CHM 132 - Final - 2003 Key

CHM 132 - Final - 2003 Key - Chemistry 132 Final...

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Unformatted text preview: Chemistry 132 Final Examination May 8, 2003 Name (PRINT) SO M7” 0 N 5 Workshop Leader There are 10 questions on this exam. The last two pages of this exam contain a Periodic Table and a list of constants and formulas. Show our work neat] and circle our answers. Give units for all answers requiring them. Score Part 1 l. (45 points) 2. (30 points) subtotal Part 2 3. (25 points) 4. (25 points) 5. (25 points) subtotal Part 3 - 6. (35 points) 7. (40 points) subtotal Part 4 8. (20 points) 9. (20 points) 10. (35 points) subtotal Grand total (300 max) 2 Name ('5 points) Consider an ideal gas for which, in the temperature range of interest, CP is constant and equal to 30.0 J mol“1 K‘l. (a) The temperature of 1.00 mol of the gas, contained in a cylinder with a piston exerting a constant pressure of 1.00 atm, is raised gradually and reversibly from 500 to 1000K. Calculate AV, q, w, AE, AH, and AS for this sample of gas. W3M£T AV: MEAT = (:)(.0t21)(§ao) P 1 l): “FAT : («)(3oxm)[email protected] N:- , _‘ "' Kw Av = ~Ct)(4rl.l)(lol3)= @ L n w 0 Y N 3 Name (continued) (b) Suppose the same change of state described in (a) had been carried out irreversibly. Explain how AV, AE, AH, and AS for the gas would differ (if at all) from the values obtained in (a). All 5? men WVWQ ww‘ablas w «kam 0F M PMTJ M19 2311 s’nJ-rs - {M M m'a/ M3 W4 W?” W 4902» 04: Push . z 4 Name 2. i. 5 points) The following thermodynamic cycle has been developed by a brash engineer from Texas, who claims that he can "roll up his sleeves and fix that Carnot cycle." He calls his process the Perot cycle, and it is summarized below: Process AB: Irreversible expansion 3 of one mole of ideal gas at a ' A constant external pressure of 1 atm P from a volume of 20 L to 40 L. The gas is heated in this process from 300 K to a temperature TB. Process BC: reversible cooling of C the gas at constant volume until the temperature is lowered to 300 K. Process CA: reversible isothermal V compression of the gas at 300 K from a volume of 40 L to its original volume of 20 L. . (a) Calculate the work done on the system in the process AB. Pay attention to the sign of the result. W; ’Vexlr 5" 1' —(t)(7’0)[l0l3)fz Wis wamlm.M/wfl Asa—v0 0% W “0946‘“! @ flux—Elf- F— W www- a “as (b) Calculate the work done on the system in the reversible isothermal compression, process CA. Pay attention to the sign of the result. INCA: —-MZT 9/“ (if) :. -(l)($,2l)(%oo)QM = + (83080») 9M2 T H730 tr (continued) (c) Calculate the net work done 91; the system over the entire cycle NchMMQ 4407M: ' -— 24 6 T (d) Based on your knowledge of the Perot cycle an ot cycle, do you think the Perot cycle can be more efficient than the Carnot cycle? Why or Why not? The @125“ 0016? m Mme wags. ' . 3. (25 points) Speciation of metal ions is a very important issue in aqueous resources. For example, the toxicities of metal—containing compounds depend on the oxidation state of the metal, while the distribution of species can depend on the pH. Consider the equilibrium between Mn2+ and Mn02(s): 2Mn2+(aq) + 6H20 + 02(g) —> 2Mn02(s) + 4H3O+ (a) Using the f0110wing information, calculate the equilibrium constant at 298 K for the Mn2+(aq)/Mn02(s) reaction: Mn02(s) + 4H3O+ + 2e' —> Mn2+ + 6H20 8° = 1.208 v 02 ) + 4H30+ + 4e' —> 6H20 8° = 1.229 v Assume that the activities of pure liquids and solids are 1.00, and that the partial pressure of O2 is 1 atm. C" F04 PM W = 1.124 - Irwin/>- .ouv k C 0°5= bi" = if: . (4—)(wm) ’1‘ ‘09“? .omL k = 10”?” (b) Calculate [Mn2+] at a pH of 8.00, a value typical of ocean water. Assume that p(02) is unchanged from part (a). z: [‘47. i 7 Name (continued) (c) Now suppose manganese deposits are confined in a bay in which an acid spill has taken place and the local pH has fallen to 2.30. On the basis of LeChatelier’s Principle, how would you expect the [Mnfi] to change? Calculate [Mn2+] under these conditions, assuming that p(02) is unchanged from part (a). Lemma; Vfinuw :11) mar ha @4433 UN] «I’M ggm‘lt‘bn‘m $4,le ‘u / slam“! h) N '094’ 8 Name (25 points) A 0.100 M aqueous CaCl2 solution at pH = 7.00 is electrolyzed using platinum electrodes. A current of 1.50 amps passes through the solution for 50.0 hours. (a) Write two possible half reactions occurring at the cathode of this cell. Which reaction actually occurs? Why? Cathie => YEA/Jo law a2++12‘——>Cai €°=—Zflbv .—..> H’L €° 30.00 —.,OS'°I7- | "0 4.. ._.._. -' = ‘ lv 1 0’?(lO-7)L 4‘ OCLM (b) Write two possible half reactions occurring at the anode of this cell. Which reaction actually occurs? Why? Za'+——-> Ceruf €= *LZev - C I, W H7, = We SC ‘41? -_; ixLCOZ-XS'DXéoKéos Ze' Q64€§ .9— W Ca2+ (aq) + ‘ ——> Ca(s) 8° = -2.76v C12(g) +2e' —> 2C1‘(aq) 3° = 1.36 1/2 02(g)+2H30++2e- —> 3 ,, 8° = 1.23 9 Name 5. (25 points) Consider the following reaction between NH3(g) and 02(g): 2NH3(g) + 7/2 02(8) -> 2 N02(g) + 3 H20(g) 411;! kJ mol") S° (J K'1 moi") NH3(g) ~46.11 ‘ 192.34 02(g) 0. 205.03 N02(g) 33.18 239.95 H20(g) -241.82 188.72 From the data shown above, calculate the following quantities at 100°C. , (a) AS° = 74234.4() +3(1?€.72) -2(mz.%+)' - 331209.02) (b) AH° : 2‘(;;,|3) +3C“ 241.874 --2 (“41.“) (0) AG" : ”g6 "' 37g ‘G.Z ( >127: (d) Kc, “’00 $46000 \(ab: 6+ (filly???) (e) Calculate the enthalpic ande entropic cfactors contributing to Keq at 100°C, and evaluate Keq. Give a brief reason for the relative magnitudes of these factors. _.A+t°_{zr “bow/(8 2.x 37%) GKWYCL W ‘2 e 10 Name (35 points) Willard Libby won a Nobel Prize for determining that the radioactive nuclide 14C could be used to determine the age of archaeological objects. He hypothesized, and subsequently validated with experimental data, that the activity of 14C incorporated into living organisms was maintained by natural processes at a level of 15.3 disintegrations per minute per gram of carbon. At the moment when a sample of carbon is removed from the life cycle, the 14C begins to decay with a half—life of 5570 years. You are the scientific curator of a museum, and you are offered the opportunity to buy a scroll claimed to be 2300 years old. You take a one gram sample of carbon derived from the papyrus of the scroll and measure the 14C disintegration rate. Calculate the disintegration rate the one gram sample of carbon should have in order to be 2300 years old. _ lab _-af,%xt AU?) = “'08 = A(0)e L ‘ .643 z (l§.3 401)“) 3 §§70 (7,; oo) : (19.3 .tpwxnm) 1 1 Name (40 points) Air bags are based on the fact that sodium azide, NaN3, decomposes rapidly through the following reaction: 2NaN3(s) —> 2Na(s) + 3N2(g) (1) The reaction is initiated by an impact-driven detonator that heats the NaN3 rapidly to about 200°C, allowing it to decompose via reaction (1). In commercial applications, a 100 g charge of NaN3 can produce about 30 L of nitrogen at room temperature in about 40 milliseconds. You are a chemist working for General Motors Research Laboratory developing better technology for air bags. You wish to speed up the decomposition of NaN3 by a factor of three. You make laboratory measurements of the rate of reaction (1) and find that the activation energy is 15 kJ/mol. Calculate the temperature at which you should carry out the detonation of NaN3 to give you the desired increase in rate. Tim Mach‘m‘oawg 0J- e/t'voJ-(bl T. W 840; W Ksot’m {ling m air “3 oil“ 2C2 [9000 473 T2. 8. 12 Name (20 points) Give brief, one-sentence explanations for the following observations: (a) The spins of the three 2p electrons in the nitrogen atom are unpaired. HM‘S Ml: sou/9 B3 4-1:. ‘4:— ‘Pqu’ deem w W \ extoflmto wfih. ha W Ar. 9‘ (b) The ground state electron configuration of M0 is [Kr](55)1(4d)“’o equal” (9)7400)“ . gum-7 New We” (“VAL 449 3W W9 ”‘ Wm“ a? 8‘3 mom Ray? Fame/£2. CW Sywyl‘ngwhv] 01L 6(203331’9 (c) The ionization potential of beryllium (Z=4) is higher than that of boron (2:5) 82 W (ZS) @1ch gm“ 7’ 8 WW peachy s\« (2?) mm; (75) (2?). twain»? m ”who Ww‘l'w) fate-a dud saw) :9 Whita WWW (d) The geometry around the carbon atoms in ethylene, C2H4, is trigonal planar. FWM hi? VSFPR unoyv-P H’C2C‘H 2 WOW Yaw; Aafiw m ‘— WM Mom/d" CAJA C ochm 425:2” PM" 1 3 Name (20 points) For the following ABn molecules and ions, give the steric number, the orientation of electron pairs on A, and the actual geometry. (a) SeF4 Valance Med-WM arm/Mao $2 = Q an-m Se 4- 4- 847m l '. gelcd'wv‘ fox/5r; : Salon} [AI/WW. "mama bfflmlmw‘J F i r: Sc , ;;e~: M F I F (b) 1; vmw oudvm mums! W {dun =7+| +z( § ete‘of‘hm 3mm = flak IN/WW : '0 W I j“ “W 3923‘“? 1: 1 3'5 (c) soz Mew wwewaz s = 6 +2€m MW Lem; fivvoh/VC: @323“? a 53;. .: coma- ;_ 0cm W (WM Wu W?“ ugéptz E. dam own = «we nmw o,’ 7790M @taww Mm %5”” (d) cs 0 0040+ WU PC 9+2ch n‘é/ofv'kslv W W = §2C=§ 14 Name . 10. (35 points) The boron oxide molecule, BO, has been discovered in electrical discharges used in semiconductor manufacturing. (a) What are the relative values of the ionization potentials of boron and oxygen? B 1312?, O ZQZZF‘i' LID/60‘ macaw/t MC 0M 0 W9 17'; I P large/v (b) Draw the molecular orbital energy level (correlation) diagram for BO, indicating which orbitals are occupied. Consider only orbitals arising from n = 2 atomic orbitals. 10. 1 5 Name (continued) (c) What are the bond orders of EU, B0, and B0‘ ? Compare the bond lengths for BO+, B0, and B0. 2[§I~o> 130+: i Z-Cflr 0): "- L; (6 ’0) Emu} 1280” < 1280 < 230* ((1) Draw the shapes of the 02p, 6‘21), nzp and 152p orbitals of B0. ...
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