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Understanding Spontaneous Change: Entropy and Free Energy

Explore the principles of spontaneous processes, entropy, and free energy in chemistry. Learn how to predict and evaluate changes in entropy and make calculations using the second law of thermodynamics.

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Understanding Spontaneous Change: Entropy and Free Energy

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  1. Petrucci • Harwood • Herring • Madura GENERAL Ninth Edition CHEMISTRY Principles and Modern Applications Chapter 19: Spontaneous Change: Entropy and Free Energy Dr. Mendenhall Lecture 1 April 5, 2010 General Chemistry: Chapter 19

  2. Objectives • Define terms of Entropy (S) • Define, explain, and apply the second law of thermodynamics. General Chemistry: Chapter 19

  3. 19-1 Spontaneity: The Meaning of Spontaneous Change General Chemistry: Chapter 19

  4. Spontaneous Process • A process that occurs in a system left to itself. • Once started, no external actions is necessary to make the process continue. • A non-spontaneous process will not occur without external action continuously applied. 4 Fe(s) + 3 O2(g) → 2 Fe2O3(s) H2O(s) H2O(l) General Chemistry: Chapter 19

  5. Spontaneous Process • Potential energy decreases. • For chemical systems the internal energy Uis equivalent to potential energy. • Berthelot and Thomsen 1870’s. • Spontaneous change occurs in the direction in which the enthalpy of a system decreases. • Mainly true but there are exceptions. General Chemistry: Chapter 19

  6. 19-2 The Concept of Entropy • Entropy, S. • The greater disorder of the system, greater the entropy of the system. • The more ordered a systems, the smaller it’s entropy. ΔS > 0 spontaneous ΔU = ΔH = 0 General Chemistry: Chapter 19

  7. qrev ΔS = T Entropy Change For changes occurring at constant temperature Ssolid < Sliquid << Sgas General Chemistry: Chapter 19

  8. Example Predict whether the entropy change is greater than or less than zero for Each of the following processes: A) Freezing liquid bromine B) Evaporating a beaker of ethanol at room temperature C) Dissolving sucrose in water D) Cooling nitrogen gas from 80ºC to 20ºC General Chemistry: Chapter 19

  9. 19-4 Criteria for Spontaneous Change:The Second Law of Thermodynamics. ΔStotal = ΔSuniverse = ΔSsystem + ΔSsurroundings The Second Law of Thermodynamics: ΔSuniverse = ΔSsystem + ΔSsurroundings > 0 (spontaneous process) ΔSuniverse = ΔSsystem + ΔSsurroundings = 0 (equilibrium process) The entropy of the universe increases in a spontaneous process & remains unchanged in an equilibrium process. General Chemistry: Chapter 19

  10. Free Energy and Free Energy Change • Hypothetical process: • only pressure-volume work, at constant T and P. qsurroundings = -qp = -ΔHsys • Make the enthalpy change reversible. • large surroundings, infinitesimal change in temperature. • Under these conditions we can calculate entropy. General Chemistry: Chapter 19

  11. Absolute Entropies • Third law of thermodynamics. • Most ordered arrangement is a perfect crystalline substance at absolute zero. • The lowest entropy any substance can obtain is that of a pure perfect crystal at 0 K is zero. • Standard molar entropy. • Tabulated in Appendix D. ΔS = [ pS°(products) -rS°(reactants)] General Chemistry: Chapter 19

  12. Example From the absolute entropy values in Appendix D, calculate the standard entropy changes for the following reactions at 25ºC. A) CaCO3(s) CaO(s) + CO2(g) B) N2(g) + 3H2(g) 2NH3(g) C) H2(g) + Cl2(g) 2HCl(g) General Chemistry: Chapter 19

  13. ΔH ΔS = Ttr ° ΔHfus 6.02 kJ mol-1 ΔSfus = = = 2.2010-2 kJ mol-1 K-1 Ttr 273.15 K Evaluating Entropy and Entropy Changes • Phase transitions. • Exchange of heat can be carried out reversibly. H2O(s, 1 atm) H2O(l, 1 atm) ΔHfus = 6.02 kJ at 273.15 K General Chemistry: Chapter 19

  14. ΔHvap  87 kJ mol-1 K-1 ΔS = Tbp Trouton’s Rule General Chemistry: Chapter 19

  15. Raoult’s Law ° PA = APA General Chemistry: Chapter 19

  16. Free Energy and Free Energy Change For the universe: TΔSuniv. = TΔSsys – ΔHsys = -(ΔHsys – TΔSsys) -TΔSuniv. = ΔHsys – TΔSsys For the system: G = H - TS ΔG = ΔH - TΔS ΔGsys = - TΔSuniverse General Chemistry: Chapter 19

  17. Criteria for Spontaneous Change ΔGsys < 0 (negative), the process is spontaneous. ΔGsys = 0 (zero), the process is at equilibrium. ΔGsys > 0 (positive), the process is non-spontaneous. J. Willard Gibbs 1839-1903 General Chemistry: Chapter 19

  18. Table 19.1 Criteria for Spontaneous Change General Chemistry: Chapter 19

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