notes_Lecture_10_102210

notes_Lecture_10_102210 - Lecture 10: Free Energy and...

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Lecture 10: Free Energy and Equilibrium Announcements – HW#3 Due Today (11:00 PM) – Review sheet posted on web Reading: Zumdahl 10.10 and 10.11 Outline – Free energy and pressure (10.10) – Equilibrium is achieved when the total Gibbs energy for a system is minimized (10.10) Δ G, Δ G o , and K (10.11)
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Example • Determine the Δ rxn for the following: C 2 H 4 (g) + H 2 O(l) C 2 H 5 OH(l) • Tabulated Δ f (@ 298 K): Δ f (C 2 H 5 OH(l)) = -175 kJ/mol Δ f (C 2 H 4 (g)) = 68 kJ/mol Δ f (H 2 O (l)) = -237 kJ/mol
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Example (cont.) • Using these values: C 2 H 4 (g) + H 2 O(l) C 2 H 5 OH(l) Δ rxn = Δ f (C 2 H 5 OH(l)) - Δ f (C 2 H 4 (g)) f (H 2 O (l)) Δ - Δ = ° Δ . . react o f prod o f rxn G m G n G Δ rxn = -175 kJ - 68 kJ -(-237 kJ) Δ rxn = -6 kJ < 0 ; therefore, spontaneous
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Δ rxn Reaction Rate • Although Δ rxn can be used to predict IF a reaction will be spontaneous as written, it does not tell us how fast a reaction will proceed. Example: C(s, diamond) + O 2 (g) b CO 2 (g) Δ G°= -397 kJ C(s, graphite) + O 2 (g) b CO 2 (g) Δ G°= -394 kJ But diamonds are forever…. Δ G ° rxn rate CO 2 (g) b C(s, graphite) + O 2 (g) Δ G°= 394 kJ C(s, diamond) b C(s, graphite) Δ G°= -3 kJ < 0
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Thermo vs. Kinetics • “Spontaneous” is not synonymous with fast . • Thermodynamics determines the equilibrium state of a system. ..the proportions of products and reactants at equilibrium. • Kinetics determines the pathway to equilibrium.
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Two ways to calculate Δ rxn 1) From our expression for Δ S univ : Δ G ° rxn = Δ H ° rxn - T Δ S ° rxn 2) From Gibbs free energies of formation: Δ G ° rxn = n Δ G f ° (products) - m Δ G f ° (reactants) What if we’re not at standard conditions??
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Free Energy and Pressure • To discuss the extension of free energy beyond standard conditions, your book considers a gas- phase reaction with all gases behaving ideally. This is an easier system to envision than a solution- phase reaction, but the work-up is similar and the result is the same. • Remember, we are still thinking about increasing entropy S large volume > S small volume S small pressure > S large pressure
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Free Energy and Pressure (cont) • It can be shown that for a gas: G = G o + RTln(P) • As P increases , ln(P) increases , and G becomes more positive (and less spontaneous ) • As P decreases
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This note was uploaded on 09/29/2011 for the course CHM 101 taught by Professor Ma during the Spring '11 term at Beacon FL.

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notes_Lecture_10_102210 - Lecture 10: Free Energy and...

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