exam_answers_152A_Exam_2_Key_Version_B

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Unformatted text preview: WMM\mm-wmw.wr w® CHEMISTRY 152A HOUR EXAM ll (Version B) Winter 2008 Wednesday, February 27, 2008 Name: I TA Section: Student Number: TA Name: Score (page) 1 2 GOOD LUCK! 3 Watch sig. figs and units! 4 5 Total (x1100) l. CONCEPTUAL I SHORT ANSWER: Answer all questions (subtotal = 52 pts) 6:22.79 (4) ‘i. (a) Give the sign (+ or -) of the cell voltage a and of AG for both an ' electrolytic cell and a galvanic cell. Galvanic: e_+ AG; Electrolytic: e - AG __-fi; I 2) . (b) (Circle correct in an electrolytic cell, the electrons move away from the (cathode, , while in a galvanic cell, the electrons move away from the 6115259 cathode). ‘ (7) (c) (Circle correct) Reduction always occurs at the (anode, while oxidation always occurs at theoathode). ‘ (a) 2. (a) For the solution of the wave equation for the particle-in-a box, why is n=0 not an allows u ntum umber? . . - : /~2tq/§n’sn.£aj 7V; I’m’w“ t) ’Mr' A a m”:“¢}_43‘ W? («near I, 9v * 5’: 0 Meg»: Vac, fan;qu gym all-5' Analog. p / r awe. live-9»: 4/50 1 5'? 141%0 , pew/{r4 42.2.. 491v, aha/44..- I v: fiwdcg ma’ll‘fim {pg 094/9 muszlJp-ax, A”) (5) (b) What happens to the spacing between energy levels AE = E(n)-E(n-1) as L (the length of the box) is made smaller? 5 9; .1— L1, 2 ’0' cmeysy 7pc ca") I'he-MSPS- 3. ln thephoto-electric effect, the kinetic energy of the electron ejected from the (3) metal surface depends on: ' (a) @F) The chemical element of the metal (b) T45) The intensity (brightness) of the light source. (0) F) The energy of the incoming photon in excess of the work function of the metal. a» /$?fl fliy a w (‘79 4. For the chemical reaction A ~> 8, give the appropriate conditions for spontaneity as a function of temperature for each of the following cases ' (Possibitities: spon at atl temperatures, becomes spont at higher temperature, becomes spont at lower temperature, never spent at any i temperature: (a) VAS<0 and AH >0 A0qu siw’l‘ “7:”? ’4'? ' : (b)AS<0 and AH <0 We“ a 4’15; f (c)AS>0 and AH >0 5pm)! «#7145; ‘9” ’79‘ ‘ (d)AS>0 and AH <0 Sfon/I. ea q// Ara/674m mmsvramw: u:-..-.-~.-......-..7.t,,t.. ,, .r . (3) 5. For the fotlowing vaiues of AG" , give the corresponding values for the equilbrium constant K (K >1, or K<1 or K=1) if AGO = 0. then K =? 1 if AGO > c, then K = 7 <1 if AGO < 0, then K 2? >4. (z) 6. (a) (T (B The change in the Gibbs free energy AG represents the minimum possibie useful work obtainable from a process at constant temperature and pressure. . ( z) (b) Which of the following can be assigned an absolute value (circle correct): A l: H e e e (5,) 7. Balance the following redox reaction, showing the separate oxidation and reduction haif reactions, then the combined net redox reaction: - l l Pb(s) + Pb02(s) + H“(aq) + Hso; (aq) --~--> Parasol, (s) + H200) I we’d: PA + 6/50,; “s? fléJ‘oy 4Ze'+/V " , Ma“ Ma; Haw/#0,; +ze1—-s féfoy+dflaj : 7’:- 7‘ ~ £5“ PL/s) +1402“; +Z/7/flgj)+ 395093;) .__.) 219452,“) 424/194,) EV «Mg-0);,”mvvgunm.i.1.” ,. . . (2:42 5729 A—z—‘y@ {(0 8. Next to each of the following founders of quantum theory, write the equation most associated with their name in this context: Heisenberg flit) 4‘ (AP) ‘9 19/2 Schroedinger W will De Broglie x7 = % u Einstein (not E= mcz) g We V 2 3 4'" ’ 4‘6 ' (a; s. For a concentration cell with 0.1 M Fc2+ (cathode) vs 0.01 M Fe2+ (anode), give an expression for the cell vottage sin terms of the standard cell voltage 8° and these concentrations in the reaction quotient Q m :- 2 / 74—»; gzgofio’ofiI/‘v @ :[017 to 1: M "/00, [mm] £30 8 50.00 wow )o ~4——- : -0 093/ t '—«--—--‘ " :7 . 5" 7, 9 ‘7 ’5’ Z {1) +0 29 V (2) 10. (a) Name the two physical phenomena for which the explanation led to the overthrow of classical physics and the beginning of quantum mechanics in the early 209' century. I / 1674 (/8042 6,446,: 2. auv oyéy/Q/Xe (z) (b) Namea article-like ro e of e— ‘ ' ,and a W of electrons L F2 é_€/er/¢l.c 9/4”! 04' ocK' w/E/ x’é ayf' fired: Z R) ,4“ acrgggjeflayfltfl A; 5 (z, 11. (a) in the Bohr model for the energy levets of the H atom, why is the sign of the energy negative? cw” 5744 arm?) XS. #4 +6, 6 “MM 50 q// Acton—Di x44; 9/ 9’4” eve- w/ Anya-I #54,. (p) (b) How does the energy spacing between energy levels AE vary with . . 9 - v ./ 2 increasmgn. if”) =_ gave/Va 6?]— 50 incmflvf'm my»; c/asrr A5 5/): My . @ i l l i it i l i it Y. i l . XV i 1 is i l i WWWWW”wam‘mvmuauxmrmw-wv-v-mam - v 52$ 1’67 @ (13%) ll. QUANTITATIVE: Do all four (4) of the following: (12 pts each) (/2) 1. Calculate the value of the solubility product ( Ksp ) for cadmium sulfide CdS, given the following reduction potentials: CdS + 2e‘ ca +32" 2° = -1.35v Col2+ +2e.’ m-» Cd 2° = -o.215v CdS+2e‘ ~> Cd+Sz' E" = ~1.35V Cd -+ Cd“ +2e- E” = 0.215V Cars—>th +52“ E" =—1.14V . 1 E56,, = 005” log K5,, —l.135 = 0 059 log KW 2(—l.l35) K =10 0-055” =10“3“' = 3.90x10~39 Sp ( 2) 2. The reaction and equilibrium constant for the H2 fuel cell at 25 °C (298 K) are: 2 H2 (9) + 02 (9) '~-—--—> 2 H20 (I) K = 2.50 x1075 (a) Calculate the standard cell voltage 8° and AG° at 298 K for this reaction. a 0.059 co” = 110g K = 0.0591 71 4 I l l log 2.50x10” =1.11V AG” = ~nFE;,, = -4(96,485)(1.11)= —4.30x102k1 (b) Predict the signs of AH° and AS° for this reaction. AS° = negative AH” = positive (c) How does the maximum amount of work obtainable from this fuei cell ‘ vary as the temperature decreases? (increases, decreases, remains the same) wmax increases as T decreases . WM'W'Mmm-awu‘lm 9 ‘ fig“ (22) 3. Consider the reaction: .AG°=0 5'21- K!7@ Fe203(s) + 3H2(g) '9 2Fe(s) + 3H20(g) Assuming that AH° and 138° do not depend on temperature, calculate the temperature at which K=1.00 for this reaction. AH° (kJ/mol) ,As" (JIKmol) F6203 ~733 75. H2 121 Fe 32 H20 -255 175 AH“ = nzpmms - nzmm = 3(«255kJ/mol) . (-733kJ/mol) = -32 kJ/mol AS° = nzpmdms - nzmm = [2(32 J/mol-K) + 3075 J/moLKn * [75 J/moloK +3(121J/mol'K)] AG° = AH° -TAS° T = AH” / AS° 32000 kJ «101% 151 J-K"-mol”l = -leK 4. Calculate the wavelength ’1‘» ( in meters) of a photon capable of exciting an electron from the ground state (n; = 1) to a final state n: = 4 of a one—dimensional box of length L = 30.0 pm (Note: pm = 10'12 m) 4”? 12],? AE = E u E, AE = V ’ 8(9.109x10'3'ng30x10"2m)2 8(9.I09x10“3'ng30x10"2m)2 15(6.626x10‘3‘J-s)2 [kg-m2 -s‘2 8(9.109x10’3'ng30x10_'2m)2 U he {6.62610-“J. [3. 8 l 2:“: x 3 0x10 m/s =1.98x10_wm E 1.00x10'w J: 1.00x10‘15J ...
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This note was uploaded on 04/07/2008 for the course CHEM 152 taught by Professor Chiu during the Spring '08 term at University of Washington.

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