Transcript Lecture 20
Characteristics of Ssurr The sign of Ssurr depends on whether the reaction or process is exothermic (Ssurr >0 or “favorable”) or endothermic (Ssurr < 0 or “unfavorable”). The magnitude of Ssurr depends on the temperature. To increase the effect of Ssurr , lower the temperature. To decrease the effect of Ssurr , raise the temperature. LeChatelier’s principle! 1 Another Question to Consider Calculate S and Suniv when 50.0 g of ice at 0C is dropped into 100.0 g of water at 75C in a perfectly insulated vessel and briefly discuss these numbers to show that all of the ice will indeed spontaneously melt. Hfusion for ice is 6.02 kJ/mol at 0C and Cp for water is 75.3 J/Kmol Assume Cp is constant over the temperature range in question. 2 3 4 Clicker Question A stable diatomic molecule spontaneously forms from its atoms at constant pressure and temperature. How does temperature affect the reaction? a) An increase in temperature makes the reaction more favorable. b) An increase in temperature makes the reaction less favorable. c) We cannot tell from the information we know. d) I do not know. 5 Clicker Question Consider the spontaneous reaction: 2KClO3(s) 2KCl(s) + 3O2(g) Is the reaction endothermic or exothermic? a) b) c) d) Endothermic Exothermic More information is needed. I do not know. 6 The meaning of Grxn At constant temperature and pressure the sign indicates the spontaneity of the reaction. Grxn < 0 means spontaneous Grxn > 0 means not spontaneous (or spontaneous in the opposite direction) If we change the temperature we can change the sign of Grxn The magnitude of Grxn tells us the relative extent of the reaction. If we change the temperature we can change the magnitude of Grxn. 7 Determining Grxn 2CO(g) + O2(g) 2CO2(g) Consider: 1. Predict if S° is positive or negative (we will determine it next). 2. Predict if the reaction is exothermic or endothermic (we will determine it next). 3. Can we predict the sign of G°? 4. At what temperatures is this reaction spontaneous? 8 Determining Grxn G (kJ/mol) H (kJ/mol) S (J/Kmol) 2CO(g) + O2(g) 2CO2(g) -137 0 -394 -110.5 0 -393.5 198 205 214 Consider: 1. Predict if S° is positive or negative (then determine it). 2. Predict if the reaction is exothermic or endothermic (then determine it). 3. Determine G° two ways. 4. At what temperatures is this reaction spontaneous? 9 Chapter 10: #130 One mole of a monatomic ideal gas (for which S = 8.00 J/Kmol at –73.0°C) was heated at a constant pressure of 2.00 atm from –73.0°C to 27.0°C. Calculate ΔH, ΔE, w, q, ΔS due to the change in volume, ΔS due to the change in temperature, and ΔG. 10 Clicker Question Given the following data, determine the boiling point of methanol (CH3OH). Hf (kJ/mol) S (J/Kmol) CH3OH(g) -201 240 CH3OH(l) -239 127 a) b) c) d) e) 63 K 63C 336C 1198 K None of these 11 Recall: Questions To Consider Why do systems reach equilibrium? That is, why doesn’t the ball go all the way down the hill? 12 Free energies change as a reaction progresses 13 “Ball on a Hill”: Gibbs Free Energy 14 The change in free energy to reach equilibrium 15 16 17 Clicker Question Given the following data, determine the equilibrium constant for the “pop bottle” reaction at 25C. G (kJ/mol) a) b) c) d) e) 2H2(g) + O2(g) 2H2O(g) 0 0 -237 3.50 x 1041 1.22 x 1083 8.17 x 10-84 2.86 x 10-42 I do not know how to do this problem. 18 Exothermic vs. Endothermic 19 Chapter 10: #139 Benzene (C6H6) has a melting point of 5.5C and an enthalpy of fusion of 10.04 kJ/mol at 25.0C. The molar heat capacities at constant pressure for solid and liquid benzene are 100.4 J/Kmol and 133.0 J/Kmol, respectively. For the process: C6H6(l) C6H6(s) Calculate ΔS and ΔSsurr at 10.0C (and explain the significance of the results). 20 Clicker Question 10 G (kJ/mol) H (kJ/mol) S (J/Kmol) 2SO2(g) + O2(g) 2SO3(g). -290 0 -371 -391 0 -396 143 205 257 Determine the value of K at 25C. a) 8 x 1024 b) 1 c) 1 x 10-25 d) I know how to do this but I am not getting any of the above answers. e) I do not know how to do this problem. 21 Clicker Question 11 2SO2(g) + O2(g) 2SO3(g). Knowing that G= -142 kJ/mol for the above reaction, predict the direction the system will shift to reach equilibrium if the system has the initial conditions: P(SO2) = 0.10 atm, P(O2) = 0.20 atm, and P(SO3) = 5.0 atm. a) Shifts to the left. b) Shifts to the right. c) The reaction is at equilibrium with those initial conditions. 22 Clicker Question 12 2SO2(g) + O2(g) 2SO3(g). Knowing that G= -142 kJ/mol for the above reaction, determine P(O2) so that the system is at equilibrium if P(SO2) = P(SO3). a) b) c) d) 0.94 atm 1.3 x 10-25 atm. 3.0 x 10-2 atm I know how to do this but I am not getting any of the above answers. e) I do not know how to do this problem. 23 Clicker Question Consider the spontaneous reaction: N2(g) + 3H2(g) 2NH3(g) Is the reaction endothermic or exothermic? a) b) c) d) Endothermic Exothermic More information is needed. I do not know. 24 25 Consider This! G (kJ/mol) H (kJ/mol) S (J/Kmol) 2KClO3(s) 2KCl(s) + 3O2(g). -290 -408 0 -391 -436 0 143 83 205 At what temperatures is this reaction spontaneous? 26 Clicker Question Is ____ a state function? Ssurr a) b) c) d) Yes No Yes No Suniv Yes No No Yes 27