Gibbs Free Energy

1. Gibbs Free Energy 19.6

2. Gibbs Free Energy • The method used to determine spontaneity involves both ∆Ssys+ ∆Ssurr • Gibbs deals with only the system, so its easier • For a process occurring at a constant temperature (in Kelvin): G° = H °–TS °

3. ∆Grxn and Spontaneity • ∆G <0 spontaneous • ∆G > 0 nonspontaeous • ∆G = 0 at equilibrium

4. What is “Free Energy”? • The maximum energy available to do work • “Free” means “available”

5. Free Energies of Formation Note that ∆G˚f for an element = 0

6. Calculating ∆G°rxn • G° = H°–TS° • Appendix L: ∆H°rxn and ∆S°rxn • Units: kJJ/K • H2O (l) --> 2H2(g) + 02(g) @ 25 °C • Calculate the change of Gibbs Free Energy and determine if spontaneous.

7. Another way to calculate ∆G° • Gº = ΣGºf (products) - ΣGºf (reactants) • Calculate for the combustion of butane. • __C4H10 + __O2 (g) --> __CO2(g) +__ H2O(g)

8. “Enthalpy Driven” Reactions • Reactions that are spontaneous because energy is released • ∆Hsystem < 0

9. “Entropy Driven” Reactions • Reactions that are spontaneous because the entropy of the system increases. • ∆Ssystem > 0

10. Enthalpy & Entropy Driven Reactions • Are spontaneous under all conditions of temperature G° = H° – TS° negative - positive • Gibb’s Free E will always be negative, so the rxn is always spontaneous

11. Enthalpy & Entropy Disfavored Reactions • Are nonspontaneous under all conditions of temperature G° = H° – TS° positive - negative • Gibb’s Free E will always be positive, so the rxn is always nonspontaneous

12. Temperature Dependence on Spontaneity • G depends on T G° = H°–TS° Signs: - - ? Rxn that are Enthalpy driven, but Entropy dis-favored are only spontaneous under _____ T

13. Temperature Dependence on Spontaneity • G depends on T G° = H°–TS° Signs: + + ? Rxn that are Entropy driven and Enthalpy dis-favored are only spontaneous under _____ T

14. Temperature Dependence on Spontaneity Consider: H2O(l) --> H2O(g) • What are the signs of ∆H and ∆S? • Under what T is rxn spontaneous? • At what specific T is rxn spontaneous? • G° = 0 = H°–TS°

15. Summary for Spontaneity Yes, under all conditions - + No, under all conditions + - Only at high temps + + Only at low temps - -