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- 1. Chem 405Chemistry Principles for Engineers 29 October 2012
- 2. Announcements• Exam 3 –Thursday, 7:10-9pm – Hamilton Smith Hall, room 127 Lab Sections 1, 2, 3 – McConnell Hall, room 208 Lab Sections 4, 5, 6• Exam 3 Study Topics on Blackboard• OWL Exam 3 folder due Thursday, 6pm• Tuesday review session cancelled – Extended Wednesday office hours
- 3. When Will a Chemical Reaction be Product-Favored? − ΔH system ΔH system ΔSsurroundings = ΔSsystem > T T Enthalpy Entropy Product-Favored? ΔHsystem < 0 ΔSsystem > 0 Yes (Exothermic) ΔSuniverse > 0 Only at low T ΔHsystem < 0 ΔSsystem < 0 ensures positive ΔSsurr (Exothermic) overcomes negative ΔSsys Only at high T ΔHsystem > 0 ΔSsystem > 0 ensures positive ΔSsys (Endothermic) overcomes negative ΔSsurr ΔHsystem > 0 ΔSsystem < 0 No (Endothermic) ΔSuniverse < 0
- 4. Directionality of Processes Enthalpy Entropy Product-Favored? Negative Positive Yes Negative Negative Only at low T Positive Positive Only at high T Positive Negative NoExothermic reactions tend to be product-favored Some endothermic processes may also be product-favoredEntropy tends to increase in a product-favored reaction Some processes involving a decrease in entropy (of the systems) may also be product-favored Directionality is dependent on ΔH, ΔS, and T
- 5. Gibbs Free EnergyThermodynamic term to relate enthalpy and entropy: ΔGsystem = -TΔSuniverse Since ΔSuniverse increases for a product-favored process, ΔGsystem decreases for a product-favored process (@ constant T and P)
- 6. Gibbs Free EnergyThermodynamic term to relate enthalpy and entropy: ΔGsystem = -TΔSuniverse Since ΔSuniverse increases for a product-favored process, ΔGsystem decreases for a product-favored process (@ constant T and P) - ΔH system ΔS universe = ΔSsurroundings + ΔSsystem = + ΔSsystem T
- 7. Gibbs Free EnergyThermodynamic term to relate enthalpy and entropy: ΔGsystem = -TΔSuniverse Since ΔSuniverse increases for a product-favored process, ΔGsystem decreases for a product-favored process (@ constant T and P) - ΔH system ΔS universe = ΔSsurroundings + ΔSsystem = + ΔSsystem T - ΔH system ΔG system = -T + ΔSsystem = ΔH system − T∆S system T
- 8. Gibbs Free EnergyThermodynamic term to relate enthalpy and entropy: ΔGsystem = -TΔSuniverse Since ΔSuniverse increases for a product-favored process, ΔGsystem decreases for a product-favored process (@ constant T and P) - ΔH system ΔS universe = ΔSsurroundings + ΔSsystem = + ΔSsystem T - ΔH system ΔG system = -T + ΔSsystem = ΔH system − T∆S system T ΔG system = ΔH system − T∆Ssystem ** T in Kelvin
- 9. Gibbs Free EnergyΔG system = ΔH system − T∆Ssystem ΔG < 0 product-favored ΔG > 0 reactant-favored
- 10. Gibbs Free Energy ΔG system = ΔH system − T∆Ssystem ΔG < 0 product-favored ΔG > 0 reactant-favoredEnthalpy Entropy Gibbs Free Energy Product-Favored?Negative Positive Always Negative YesNegative Negative - at low T; + at high T Only at low TPositive Positive - at high T; + at low T Only at high TPositive Negative Always Positive No
- 11. Calculating Gibbs Free Energy 1. Under standard conditions: ΔG o = ΔH o − T∆S o ** T in Kelvin can calculate ΔG at different temperatures 2. From standard Gibbs free energies of formation:ΔG o = ∑ {(moles of product) × ΔG o (product)} − ∑ {(moles of reactant) × ΔG o (reactant)} f f gives ΔG at 25 oC (298 K)
- 12. Transitions Between Product-Favored and Rectant-Favored Processes Enthalpy Entropy Gibbs Free Energy Negative Negative - at low T; + at high T Positive Positive - at high T; + at low T Transition at ΔGo = 0 0 = ΔH o − T∆S o ΔH o T= ∆S oused to calculate the temperature at which the transition occurs

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