Prentice-Hall © 2007 General Chemistry: Chapter 19 Slide 1 of 44 Dr. Mendenhall Lecture 1 April 5, 2010 CHEMISTRY Ninth Edition GENERAL Principles and Modern Applications Petrucci Harwood Herring Madura Chapter 19: Spontaneous Change: Entropy and Free Energy

Prentice-Hall © 2007 General Chemistry: Chapter 19 Slide 2 of 44 Objectives 1.Qualitatively and quantitatively predict whether reactions are spontaneous or nonspontaneous based on their  H or  S values. 2.Determine  G from tabulated data 3.Use  G =  H =T  S, to determine  G at various temperatures.

Prentice-Hall © 2007 General Chemistry: Chapter 19 Slide 3 of 44 Criteria for Spontaneous Change: The Second Law of Thermodynamics. ΔS total = ΔS universe = ΔS system + ΔS surroundings The Second Law of Thermodynamics: ΔS universe = ΔS system + ΔS surroundings > 0 All spontaneous processes produce an increase in the entropy of the universe.

Prentice-Hall © 2007 General Chemistry: Chapter 19 Slide 4 of 44 Ex. Entropy Changes in the System Predict whether the entropy change of the system in each of the following reactions is positive or negative: A) Ag + (aq) + Cl - (aq) AgCl(s) B) NH 4 Cl(s) NH 3 (g) + HCl(g) The Ag+ and Cl- ions are free to move in solution, whereas AgCl is a solid. Furthermore, the number of particles decreases from left To right. Therefore,  S is negative. Since the solid is converted to a gas,  S is positive.

Prentice-Hall © 2007 General Chemistry: Chapter 19 Slide 5 of 44 Free Energy and Free Energy Change  Hypothetical process:  only pressure-volume work, at constant T and P. q surroundings = -q p = -ΔH sys  Make the enthalpy change reversible.  large surroundings, infinitesimal change in temperature.  Under these conditions we can calculate entropy.

Prentice-Hall © 2007 General Chemistry: Chapter 19 Slide 6 of 44 Free Energy and Free Energy Change TΔS univ. = TΔS sys – ΔH sys = -(ΔH sys – TΔS sys ) -TΔS univ. = ΔH sys – TΔS sys G = H - TS ΔG = ΔH - TΔS For the universe: For the system: ΔG sys = - TΔS universe

Prentice-Hall © 2007 General Chemistry: Chapter 19 Slide 7 of 44 Criteria for Spontaneous Change ΔG sys < 0 (negative), the process is spontaneous. ΔG sys = 0 (zero), the process is at equilibrium. ΔG sys > 0 (positive), the process is non-spontaneous. J. Willard Gibbs

Prentice-Hall © 2007 General Chemistry: Chapter 19 Slide 8 of 44 Table 19.1 Criteria for Spontaneous Change

Prentice-Hall © 2007 General Chemistry: Chapter 19 Slide 9 of 44 Standard Free Energy Change, ΔG°  The standard free energy of formation, ΔG f °.  The change in free energy for a reaction in which a substance in its standard state is formed from its elements in reference forms in their standard states.  The standard free energy of reaction, ΔG°. ΔG° = [  p ΔG f °(products) -  r ΔG f °(reactants)]

Prentice-Hall © 2007 General Chemistry: Chapter 19 Slide 10 of 44 Example Calculate the standard free-energy (  G) changes for the following reactions at 25ºC. A) CH 4 (g) + O 2 (g) CO 2 (g) + 2H 2 0(l) B) 2MgO(s) 2Mg(s) + O 2 (g) C) H 2 (g) + Br 2 (g) 2HBr(g)

Prentice-Hall © 2007 General Chemistry: Chapter 19 Slide 11 of 44 Example a) Predict the direction in which the  G for the equilibrium reaction of N 2 (g) + 3H 2 (g) 2NH 3 (g) will change with the increase of temperature. B) Calculate  G at 500ºC Assuming that  H and  S do not change with temperature.