Chemistry 126 Final Examination, Prof. Hanson, May, 2004 Section B or D (circle one) Seat Coordinate Name DO NOT OPEN THIS EXAM UNTIL INSTRUCTED TO DO SO Each asterisk () is 5 points. There are 40 s, for a total of 200 points. The first 100 points of this exam (pages 3 6) cover thermodynamics and electrochemistry. The kinetics section starts on page 7. Feel free to do the problems in any order. There are 11 pages total. Please check that all pages are present when you are instructed to do so. The pledge is on page 11. Note: Discussion of the contents of this exam with anyone in either section of the course who has not taken it will be considered an honor system violation. Selected Mean Bond Dissociation Energies (kj/mol) H H 432 Br Br 193 C C 347 C H 413 C O 358 C=O 799 C O 1072 N O 201 O H 467 O O 146 O=O 495 Selected Standard Thermodynamic Data H o f (298 K) S o (298 K) (kj/mol) (J/mol K) C(diamond) 0.00 5.74 CH 4 (g) 74.81 186.26 O 2 (g) 0.00 205.14 CO(g) 110.52 197.67 CO 2 (g) 393.51 213.74 H 2 O(l) 285.83 69.91 H 2 O(g) 241.82 188.83 Selected Standard Reduction Potentials at 25 o C (Volts) Ag 2+ + e Ag + 1.99 Au 3+ + 3e Au 1.50 Cl 2 (g) + 2 e 2 Cl 1.36 O 2 (g) + 4 H + + 4 e 2 H 2 O 1.23 Pt 2+ + 2 e Pt(s) 1.20 NO 3 (aq) + 4 H + (aq) + 3 e NO (g) + 2 H 2 O 0.96 Ag + + e Ag(s) 0.80 AgCl(s) + e Ag(s) + Cl 0.22 2 H + + 2 e H 2 (g) 0.00 1
2 1 1A 18 8A 1 H 1.008 2 2A 13 3A 14 4A 15 5A 16 6A 17 7A 2 He 4.003 3 Li 6.941 4 Be 9.012 5 B 10.81 6 C 12.01 7 N 14.01 8 O 16.00 9 F 19.00 10 Ne 20.18 11 Na 22.99 12 Mg 24.31 3 3B 4 4B 5 5B 6 6B 7 7B 8 9 10 8B 11 1B 12 2B 13 Al 26.98 14 Si 28.09 15 P 30.97 16 S 32.07 17 Cl 35.45 18 Ar 39.95 19 K 39.10 20 Ca 40.08 21 Sc 44.96 22 Ti 47.88 23 V 50.94 24 Cr 52.00 25 Mn 54.94 26 Fe 55.85 27 Co 58.93 28 Ni 58.69 29 Cu 63.55 30 Zn 65.39 31 Ga 69.72 32 Ge 72.61 33 As 74.92 34 Se 78.96 35 Br 79.90 36 Kr 83.80 37 Rb 85.47 38 Sr 87.62 39 Y 88.91 40 Zr 91.22 41 Nb 92.91 42 Mo 95.94 43 Tc (98) 44 Ru 101.07 45 Rh 102.91 46 Pd 106.42 47 Ag 107.87 48 Cd 112.41 49 In 114.82 50 Sn 118.71 51 Sb 121.76 52 Te 127.60 53 I 126.90 54 Xe 131.29 55 Cs 132.91 56 Ba 137.33 71 Lu 174.97 72 Hf 178.49 73 Ta 180.95 74 W 183.85 75 Re 186.21 76 Os 190.2 77 Ir 192.22 78 Pt 195.08 79 Au 196.97 80 Hg 200.59 81 Tl 204.38 82 Pb 207.2 83 Bi 208.98 84 Po (209) 85 At (210) 86 Rn (222) 87 Fr (223) 88 Ra 226.03 103 Lr (260) 104 Rf (261) 105 Db (260) 106 Sg (263) 107 Bh (262) 108 Hs (265) 109 Mt (266) 110 111 112 114 116 118 Lanthanide Series 57 La 138.91 58 Ce 140.12 59 Pr 140.91 60 Nd 144.24 61 Pm (145) 62 Sm 150.36 63 Eu 151.96 64 Gd 157.25 65 Tb 158.93 66 Dy 162.50 67 Ho 164.93 68 Er 167.26 69 Tm 168.93 70 Yb 173.04 Actinide Series 89 Ac 227.03 90 Th 232.04 91 Pa 231.04 92 U 238.03 93 Np 237.05 94 Pu (244) 95 Am (243) 96 Cm (247) 97 Bk (247) 98 Cf (251) 99 Es (252) 100 Fm (257) 101 Md (258) 102 No (259)
What is the probability of drawing two hearts from the top of a well-shuffled standard deck of cards (containing 13 cards of each suit, hearts, diamonds, spades, and clubs) Explain what is meant by fluctuation in terms of equilibrium. Explain the meaning of the diagram on the right (particularly in light of your answer to the last question). Calculate the number of ways (W) the system shown in the above diagram could be found. /20 3
One of the important ideas we were trying to emphasize in this course is how experimental data can be used to gain understanding of a chemical reaction. So, for example, say 50 ml of 2.0 M Na 2 CO 3 and 100 ml of 2.0 M HCl were combined in a calorimeter, and the following data were collected: Observation: The solution got warm, and lots of bubbles were produced. Initial temperature of both solutions: 21.0 o C Final temperature of the combined solution: 26.0 o C Known heat capacity of the empty calorimeter: 25.5 J/K Specific heat of the resultant solution: 4.60 J/g-K Density of the final solution: 1.05 g/ml Identify the surroundings. What is the heat capacity of the surroundings What is q sur Write the net ionic equation for this reaction and calculate its associated H (in kj). What does the sign of H tell us about this particular reaction Would you expect S to be positive or negative in this case Why /35 4
Consider the following reaction: CO 2 (g) C(diamond) + O 2 (g) Predict the signs of H o and S o in this case (no calculation should be necessary) and draw a graph of G vs. T depicting the reaction consistent with your analysis. Do you predict there to be a temperature above which this reaction is favorable For the reaction: 2 CO (g) + O 2 (g) 2 CO 2 (g) Using tabulated bond dissociation energies from the cover, predict U in this case. Using tabulated values for standard enthalpies of formation, calculate H o at 25 o C. Compare your value of U to your value for H o. [HINT: They should be similar.] Why are they not the same /25 5
An electrochemical cell is designed to utilize the following reaction: O 2 (g) + H + (aq) + Ag(s) H 2 O(l) + Ag + (aq) Balance this equation. Determine the standard cell potential at 25 o C in this case. Predict the cell potential at 25 o C provided the partial pressure of O 2 is 0.20 atm, the ph at the cathode is 6.00, the ph at the anode is 4.00, and the Ag + concentration at the silver electrode is 0.080 M. /20 6
Consider the hypothetical reaction between nitrogen dioxide and carbon dioxide in which both compounds are reduced and oxygen gas is produced following the equation given below: A hypothetical mechanism is given below: NO 2 + CO 2 NO + CO + O 2 Label each step (forward as well as reverse) as unimolecular or bimolecular. What are the intermediates (if any) in this mechanism (Write none if there are none.) What is the rate-determining step for this reaction (Step 1, 2, or 3) Determine the predicted rate law for the reaction based on this mechanism. What is the predicted order of the reaction in relation to NO 2 /30 7
Now imagine going into the laboratory to study this reaction. Using the method of initial rates, you get the following results: NO 2 + CO 2 NO + CO + O 2 Run [NO 2 ] o [CO 2 ] o (d[no 2 ]/dt) o 1 0.0064 0.0022 0.000026 2 0.0128 0.0022 0.000049 3 0.0128 0.0045 0.000111 Determine the rate law (including the value for k, with units) for the reaction based on this information. Are these results consistent with the proposed mechanism given on the previous page If your answer to this question was yes, propose another mechanism that is consistent with these experimental results; if your answer was no, propose a mechanism that is consistent with these results. [HINT: Your mechanism need not involve more than two or three steps. Possible intermediates include O, N 2, NO 3, and N 2 O 4.] /25 8
The questions on this page relate to the data shown on the next page for a reaction with the following chemical equation: 2 A B + C What is the order of the reaction with respect to substance A What is the value of the rate constant in this case What is the approximate initial rate of the reaction At what time would you expect the concentration of A to be 0.001 M /35 9
Data for the questions on the previous page 10
Finally, all of what we have been learning this semester is based on the tenet that simple probability and mindless chance underlie all processes, both living and nonliving. It seems to me that this might have some broader implications, for example, on the role of God in historical or current events, or the power of prayer. Briefly discuss your understanding of how what you have learned this semester in Chemistry 126 relates to your own personal religious or philosophical perspective. I hope you have a terrific summer. Best wishes for next year, whatever that may bring you. Honor system note: I happen to be a very strong proponent of the honor system. Please do not undercut my faith in it. Obviously not all of the concepts were on the exam. Students who take the exam on Wednesday are bound by the honor system to not discuss the exam with students planning to take the exam on Friday. Basically, for the purposes of the honor system, you must consider this exam similar to a take-home exam in the sense that it starts on Wednesday and goes through Friday. Discussing the exam with someone who is planning to take it after you yourself have taken it would not be fair and would clearly violate the pledge: that during this examination I have neither given nor received assistance, and that I have seen no dishonest work. Don t say what s on it; don t say what s not on it. Students on Friday will be asked to consider the pledge in this manner. If you are taking this exam on Wednesday and have any question as to whether a violation has occurred between Wednesday and Friday, please check the box and do not sign the pledge. PLEDGE: I pledge my honor that during this examination I have neither given nor received assistance, and that I have seen no dishonest work. [ ] I have intentionally NOT signed the pledge Signature 11