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Chapter 18 Free Energy and Thermodynamics 403 Reactant/Product K from Appendix IIB) 210.8 205.2 NO₂(₈) 240.1 Be sure to pull data for the correct formula and phase. = - = - + = [2(240.1J/K)] - [2(210.8J/K) + then = [480.2J/K] [626.8J/K] = -146.6J/K (a) = - = 116.2kJ = -72.5kJ = -7.25 X 10⁴ J; so the reaction is spontaneous. (b) = = -116.2kJ = -11.4kJ = 1.14 X 10⁴ J; so the reaction is spontaneous. (c) = = 116.2kJ = +9.1kJ = +9.1 X 10³ J; so the reaction is nonspontaneous. Check: The units (kJ) are correct. The enthalpy term dominates at low temperatures, making the reaction spontaneous. As the temperature increases, the decrease in entropy starts to dominate and in the last case the reaction is nonspontaneous. 18.67 Because the first reaction has Fe₂O₃ as a product and the reaction of interest has it as a reactant, we need to reverse the first reaction. When the reaction direction is reversed, AG changes. Fe₂O₃(s) 2Fe(s) + = +742.2kJ Because the second reaction has 1 mole of CO as a reactant and the reaction of interest has 3 moles of CO as a reactant, we need to multiply the second reaction and the AG by 3. + = 3(-257.2kJ) = -771.6kJ Hess's law states that the AG of the net reaction is the sum of the of the steps. The rewritten reactions are: Fe₂O₃(s) 2Fe(s) + = +742.2kJ + = -771.6kJ Fe₂O₃(s) 2Fe(s) + = -29.4kJ Free Energy Changes, Nonstandard Conditions, and the Equilibrium Constant 18.69 (a) Given: I₂(s) at 25.0 °C Find: Conceptual Plan: = Solution: Reactant/Product from Appendix IIB) I₂(s) 0.0 19.3 Be sure to pull data for the correct formula and phase. = = - = [1(19.3kJ)] [1(0.0kJ)] = +19.3kJ; the reaction is nonspontaneous. Check: The units (kJ) are correct. The answer is positive because gases have higher free energy than do solids and the free energy change of the reaction is the same as free energy of formation of gaseous iodine. Copyright © 2017 Pearson Education, Inc.