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402 Chapter 18 Free Energy and Thermodynamics Solution: Reactant/Product from Appendix IIB) H₂(g) 0.0 Fe₂O₃(s) -742.2 Fe(s) 0.0 -228.6 Be sure to pull data for the correct formula and phase. = - = + + = [2(0.0kJ) + 3(-228.6kJ)] - [3(0.0kJ) + = [-685.8kJ] - [-742.2kJ] = +56.4kJ So the reaction is nonspontaneous. The value is similar to that in Problem 18.61. Check: The units (kJ) are correct. The answer is the same as that in Problem 18.61 within the error of the calculation. (d) Given: + 3H₂(g) at 25 °C Find: and spontaneity and compare to Problem 18.61 Conceptual Plan: = - then compare to Problem 18.61 Solution: Reactant/Product from Appendix IIB) 0.0 0.0 NH₃(g) -16.4 Be sure to pull data for the correct formula and phase. = = - + = [1(0.0kJ) + = -32.8kJ] [0.0kJ] = -32.8kJ So the reaction is spontaneous. The result is the same as that in Problem 18.61. Check: The units (kJ) are correct. The answer matches the one in Problem 18.61. Values calculated by the two methods are comparable. The method using and is longer, but it can be used to determine how changes with temperature. 18.65 Given: 2NO(g) + Find: and spontaneity at (a) 298 K, (b) 715 K, and (c) 855 K Conceptual Plan: = - then = - then J/K kJ/K then T AG° 1kJ AG = 1000J Solution: Reactant/Product from Appendix IIB) NO(g) 91.3 O₂(₈) 0.0 33.2 Be sure to pull data for the correct formula and phase. = = + = - [2(91.3kJ) + 1(0.0kJ)] = [182.6kJ] = -116.2kJ then Copyright © 2017 Pearson Education, Inc.