CEM 251 (Fall-2003) Final Exam (100 pts) Name: -------------------------------------------------------------------------------, SSN -------------------------------- LAST NAME, First (Circle the alphabet segment of your LAST NAME): A-C D- I-L M-S T-Z Please answer the following questions: Part I: Multiple Choices (45 pts: 15 @ 3 pts each). Circle the ONE best answer: 1. Which group displays all three states of matter at room temperature? a) 14 b) 15 c) 16 d) 17 2. Which of the following elements can exist as an allotrope? a) b) F c) e d) P 3. The best explanation for the fact that oxygen forms only OF 2 with fluorine while sulfur forms SF 2, SF 4, and SF 6 is that a) Oxygen is more electronegative than sulfur. b) Oxygen is smaller than sulfur. c) Sulfur has d orbitals available for bonding d) Only oxygen forms hydrogen bonds 4. Which group contains elements which would display both +2 and +4 oxidation states due to the inert pair effect? a) 2 b) 14 c) 16 d) 18 5. Choose the element with the greatest first ionization energy. a) Xe b) e c) N d) O 6. Which one of the following statements is incorrect? a) Al is the most abundant metal by mass in the Earth s crust. b) The most useful production of Al involves the electrolysis of molten Al 2 Cl 6. c) The B--B bond in diborane, B 2 6, consists of two electrons shared among three atoms. d) Boric acid, B(O) 3, accepts a hydroxide ion, forming the complex ion [B(O) 4 ] -. 7. In the electrolysis of molten NaCl, what are the products at the anode and the cathode, respectively? a) Cl 2 (g), Na (l) b) Cl -, Na + c) Na (l), Cl 2 (g) d) Cl (g), Na (l) 1
8. Using Nernst equation, what is the E cell, at 25ºC for the reaction: Zn (s) + Cu 2+ (aq) Zn 2+ (aq) + Cu (s) Given that Eº cell = +1.100 V, [Cu 2+ ] = 1.0 M and [Zn 2+ ] = 0.10 M a) 1.13 V b) 1.16 V c) 1.10 V d) 1.07 V 9. What is the standard cell potential, E, for the following half-reaction? Pb 2+ (aq) + 2 e Pb (s) Given that Cu (s) + Pb 2+ (aq) Cu 2+ (aq) + Pb (s) E = +0.650 V and Cu 2+ (aq) + 2 e Cu (s) E = +0.337 V a) + 0.987 V b) -0.987 V c) +0.313 V d) -0.313 V 10. Arrange the following in the expected order of decreasing melting point (highest melting point first, etc.) KBr (A) MgCl 2 (B) MgBr 2 (C) NaCl (D) a) A > B > C > D b) C > D > A > B c) B > C > D > A d) C > B > D > A 11. What is the proper order of decreasing F-X-F angle in the species below? XeF 2 (A) BF 3 (B) ClF 2 + (C) XeF 4 (D) a) A > B > C > D b) D > C > B > A c) B > C > A > D d) C > B > D > A 12. Which of the following species has a distorted octahedral geometry? a) ClF 6 + b) SF 6 c) PF 6 - d) XeF 6 13. Consider the species ClF 2 -, XeOF 4 and IF 5 2-, what is the hybridization of the central atom, respectively? a) sp, sp 3 d, and sp 3 d b) sp 3 d, sp 3 d 2, sp 3 d 3 c) sp, sp 3 d, and sp 3 d 2 d) sp 3, sp 3 d, and sp 3 d 14. In the above problem, what is the molecular shape of the species, respectively? a) linear, square pyramidal, and pentagonal planar b) trigonal bipyramidal, octahedral, pentagonal bipyramidal c) bent, trigonal bipyramidal, square pyramidal d) linear, trigonal bipyramidal, distorted octahedral 2
15. The unit cell of the nickel arsenide lattice is illustrated to the right. The compound is composed of nickel and arsenic. Based on the unit cell shown here (Ni atoms are located inside the cube and As atoms are located at the corners, and at the half-way of the cube edges). What is the formula of nickel arsenide? a) NiAs b) Ni 3 As 2 c) NiAs 6 d) NiAs 2 Part II. Reactions & Reactivity (35 pts) 1. Reactions (10 pts: 5 @ 2 pts each) Predict the composition of the product(s), its state and balance the reactions: a) XeF 4 (g) + SF 4 (g) eat b) NO 2 (g) + 2 O (l) c) Ca (s) + P 4 (s) eat d) OF 2 (s) + 2 O (l) e) + N B - + N Cl (aq) - B + N B- 2. Reactivity (25 pts: 5 @ 5 pts each): Answer ONLY 5 of the following: a) Explain why BF 3 is a weaker acid than BCl 3? 3
b) Describe the preparation, uses, and structures of silicon polymers. c) In BF 3, the B-F bond length is 130 pm, but in BF 4 - it is approximately 145 pm. Explain this difference in the B-F bond lengths. 4
d) Briefly discuss the extraction of magnesium from its ores (for full credit, chemical reactions must be included in your answer). e) Compare and contrast (similarities, differences & structures) between ice and 2 O (l) 5
f) Using MOT, Explain why the bond the bond length in NO (115 pm) is longer than that in nitrosonium ion, NO + (106 pm). Also, explain the magnetic properties of the two species. Part III (21 pts) Show all work for full credit. Please express all answers with proper units and correct number of significant figures. 1. (8 pts) Construct Born-aber cycle for NaCl 2, calculate the standard enthalpy of formation at 298 K ( f ) of NaCl 2, and comment on the factors which make it an unstable compound. Use the following thermochemical data: a (Na, s) = 107 kj/mol, IE 1 (Na,g) = 502 kj/mol, IE 2 (Na,g) = 4569 kj/mol, D(Cl 2, g) = 242 kj/mol, EA (Cl, g) = -360 kj/mol, the calculated lattice energy for NaCl 2 is -2250 kj/mol. + 2450 kj/mol 6
2. (7 pts) Construct a potential diagram for the reduction of aqueous solution of Tl 3+ to Tl +, followed by reduction to Tl. Values of E for the Tl 3+ /Tl + and Tl + /Tl couples are +1.25 and -0.34 V, respectively. Calculate E for the T 3+ /Tl couple and comment on the thermodynamic viability of this process. +0.72 V 3. (6 pts) Use the following data to estimate the bond dissociation enthalpy of BrCl: D(Br-Br) = 224 kj/mol; D(Cl-Cl) = 242 kj/mol; χ P (Br) = 3.0; χ P (Cl) = 3.2. Compared to BrF, which one do you think would have a higher bond dissociation enthalpy and why? 237 kj/mol 7
PREFERENCE SEET FOR CEM 251 Final Exam Fall 2003 You will have 140 minutes to complete this exam. The exam has 7 pages plus Periodic Table and Reference page. When you are told to do so, tear off the Periodic Table cover sheet and use as required during exam. Useful Information: 1.0 ev 96.5 kj/mol & J = CV F = 96,500 C/mol R = 8.314 x 10-3 kj/mol Useful Equations: G = - RT ln K = - zfe Nernst Eqn: E = E - (RT/zF) ln [reduced form]/[oxidized form] D = χ P (Cl) χ P (Br) 1 2 3 4 5 6 7 1 1.01 3 Li 6.94 11 Na 22.99 19 K 39.1 37 Rb 85.47 55 Cs 132.9 87 Fr (223) 4 Be 9.01 12 Mg 24.30 20 Ca 40.08 38 Sr 87.62 56 Ba 137.3 88 Ra 226.0 Periodic Table of the Elements 21 Sc 44.96 39 Y 88.91 57 La 138.9 89 Ac 227.0 22 Ti 47.88 40 Zr 91.22 72 f 178.5 104 Rf (261) 23 V 50.94 41 Nb 92.91 73 Ta 181.0 105 Db (262) 24 Cr 52.00 42 Mo 95.94 74 W 183.8 106 Sg (263) 25 Mn 54.94 43 Tc (98) 75 Re 186.2 107 Bh (262) 26 Fe 55.85 44 Ru 101.1 76 Os 190.2 108 s (265) 27 Co 58.93 45 Rh 102.9 77 Ir 192.2 109 Mt (266) 28 Ni 58.69 46 Pd 106.4 78 Pt 195.1 110 Uun (269) 29 Cu 63.55 47 Ag 107.9 79 Au 197.0 111 Uuu (272) 30 Zn 65.38 48 Cd 112.4 80 g 200.6 112 Uub (277) 5 B 10.81 13 Al 26.98 31 Ga 69.72 49 In 114.8 81 Tl 204.4 6 C 12.01 14 Si 28.08 32 Ge 72.59 50 Sn 118.7 82 Pb 207.2 7 N 14.01 15 P 30.97 33 As 74.92 51 Sb 121.8 83 Bi 209.0 8 O 16.00 16 S 32.06 34 Se 78.96 52 Te 127.6 84 Po (209) 9 F 19.00 17 Cl 35.45 35 Br 79.90 53 I 126.9 85 At (210) 2 e 4.00 10 Ne 20.18 18 Ar 39.95 36 Kr 83.80 54 Xe 131.1 86 Rn (222) 58 Ce 140.1 90 Th 232.0 59 Pr 140.9 91 Pa 231.0 60 Nd 144.2 92 U 238.0 61 Pm (145) 93 Np 237.0 62 Sm 150.4 94 Pu (244) 63 Eu 152.0 95 Am (243) 64 Gd 157.2 96 Cm (247) 65 Tb 158.9 97 Bk (247) 66 Dy 162.5 98 Cf (251) 67 o 164.9 99 Es (252) 68 Er 167.3 100 Fm (257) 69 Tm 168.9 101 Md (258) 70 Yb 173.0 102 No (259) 71 Lu 175.0 103 Lr (260) 8