Free Energy for the reaction:
FeCl2 → Fe Cl-

8. Calculation of ΔG from ΔGo
ΔG = ΔGo + RTlnQ
Free energy activity Ex
FeCl2(s) → Fe Cl-
If [Fe2+] = M and [Cl-] = M calculate ΔG.
(Note that this turns out to be spontaneous at low concentration.)

10. Relationship between ΔG and K
ΔGo = -RTlnK
If K > 1, ΔG < 0, rxn is spontaneous at standard conditions.
If K < 1, ΔG > 0, rxn is nonspontaneous at std cond.
If K = 1, ΔG = 0, rxn is at equilibrium at std. cond.

11. Ex - Calculate ΔGo for the ionization of water at standard conditions.

12. Notes quiz