Department of Chemistry and Biochemistry CHM 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

Department of Chemistry and Biochemistry CHM 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.

Department of Chemistry and Biochemistry CHM 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)

Department of Chemistry and Biochemistry CHM 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

Department of Chemistry and Biochemistry CHM 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 :

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

Department of Chemistry and Biochemistry CHM 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.

Department of Chemistry and Biochemistry CHM 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.

Department of Chemistry and Biochemistry CHM 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.

Department of Chemistry and Biochemistry CHM 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.

Department of Chemistry and Biochemistry CHM 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,

Department of Chemistry and Biochemistry CHM 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

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

Department of Chemistry and Biochemistry CHM 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)