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Chapter 19: Spontaneous Change: Entropy and Free Energy

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. Objectives. Define terms of Entropy (S)

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Chapter 19: 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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