14 What major factor leads to a decrease in entropy as the reaction shown takes place? A. Increase in number of molecules during changeB. Decrease in number of molecules during changeC. Change in pressure of systemD. Change in internal energy of systemAnswer: B
17 Why does the plot show vertical jumps at the melting and boiling points? A. During melting or boiling at constant temperature entropy dramatically increases because energy is removed from the system during the change.B. During melting or boiling at constant temperature entropy changes significantly because more microstates exist for the final phase compared to the initial phase.C. During melting or boiling at constant temperature entropy changes significantly because fewer microstates exist for the final phase compared to the initial phase.D. During melting or boiling at constant temperature entropy changes significantly because no change in the number of microstates occurs during the phase change.Answer: B
26 Al2O3(s) + 3 H2(g) 2 Al(s) + 3 H2O(g) Sample Exercise 19.5 Calculating ΔS° from Tabulated EntropiesCalculate the change in the standard entropy of the system, ΔS° , for the synthesis of ammonia from N2(g)and H2(g) at 298 K:N2(g) + 3 H2(g) 2 NH3(g)SolutionAnalyze We are asked to calculate the standard entropy change for the synthesis of NH3(g) from its constituent elements.Plan We can make this calculation using Equation 19.8 and the standard molar entropy values in Table 19.1 and Appendix C.SolveUsing Equation 19.8, we have ΔS° = 2S°(NH3)–[S°(N2) + 3S°(H2)]Substituting the appropriate values from Table 19.1 yieldsΔS° = (2 mol)(192.5 J/mol–K)–[(1 mol)(191.5 J/mol–K) + (3 mol)(130.6 J/mol–K)]= –198.3 J/KCheck: The value for ΔS° is negative, in agreement with our qualitative prediction based on the decrease in the number of molecules of gas during the reaction.Practice ExerciseUsing the standard molar entropies in Appendix C, calculate the standard entropy change, ΔS°, for the following reaction at 298 K:Al2O3(s) + 3 H2(g) 2 Al(s) + 3 H2O(g)Answer: J/K26
27 Plan today Gibbs Free energy WS Graph problems from WS Problems from bookReminder: Quest is a quiz grade, next text is Thursday after break!