Gibbs Free Energy 19.6

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

∆G rxn and Spontaneity ∆G <0 spontaneous ∆G > 0 nonspontaeous ∆G = 0 at equilibrium

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

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

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

Another way to calculate ∆G°  Gº = Σ  Gº f (products) - Σ  Gº f (reactants) Calculate for the combustion of butane. __C 4 H 10 + __O 2 (g) --> __CO 2 (g) +__ H 2 O(g)

“Enthalpy Driven” Reactions Reactions that are spontaneous because energy is released ∆H system < 0

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

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

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

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 -

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 +

Temperature Dependence on Spontaneity Consider: H 2 O(l) --> H 2 O(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°

Summary for Spontaneity  H°  S° Spontaneous Yes, under all conditions No, under all conditions Only at high temps Only at low temps