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CHM 101 – Chapter Nineteen

CHM 101 – Chapter Nineteen. Spontaneous Processes Entropy & the Second Law of Thermodynamics The Molecular Interpretation of Entropy Entropy Changes in Chemical Reactions Gibbs Free Energy Free Energy and Temperature. Molecular Interpretation of Entropy.

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CHM 101 – Chapter Nineteen

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  1. CHM 101 – Chapter Nineteen Spontaneous Processes Entropy & the Second Law of Thermodynamics The Molecular Interpretation of Entropy Entropy Changes in Chemical Reactions Gibbs Free Energy Free Energy and Temperature CHM 101 - Reeves

  2. Molecular Interpretation of Entropy The Third Law of Thermodynamics defines zero entropy: The entropy of a perfectly ordered crystalline solid at 0K is 0. Under all other circumstances, absolute entropies are positive. CHM 101 - Reeves

  3. Molecular Interpretation of Entropy Absolute entropies have been measured for many substances. Appendix C provides a comprehensive list. Arrange the following in order of increasing entropy (S) C (graphite) C (diamond) C (g) CH3CH2CH3(g) CH4(g) CH3CH2OH(l) CHM 101 - Reeves

  4. A reversible change is one for which a very slight (infinitesimal) change in condition reverses the direction of the change. Consider melting ice. The Second Law of Thermodynamics H2O(s) H2O(l) DH = 6 kJ CHM 101 - Reeves

  5. The Second Law of Thermodynamics The entropy change (DS) for any process is defined as: The Second Law of Thermodynamics states that in any spontaneous process, the entropy of the Universe always increases. Thus: In the case of melting one mole of ice at the infinitesimal temperature difference described above : CHM 101 - Reeves

  6. The Second Law of Thermodynamics Most changes are irreversible, And a slight change does not change the direction of the process. CHM 101 - Reeves

  7. The Second Law of Thermodynamics In the case of a finite difference where Tsurr>Tsys If the temperature of the surroundings is less than that of the system (say Tsurr = -1oC), then the heat flows in the opposite direction and DS is still positive. CHM 101 - Reeves

  8. Entropy Changes in Chemical Reactions • Although absolute entropies (S) are always positive, entropy changes (DS) for chemical reactions can be either positive or negative. • Since the entropies of gases are so much larger than entropies of solids or liquids, the sign of DS will depend on whether there are more gaseous moles of reactants or products. • If there are more moles of gaseous product, DS will usually be positive. Conversely, more moles of gaseous reactants indicates a negative DS. CHM 101 - Reeves

  9. CHM 101 – Chapter Nineteen Entropy changes in Chemical Reactions Calculate the entropy change (DS) the reaction of gaseous hydrogen peroxide (H2O2) with hydrogen to form liquid water. CHM 101 - Reeves

  10. Entropy Changes in Chemical Reactions • An exothermic reaction (DHrxn< 0) releases heat, dispersing energy that had been localized in the chemical bonds of the reactants. • As a result, the surroundings experience a positive entropy change: • An increase in the entropy of the system (DSrxn>0) disperses the reactant atoms into products that can be arranged in many more configurations. CHM 101 - Reeves

  11. The Gibbs Free Energy Recall that according to the Second Law: J Willard Gibbs summarized this result by defining the Free Energy (G) as G = H - TS, or at constant T, Thus for any spontaneous process at const T & P, CHM 101 - Reeves

  12. The Gibbs Free Energy Because DG = DH - TDS, the sign of the Free Energy change (DG) depends on the signs of the enthalpy (DH) and entropy (DS) changes. CHM 101 - Reeves

  13. The Gibbs Free Energy Calculate the standard free energy change (DG0) associated with boiling water at 25oC and 1 atm CHM 101 - Reeves

  14. The Gibbs Free Energy Estimate the temperature at which liquid water is in equilibrium with its vapor at 1 atm. pressure. H2O(l) H2O(g) CHM 101 - Reeves

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