Prelim_I_Solns - Chemistry 207 Cornell University Summer...

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Unformatted text preview: Chemistry 207 Cornell University Summer 2005 (6-week session) Preliminary Exam 1 Nmne AWCNKC ‘Ceq \J TA READ THESE INSTRUCTTIONS CAREFULLY 1. DO NOT OPEN THIS EXAM UNTIL YOU ARE TOLD TO BEGIN. 2. No graphing calculators are allowed. 3. You may use a 3“ x 5" notecard with any handwritten information on it you choose. 4. Entries my be made via pen or pencil. 5. Be sure to fill out the top of this sheet legibly. Do so now. 6. Place all answers in the spaces provided. Units and significant figures are important. 7. Partial credit will be awarded for written questions with answers which are substantially correct, but contain easily identified errors. Partial credit will not be awarded for isolated work that is not readable, or is not part of a reasonable progression toward the correct answer. 8. Where a problem has a numerical answer credit will be given only if steps indicating a logical progression toward the answer are stated or indicated clearly. Show all of your work in an organized manner. 9. Complete, logical sentences should be used when answering questions requiring comments or discussion. 10. A Periodic Table and listing of potentially useful constants is attached at the end of the exam. I I. There are 9 pages to this exam. Verify that your exam contains the correct number of pages now. 1. — (50 points) 2. _ (23 points) 3. (88 points) 4. (22 points) Total: —_ (183 points) Percentage: 1. Metallic titanium and its alloys (especially those with aluminum and vanadium) combine the advantages of high strength and light weight, making them particularly useful in applications where these attributes are desired and the cost of their use is not prohibitive. Examples include the aerospace industry, where titanium alloys are used in the manufacture of airplane bodies and engines; the high performance bicycle industry (both on- and off-road models) where titanium alloy bike frames are de rigueur; and as orthopedic implants, where its great strength makes it an effective hip or leg implant. The major natural sources of titanium are the ores rutile, which contains titanium(IV) oxide, and ilmenite (TiFeog). (a) (8 points) An intermediate in the preparation of elemental titanium from titanium(IV) oxide is a gaseous chloride of titanium that contains 25.24% titanium by mass. Determine the empirical formula of this compound. "15.143 Ti ( W°\ T'\> : 0.3211Md1i ‘-\'l.%%q_ \J 14.5 o [‘J U VEtL‘- ct > -_ Z.\o‘l Man C\ 25 437.1 a T‘ 05""2 “no? sci-r; 63311. :>\TKCM i (b) (10 points) At 136 °C and atmospheric pressure the density of gaseous titanium chloride is 5.6 g/L. Under these same conditions the density of an equivalent number of moles of nitrogen gas is 0.83 g/L. Determine the molecular formula of the titanium chloride of part (a). 0.3: a. i ,~ H2. (.349 - M2): 0.01% ml a L 28.06% 3 t. Z ‘ \ wo\ (AW?\£ Eula ____‘3 :cxw-pV. <__\;_. “2) (AM:- >= \qQ 3N,“ Tau“ MOM-r M61; : We 3(m°\ :_.\/ \Tgckk L (c) (6 points) The Kroll method of producing elemental titanium involves the treatment of ilmenite or rutile with molecular chlorine and coke (carbon). In the case of ilmenite the products of this reaction are the titanium chloride of part (a), iron(IlI) chloride, and carbon monoxide. Write a balanced equation for this reaction. amigo; + “I Cl? 4: Lo c -~~—»,2 2mm, s cm at 21cm (d) (7 points) What mass of chlorine is needed to produce 1.00 ton (907.18 kg) of the titanium chloride of part (a)? Wu.“ otO'Lva k3 Vic“ (\lwm <l__ mu 0 < 3 , ‘ Tack a 10 «asst \4 _ E9. booms lkum 1; an) 3 0;) —\\.\‘M~\o k3 at (e) (4 points) In the second step of the Kroll method the titanium chloride of part (a) is separated from the iron(III) chloride by fractional distillation. The titanium chloride is then reacted with liquid magnesium at 900 °C to give titanium and magnesium chloride. Write a balanced chemical equation for this step in the refining of titanium. .— ‘Txoq Jr 2M3 -——-----"> \x + ZMSUZ (i) (15 points) Suppose the reaction chamber for part (e) contained l50.00 kg of the titanium chloride and 50.0 kg of liquid magnesium. What maximum mass of titanium could result? y .\ . V5600 \43 Trad (—“—t T oq 133“ T > = 0.116% mm T; lact. lquE) \gc J \kwM “new c it.» \ 40.00 £3 M3 (”.3150 MW \\cv~m\ T : \.o‘(_%b \LMQ\ T; N «sown zkwa uc Txcu x; \mxl; W3. 0.13mi: \iwt T; ( thfifiskj T.” J amen, \LS Ti ~ \Awm l 2. Alcohol levels in blood can be determined by titration with potassium dichromate. The net ionic equation involves the reaction of alcohol (CZHSOH) with dichromate in acidic solution to yield carbon dioxide and chromium(lIl) ion. (a) (15 points) Balance the net ionic equation. 1‘ @31- Czusm-K J: c9531 ———v C92. + Cc / \ \ I \ /\ \ —7. +1 -7_ “O -1 rq ~2. *3 7 U rebuv‘r‘wvu Z (101’ 5, Hi“ -\~ Cverjz'q Lo- <\- 7010‘) w .......__._..__..‘...._ .... , ‘ , . . ”.1 _ .7. .. . .. .- .__...-. ...._._..._..-—-—_.—.— W 1’ '3 \ Uo W‘r + Cal-\SOl-l + ZCrZQ.‘ ——~—7 ace: 4: bled“ + \L\-K2_Q ( h. .....~-- . -. _ .. . _ , . .. ., .._.=..i._ .._....., (b) (8 points) What is the Blood Alcohol Content (BAC) in mass percent if 6.72 mL of 0.05102 M aqueous potassium dichromate is required for titration of an 11.46 g sample of blood? The maximum allowed BAC under New York state law is 0.08%. Is the individual whose BAC test results are described in this problem in violation of this law? unz “A < L > 0.0S\02Mo\ 2—) \MQ\C1\-L(>\—\ . s Mable? _ mm x. T 0:07 em... 2— S clusM-K , 7. “MA Cer-I \Mo\ 0. 0610\0 3 \\.'~l\o 3 gqmflg .______._..__. , M0. QM i®‘\VK©m& 82:8, NA \Jlo\o:\e we \auo. 3. The rare- gases are noted for their relative unreactivity. The qualifying term “relative” is deliberate. While the rare- gases form compounds with great reluctance, they do form stable compounds. The first compound involving a rare-gas atom was created by Neil Bartlett in l962. His discovery of the compound Xe+PtF6‘, a yellow-orange ionic solid, initiated a flurry of activity at research institutions around the world as scientists began the search for other rare- gas compounds. One of the preliminary calculations such researchers would have undertaken is the determination of the lattice energy and overall energy change which accompany the predicted formation of such a rare- gas ionic solid. The following questions ask you to perform such an analysis for the compound Xng. (a) (10 points) Complete the following table for the most abundant naturally occurring isotope of xenon, ”254Xe. (b) (6 points) Write reactions involving condensed electron configurations to describe the formation of Xe2+ and F from their elements. . + \(e ([Kr'lizzdg‘ogfb) —7 Ma ([K‘1591L\QJ\0 517%} ‘& Z2- F (NJZJZYS)4€ ——-—=v F’ (trke’lze'zzfi) (c) (6 points) What subshcll contains the highest energy electron(s) of F"? Draw a picture of one orbital within this subshell and state its name. \ 3 2? (d) (6 points) Draw a picture of an orbital that has a principal quantum number and an angular momentum quantum number of one less that of the orbital drawn in part (0). State the name of this orbital. \L (or he) (e) (15 points) Estimate the lattice energy of XeF2(s) from the known lattice energy of Cal-72. (Hints: Assume Xe2+ is proportional in size to Xe as Ca2+ is proportional in size to Ca.) C 2'l 2* 2+ .3: : £2. 2) )4“- = (“TM >\?_;O?M = 75 W“ cm i; R7 PM 023(4) Eh-l-Vtce, 'fi’f kc: Z _ g ‘7'. _____.‘ ,_‘___ Wigfl’“ =7 chm flaw: = C51?“ amt: E\o33ric2 Ear Cafa 7. (4.2)(4) MW + \33 (w E‘\O:\JV\CQ [‘0‘ Lip-L :\ 30:18 \LS/NO\\ C-———<__——-—-——- (i) (15 points) Calculate the overall energy change in forming XCF2(S) from its elements. F1910 #7 ZFrsy El = \28 \Jmot ie (:9 —-5 16(3) + e‘ E2 : \\‘to.'~\\«.jlwo\ as 16(3) —9 kg (5) + e’ E, : ZOKUQA \lem\ 1F (3) 'l' 2.6, —7 {F13} Eq ._ Z (.351?) \szm\\ 1. — laSlakt)(Mo\ Y\ 1" — 6 <3) 4' 2? (j) —? 12F1(33 E S —_ ~ Bose \tjnml Km?» 4’ Fzéy 7 KeFZCs) \ Ewe) : -37°\L:Smo\ l (g) (6 points) Based upon your calculations shouldllEe—lg; be a stable ionic solid? Justify your answer. \les, VerZ EAAQAQ \22 (k 35t0_\s\e \Mxtc. compoomg {AWLQ QWQSSb‘W (decilcg) when N: is 'waeg. (h) (6 points) Assuming the reaction would occur, what would “drive” the overall reaction leading to the formation of XeF2(s) from its elements? The majoeés‘ivc (Q We \(ullgicq QWLKSLD grim; Xrllxc fitmériom 0% ‘Le (:2 - (i) (6 points) Using the blanks below, rank the following in order of increasing size: Kr, Rn, and Xe. Kr < \Le < Rh (j) (6 points) Using the blanks below, rank the following in order of increasing first ionization energy: Kr, Rn, and Xe. R‘A < ‘ke < kg (k) (6 points) Using the blanks below, rank the following in order of increasing lattice energy: Ker, Ran, and Xer? Kw (:1 < \Lr F1. < \kecz 4. Spectroscopy is an invaluable tool to the modern chemist, allowing the most fundamental aspects of chemical reactions to be probed. Take the reaction below as an example: H2*(g) + Hews) —> 2H*(g) + He*(g) This reaction involves the collision of a gase-phase H; ion with a gas-phase He“. As a result of this collision an electron is transferred from H; to He“, forming He“ and causing H; to break apart into two H+ ions. Processes of this sort are of interest to chemists because of the insight they reveal into such fiindamental acts as electron transfer. Electron transfer is the most important process to take place in natural and artificial chemical systems, playing a fundamental role, for example, in photosynthesis as well as in photography. Of course, to learn something valuable about this process it is necessary to have a means of probing the products, reactants, or both. One means of probing the above reaction is spectroscopically. The energy levels of He+ ions are well described by the equation En = —2.18x10""J(f—:). (a) (8 points) The He+ in the reaction above can be detected by ionizing it (i.e., converting it to He“). An experiment of this sort was performed and found to require 91.2 nm light. In what energy state (what n value) was the He+ initially? lie ._ 55—, ~2..\8Y\\Q \83 (3: > i. >~ “E ““11 (umemdd J'SXF‘QOMOEWJ- — zixeyJO—‘g: <2: “ 3: ) atzmb’qwt —-—-~-~ W “‘6 \W'iil (b) (8 points) If l-le’r initially in the energy/”state ii determined in part (a) relaxes to its ground- state what wavelength of light does it emit? '1 2. ~. as , ~‘8 55—. anew) q E. _ E- : «Blue 3 \2 at 34 , a f“ (\1 \AC : (let‘oziofi\0 3‘5>(3‘OQ*\0 Nb) : \ZQN \NM. \ 55 c.3klk\o"8; ‘— (c) (6 points) On the energy level diagram below indicate the transitions occurring in parts (a) and (b) with arrows. [1:00 n=6 n=5 n=4 —I_— n=3 _—l—— l n=2 | LI Be 6. 941 9.0I213 3 4 37 38 39 Rb Sr Y Nb 85.9678 81.61 50.9059 92 9064 ‘ lanthanide series: *Act'midc mics: an: the mass numbers of the most stable isotopes. Possibl Useful Information: E for CaCl2 Elnttice for caFZ Ca2+ radius Ca radius F radius F radius Xe radius Bond dissociation energy of F2 Bond dissociation energy of C12 Heat of sublimation for Xe EIel for Xe Eiez for Xe Een for F Eca for C1 lattice 7 8 7B 25 l l 12 Na Mg 6 21 9390 24.1050 SB 43 SB 63 I9 20 21 ZVS Ca Tzzl Cr 39.0903 40070 «9559 47.00 50.94” sI. Bh lln (261) (265) 92 U 230. 029 Periodic Table of the Elements I4 3A 4A slA I3 I4 15 1; Si 26.9A31l5 28. 0355 30. :7” 31. 066 35C 4:27 39. :18 Kr 6Z.5 3.9 6:; 713 72. 59 74A 91l6 711. 96 NB 904 33. ED 44 46 43 513 Cd loRLM 02.906 [0642 mm mm mun lltl.7l0 IISI.75 I17.60 1261.905 IJXI.29 34 P 55 56 S7 72 C5 Ba ‘La Hf Os Pt In 905 137. 317 [33. 906 17349 [00:48 133w 85 136.207 I902 192 ''22 I93. 03 I96.u 967 200. 59 204 3'33 207'1 2 208 980 ( B8 89 [05 TAc ms) 116 025 227.020 ([36?) (262) (263) 60 64 Nd NEJZ 140.908 “4.24 15036 15L965 ISG7.25 Atomic weights are based on carbon-12. For certain radioactive elements the numbers listed (in parentheses) Mn 5493001147 sac. 93032 51: I07 ms 109 110 VIt Unit (269) (163) 2258 kJ/mol 2609 kJ/mol 114 pm 197 pm 133 pm 72 pm 130 pm 158 kJ/mo] 243 kJ/mol 10.66 kJ/mol 1170.4 kJ/mol 2046.4 kJ/mol —328 kJ/mol -348.6 kJ/mol 9 646 I7 7A 10 63.546 209) (2 l0) (2R2: [III II I272) [Eu 1) (.277! 66 I58. 9.115 I62 90 I6]: 930 167. 26 99 (232) (257m) 69 7] Lu I68. T934 WY)?“ 174 967 114m k Cf 97 B zstpw gm (C247) (:47) F175?) l3 8A 2 4 07260 to N0 20 I797 0 ...
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