1. Department of Chemistry and Biochemistry CHM 101 - Reeves 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

2. Department of Chemistry and Biochemistry CHM 101 - Reeves 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.

3. Department of Chemistry and Biochemistry CHM 101 - Reeves 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) CH 3 CH 2 CH 3 (g)CH 4 (g)CH 3 CH 2 OH(l)

4. Department of Chemistry and Biochemistry CHM 101 - Reeves The Second Law of Thermodynamics A reversible change is one for which a very slight (infinitesimal) change in condition reverses the direction of the change. Consider melting ice. H 2 O(s) H 2 O(l)  H = 6 kJ

5. Department of Chemistry and Biochemistry CHM 101 - Reeves The Second Law of Thermodynamics The entropy change (  S) 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 :

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

7. Department of Chemistry and Biochemistry CHM 101 - Reeves The Second Law of Thermodynamics In the case of a finite difference where T surr >T sys If the temperature of the surroundings is less than that of the system (say T surr = -1 o C), then the heat flows in the opposite direction and  S is still positive.

8. Department of Chemistry and Biochemistry CHM 101 - Reeves Entropy Changes in Chemical Reactions Although absolute entropies (S) are always positive, entropy changes (  S) 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  S will depend on whether there are more gaseous moles of reactants or products. If there are more moles of gaseous product,  S will usually be positive. Conversely, more moles of gaseous reactants indicates a negative  S.

9. Department of Chemistry and Biochemistry CHM 101 - Reeves CHM 101 – Chapter Nineteen Entropy changes in Chemical Reactions Calculate the entropy change (  S) the reaction of gaseous hydrogen peroxide (H 2 O 2 ) with hydrogen to form liquid water.

10. Department of Chemistry and Biochemistry CHM 101 - Reeves Entropy Changes in Chemical Reactions An exothermic reaction (  H rxn < 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 (  S rxn >0) disperses the reactant atoms into products that can be arranged in many more configurations.

11. Department of Chemistry and Biochemistry CHM 101 - Reeves 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,

12. Department of Chemistry and Biochemistry CHM 101 - Reeves Because  G =  H - T  S, the sign of the Free Energy change (  G) depends on the signs of the enthalpy (  H) and entropy (  S) changes. The Gibbs Free Energy

13. Department of Chemistry and Biochemistry CHM 101 - Reeves The Gibbs Free Energy Calculate the standard free energy change (  G 0 ) associated with boiling water at 25 o C and 1 atm

14. Department of Chemistry and Biochemistry CHM 101 - Reeves The Gibbs Free Energy Estimate the temperature at which liquid water is in equilibrium with its vapor at 1 atm. pressure. H 2 O(l) H 2 O(g)