L28hr_Thermo6

L28hr_Thermo6 - Effect of temperature on free energy 1....

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Effect of temperature on free energy S T H G Δ Δ = Δ 1. Δ H>0, Δ S>0 Δ G T 3. Δ H<0, Δ S>0 Δ G T 2. Δ H>0, Δ S<0 Δ G T 4. Δ H<0, Δ S<0 Δ G T Sometime Sometime never always
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Spontaneous process: Δ G < 0 S T H G Δ Δ = Δ L28 Today: Effect of pressure on Δ G Δ G and equilibrium (Q and K) Mass Action Law: derivation Determine Δ H: Van't Hoff Equation
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12345 16% 33% 0% 11% 40% 1. under all conditions 2. under no conditions 3. at high temperature 4. at low temperature 5. This cannot be answered without additional information.
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Review: Reversible gas expansion: P = P ext = dV P w ext nRT PV = q w = V nRT P = Compare to entropy change in gas expansion of n moles = Δ f i r T dQ S 1 2 2 1 ln V V nRT V dV nRT dV V nRT PdV dV P w V V ext = = = = = 1 2 ln V V nRT w q = = heat: q or Q Δ S = q /T 2 1 ln V Sn R V Δ=
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I. Derivation: Dependence of Δ G of ideal gas on P at constant T 1. Because PV= Const 2. Δ G for expansion of ideal gas at constant T: Δ H = 0 why? H E + PV => Δ H = Δ E + Δ (PV) But: Δ E = (3/2)R Δ T = 0; PV = Const => Δ (PV) = nR Δ T= ±0 => Δ H = Δ E + Δ (PV) = 0 + 0 = 0 3. From 1. and 2. find Δ G Δ G = Δ H -T Δ S = 0 –( T nR ln {P 2 /P 1 } ) = nRTln{P 2 /P 1 } If we define: P 1 = P ref = 1 atm => ln{P 1 } = 0; P 2 P 21 2 12 1 ln ln ln VP P Sn R n R n R P ⎛⎞ Δ === ⎜⎟ ⎝⎠ Δ
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This note was uploaded on 07/22/2008 for the course CHEM 115b taught by Professor Reisler during the Spring '08 term at USC.

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L28hr_Thermo6 - Effect of temperature on free energy 1....

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