CHEMISTRY 202 Hour Exam II November 2, 2017 Dr. D. DeCoste Name Signature T.A. This exam contains 22 questions on 10 numbered pages. Check now to make sure you have a complete exam. You have two hours to complete the exam. Determine the best answer to the first 20 questions and enter these on the special answer sheet. Also, circle your responses in this exam booklet. Show all of your work and/or provide complete answers to questions 21 and 22. 1-20 (60 pts.) 21 (30 pts.) 22 (30 pts.) Total (120 pts.) Useful Information: Always assume ideal behavior for gases (unless explicitly told otherwise). 760 torr = 1.00 atm R = 0.08206 Latm/molK = 8.314 J/Kmol K = C + 273 NA = 6.022 x 10 23 E = q + w S = qrev/t H = E + PV G = H TS Here are some of the formulas we used/derived in studying thermodynamics. An individual formula may or may not apply to a specific problem. This is for you to decide! S = nrln(v2/v1) S = H/T Cv = (3/2)R Cp = (5/2)R S = ncln(t2/t1) G = G + RTln(Q) Ssurr = q/t w = P V qrev = nrtln(v2/v1) q = nc T ln(k) = H R 1 + T S R K ln K 2 1 H = R 1 T2 1 T 1
Hour Exam II Page No. 1 1. Your goal is to add enough ice at 10.0 C to liquid water at 25.0 C in a perfectly insulated Styrofoam cup calorimeter so that you end up with nothing but ice. Given the following information, which statement below best describes the relative amounts of ice and water required (in terms of masses)? Molar heat capacity of H2O(s) = 37.5 J/mol C Molar heat capacity of H2O(l) = 75.3 J/mol C ΔHfusion = 6.01 kj/mol a) You need to add at least 5 times as much ice as liquid water. b) You need to add at least 14 times as much ice as liquid water. c) You need to add at least 21 times as much ice as liquid water. d) You need to add at least 31 times as much ice as liquid water. e) You need to add at least 5 times as much liquid water as ice. ----------------------------------------------------------------------------------------------------------------------- 2, 3. You are studying our pop bottle reaction (in which we mix hydrogen and oxygen gases at 25.00 C to produce liquid water) in a bomb calorimeter. When you react the exact mixture required (no excess) to produce 1.000 mole of liquid water, the temperature of the calorimeter increases by 24.95 C. The bomb calorimeter has a heat capacity of 11.30 kj/ C. 2. Calculate the value of H in kj/mol of H2O(l) produced at 25.00 C. a) 278.2 b) 279.4 c) 281.9 d) 285.6 e) 289.3 3. Calculate the value of E in kj/mol of H2O(l) produced at 25.00 C. a) 278.2 b) 279.4 c) 281.9 d) 285.6 e) 289.3 --------------------------------------------------------------------------------------------------------------------- 4. At 25 C, the following enthalpies of reaction are known: 2ClF(g) + O2(g) Cl2O(g) + F2O(g) H = 167.4 2F2(g) + O2(g) 2F2O(g) H = 43.4 ClF(g) + F2(g) ClF3(g) H = 108.7 Calculate H for the reaction 2ClF3(g) + 2O2(g) Cl2O(g) + 3F2O(g) at 25 C a) 93.4 kj b) 15.3 kj c) 170.7 d) 232.7 kj e) 341.4 kj 5. Calculate the entropy change for a process in which 100.0 g of water at 20.0 C is mixed with 100.0 g of water at 80.0 C in a perfectly insulated container. Assume the heat capacity of water is constant at 4.18 J/gK a) 0 J/K b) 0.201 J/K c) 3.62 J/K d) 4.32 J/K e) 77.9 J/K
Hour Exam II Page No. 2 6. Consider the sublimation of dry ice: CO2(s) CO2(g) at 1 atm. Assuming H and S values are independent of temperature, determine the temperature at which this process will occur spontaneously. Use the following data: CO2(s) CO2(g) H f (kj/mol) 427.4 393.5 S (J/mol K) 51.1 213.8 a) The process will only occur spontaneously above temperatures of 65 C. b) The process will only occur spontaneously below temperatures of 65 C. c) The process will only occur spontaneously above temperatures of 65 C. d) The process will only occur spontaneously below temperatures of 65 C. e) The process is spontaneous at any temperature. 7. Consider the vaporization of methanol: CH3OH(l) CH3OH(g). Use the following data to determine the enthalpy of vaporization for methanol at its boiling point, 64.7 C. Do not assume enthalpy values are independent of temperature. CH3OH(l) CH3OH(g) H f (kj/mol) 239.0 201.0 Cp (J/mol C) 79.5 52.3 a) 32.8 kj/mol b) 36.9 kj/mol c) 38.0 kj/mol d) 39.1 kj/mol e) 43.2 kj/mol 8. Of T, Suniv, G, and V, how many are state functions? a) 0 b) 1 c) 2 d) 3 e) 4 9. Consider an ideal gas trapped in a container fitted with a frictionless massless piston. We transfer heat to the system, which increases the temperature and the volume of the gas. Which of the following correctly displays how we should solve for ΔS for this expansion? a) ΔS = ncpln(t2/t1) + nrln(v2/v1) b) ΔS = ncpln(t2/t1) + ncpln(v2/v1) c) ΔS = ncvln(t2/t1) + ncpln(v2/v1) d) ΔS = ncpln(t2/t1) + ncvln(v2/v1) e) ΔS = ncpln(t2/t1) 10. Consider the process: H2O(g) H2O(l). Which of the following statements correctly describes the signs of q and w at 1 atm and 370K? a) The signs of q and w are both negative. b) The sign of q is positive and the sign of w is negative. c) The sign of q is negative and the sign of w is positive. d) The signs of q and w are both positive. e) q and w are both zero
Hour Exam II Page No. 3 11. In a lecture demonstration, Ba(OH)2 8 H2O and NH4Cl solids were added to a beaker and reacted with one another. The beaker was placed on a wet wooden block as the mixture was reacting, and the beaker froze to the block. In addition, NH3 gas was given off. Which of the following can we say about this reaction? a) ΔH < 0, ΔS > 0 b) ΔH < 0, ΔS < 0 c) ΔH > 0, ΔS < 0 d) ΔH > 0, ΔS > 0 e) ΔS > 0; we cannot tell about ΔH 12. In the isothermal expansion of a given mass of an ideal gas, how many of the following terms can be non-zero? q, ΔG, ΔS, w, ΔH a) 0 b) 1 c) 2 d) 3 e) 4 13. How many of the following statements are true concerning the spontaneity of chemical reactions? If a reaction is endothermic it will never be spontaneous at any temperature. If a reaction is endothermic, it will be spontaneous at relatively low temperatures but not at higher temperatures. If a reaction is exothermic then it must be spontaneous at all temperatures. If a reaction is exothermic it will be spontaneous at relatively low temperatures but not at higher temperatures. a) 0 b) 1 c) 2 d) 3 e) 4 14. During the exam you rip the equation sheet off of the exam packet and, in a moment of panic, lose it. The only equation you remember dealing with ΔS is ΔS = ΔH/T. For which type of problem can you be sure this works? a) Problems dealing with the isothermal expansion or compression of an ideal monatomic gas. b) Problems dealing with the change of state of a liquid at its boiling point. c) Problems dealing with a chemical reaction. d) The equation would work with at least two of the above (a-c) problem types. e) The equation would not work with any of the above (a-c) problems types.
Hour Exam II Page No. 4 15. Consider two 1.0 mole samples of ideal, monatomic gases at 25 C in identical 100.0-L containers. Both gas samples are compressed to a volume of 50.0-L. Gas A is compressed reversibly and gas B is compressed in one-step. How many of the following statements are true? The magnitude of ΔS for Gas A is larger than the magnitude of ΔS for Gas B. The magnitude of ΔSsurr for Gas A is larger than the magnitude of ΔSsurr for Gas B. The magnitude of ΔS for Gas A is larger than the magnitude of ΔSsurr for Gas B. The magnitude of ΔSsurr for Gas A is larger than the magnitude of ΔS for Gas B. The magnitude of ΔSsurr for Gas A is larger than the magnitude of ΔS for Gas A. The magnitude of ΔS for Gas B is larger than the magnitude of ΔSsurr for Gas B. a) 0 b) 1 c) 2 d) 4 e) 6 ------------------------------------------------------------------------------------------------------------------- 16-18. Recall when we first began discussing thermodynamics we said we should be able to explain why a gas would expand when connected to an evacuated container as depicted below. In this case we have 1.0 mol of an ideal, monatomic gas at 25 C. The volume to the left of the stopcock is the same as the volume to the right of the stopcock. 16. Determine ΔSuniv for the process depicted above. a) 0 J/K b) 5.76 J/K c) 6.59 J/K d) 8.64 J/K e) 14.4 J/K 17. Determine ΔG for the process depicted above. a) 0 kj b) 4.29 kj c) 4.29 kj d) 1.72 kj e) 1.72 kj 18. For this situation, is the spontaneity of the process indicated by either ΔSuniv or ΔG? a) Yes, both ΔSuniv and ΔG are indications of the spontaneity of the gas expanding as shown above. b) No, neither ΔSuniv nor ΔG are indications of the spontaneity of the gas expanding as shown above. c) ΔG is an indication of the spontaneity of the gas expanding as shown above, but ΔSuniv is not an indication of the spontaneity. d) ΔSuniv is an indication of the spontaneity of the gas expanding as shown above, but ΔG is not an indication of the spontaneity.
Hour Exam II Page No. 5 19, 20. A 1.00-mole sample of an ideal, monatomic gas is in a container fitted with a piston (Vi = 5.00 L and Pi = 2.00 atm). The external pressure is changed to 1.00 atm and the gas expands isothermally in one step. The gas is then compressed isothermally and reversibly back to 5.00-L and 2.00 atm. 19. Determine ΔSuniv for the overall process (expansion and compression). a) 0 J/Kmol b) 1.60 J/Kmol c) 3.20 J/Kmol d) 4.16 J/Kmol e) 5.76 J/Kmol 20. Determine ΔS for the overall process (expansion and compression). a) 0 J/Kmol b) 1.60 J/Kmol c) 3.20 J/Kmol d) 4.16 J/Kmol e) 5.76 J/Kmol
Hour Exam II Page No. 6 21. You wake up early one Saturday morning to study thermodynamics, getting out your textbook, notes, and lap top. You find an old 20 oz. cola bottle that you almost fill with room temperature water for the day, and you seal the bottle. Your roommate gets up, sees you, wishes you luck, and hurries out for the day. You decide you want cooler water so you place the sealed bottle in the freezer, and, so intent on studying, you forget about it until hours later when you remove it from the freezer, now a block of ice in the bottle. You let the bottle sit on your desk and continue studying, forgetting about it until it once again becomes liquid water at room temperature. Your roommate comes back that evening, sees you at your desk with your textbook, notes, lap top, and sealed almost-filled bottle of room temperature water and says, Well, it looks like nothing has changed all day. You look at your water bottle, look at your text and see it is turned to Section 10.5 Entropy and the Second Law of Thermodynamics. You smile and say, Oh, hasn t it? a. Let s consider the system to be the water in the sealed bottle that started and ended at room temperature (but that you neglected to drink at all). Due to what happened to the water, how, if at all, do each of the following: ΔS, ΔSsurr, ΔSuniv, change (do each increase, decrease or stay the same)? Explain why. Answer this for each step of what happened to the water and overall. Full credit is reserved for a complete, coherent, conceptual explanation which discusses signs and relative magnitudes. [18 pts.] ---------------------------------------------------------------------------------------------------------------- Water cooling to 0 C and ice cooling to X C in the freezer: ΔS: negative ΔSsurr: positive ΔSuniv: positive Water freezing at 0 C in the freezer: ΔS: negative ΔSsurr: positive ΔSuniv: positive Ice warming to 0 C and water warming to 25 C in the room: ΔS: positive ΔSsurr: negative ΔSuniv: positive Ice melting at 0 C in the room: ΔS: positive ΔSsurr: negative ΔSuniv: positive Overall: ΔS = zero ΔSsurr = positive ΔSuniv = positive See textbook, lecture notes, and videos for explanations
Hour Exam II Page No. 7 21. (con t) b. You added 566 ml of water at 25 C (also your constant room temperature) and your freezer was set to 2 C. Assume the water was in the freezer long enough to reach thermal equilibrium with the freezer, and that the heat capacities of water and ice do not change over the temperature range in question. Use the following data to calculate ΔS, ΔSsurr, and ΔSuniv due to what happens to the water. Report your answers in J/K for the overall process of starting and ending with room temperature water. Show all work and explain the significance of the results. [12 pts.] Density of water at 25 C: 1.00 g/ml Cp for H2O(l) = 75.3 J/molK Cp for H2O(s) = 37.5 J/molK ΔHfusion for H2O(l) at 0 C= 6.010 kj/mol ---------------------------------------------------------------------------------------------------------------- Water cooling to 0 C and ice cooling to 2 C in the freezer: ΔSsurr = 218.12 J/K; 8.69 J/K ΔS = 207.17 J/K; 8.66 J/K ΔSuniv: 11.05 J/K; 0.03 J/K Water freezing at 0 C in the freezer: ΔS = 691.114 J/K ΔSsurr = 696.452 J/K ΔSuniv = 5.338 J/K Ice warming to 0 C and water warming to 25 C in the room: ΔS = 8.66 J/K; 207.17 J/K ΔSsurr = 7.90 J/K; 198.36 J/K ΔSuniv = 0.76 J/K; 8.81 J/K Ice melting at 0 C in the room: ΔS = 691.114 J/K ΔSsurr = 633.338 J/K ΔSuniv = 57.776 J/K Overall: ΔS = 0 J/K ΔSsurr = 83.7 J/K ΔSuniv = 83.7 J/K
Hour Exam II Page No. 8 22. Recall problem #23 from the first exam in which you considered the dissociation of hydrogen molecules to atomic hydrogen [H2(g) 2H(g)] in a container fitted with a frictionless massless piston at a constant external pressure. The claim was made in that problem that the temperature must be sufficiently high for this reaction to take place. a. Without using any data, explain why the temperature must be high for this reaction [H2(g) 2H(g)] to be spontaneous, and why at low temperatures it will not be spontaneous. Your discussion should include references (with explanations) to signs of ΔH, ΔS, ΔSsurr, and ΔSuniv and relative magnitudes when appropriate. Full credit is reserved for a complete and coherent explanation. [9 pts.] See textbook, lecture notes, and videos for explanations b. Given the following data, determine if the reaction [H2(g) 2H(g)] is spontaneous at 25.0 C and 1 atm. Explain and show all work. [4 pts.] H2(g) H(g) H f (kj/mol) 0 218.0 S (J/mol K) 130.6 114.6 ΔH = 436.0 kj ΔS = 98.6 J/K ΔG = 406.6 kj positive, not spontaneous
Hour Exam II Page No. 9 22. c. In the problem from exam 1, we considered a temperature of 4500.K. Given that Cp for H2(g) is 28.86 J/Kmol and that H(g) can be considered to be a monatomic ideal gas, determine if the reaction [H2(g) 2H(g)] is spontaneous at 4500.K and 1 atm. Assume that heat capacities are independent of temperature, but that ΔH and ΔS are not independent of temperature. Explain and show all work. [8 pts.] ΔH at 4500. K = 489.4 kj ΔS at 4500. K = 133.1 J/K ΔG = 489.4 kj (4500. K)(0.1331 kj/k) = 109.6 kj negative, spontaneous
Hour Exam II Page No. 10 22. d. Use your answers from parts b and c to determine the values of the equilibrium constants for this reaction [H2(g) 2H(g)] at both 298 K and 4500. K. Then, use these values to estimate the value of ΔH for the reaction. [6 pts.] At 298 K: K (Kp) = 5.33 x 10-72. At 4500. K: K (Kp) = 18.7. ΔH = 443.2 kj e. Given your various answers for ΔH in this problem, would it have been reasonable to assume that ΔH is independent of temperature? Would it have changed your answer to the spontaneity question in part c to assume ΔH and ΔS are independent of temperature? Justify your answer. [3 pts.] Yes, it would have been reasonable and, no, it would not have changed our answer about spontaneity.
Hour Exam II Page No. 11 21b (summary) Water Water Ice Ice Ice Water overall cooling freezing cooling warming melting warming ΔS 207.17 691.114 8.66 8.66 691.114 207.17 0 ΔSsurr 218.12 696.452 8.69 7.90 633.338 198.36 83.7 ΔSuniv 11.05 5.338 0.03 0.76 57.776 8.81 83.7 In the freezer Water Water Ice overall cooling freezing cooling ΔS 207.17 691.114 8.66 906.994 ΔSsurr 218.12 696.452 8.69 923.142 ΔSuniv 11.05 5.338 0.03 16.148 In the room Ice Ice Water overall warming melting warming ΔS 8.66 691.114 207.17 906.994 ΔSsurr 7.90 633.338 198.36 839.598 ΔSuniv 0.76 57.776 8.81 67.396