CHEM 251 (Fall-2005) Final Exam (100 pts)

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1 CHEM 251 (Fall-2005) Final Exam (100 pts) Name: , Student Clid # LAST NAME, First (Circle the alphabet segment of your LAST NAME): A-C D-H I-L M-S T-Z Please answer the following questions: Part I: Multiple Choices (30 pts: 3 pts each). Circle the ONE best answer: 1. Consider the following two half reactions: I 2 (aq) + 2 e 2 I - (aq) E = V Zn 2+ (aq) + 2 e Zn (s) E = V Which of the above four species is the best oxidizing agent? a) I 2 b) I - c) Zn 2+ d) Zn 2. The best explanation for the fact that nitrogen forms only NF 3 with fluorine while phosphorus forms PF 3, and PF 5 is that a) nitrogen is more electronegative than sulfur. b) nitrogen is smaller than phosphorus. c) phosphorus has d orbitals available for bonding d) only nitrogen forms hydrogen bonds 3. Which of the following statements is false? a) IF - 6 possesses a distorted octahedral structure. b) HClO is a weak acid in aqueous solution. c) ClO 2 is used to bleach flour and wood pulp and for water treatment. d) I - 3 possesses an angular structure. 4. Which of the following ions contribute greatly to the hardness of water? a) NO 3 - b) Ca 2+ c) Cu 2+ d) Al Consider the species Cl 2 SO, XeOF 4 and F 5 IO 2-, what is the hybridization of the central atom, respectively? a) sp 2, sp 3 d, and sp 3 d 2 b) sp 3 d, sp 3 d 2, sp 3 d 3 c) sp 3, sp 3 d 2, and sp 3 d 3 d) sp 3, sp 3 d, and sp 3 d 2 6. In the above problem, what is the molecular shape of the species, respectively? a) trigonal planar, trigonal bipyramid, and octahedral b) trigonal pyramid, trigonal bipyramid, and distorted octahedral c) T-shaped, square pyramid, and octahedral. d) trigonal pyramidal, square pyramid, and distorted octahedral. 1

2 7. What is the proper order of decreasing F-X-F angle in the species below? XeF 2 (A) BF 3 (B) ClF 2 + (C) ClF 3 (D) a) A > B > C > D b) D > C > B > A c) A > B > C > D d) B > C > A > D 8. Which one of the following statements is incorrect? a) H 2 S is a toxic gas. b) The most useful production of Mg involves the electrolysis of molten MgCl 2. c) The B-H-B bond in diborane, B 2 H 6, consists of four electrons shared among three atoms. d) The Ca 2+ salt of SiO 4 4- is the component of cement. 9. Which of the following ions are more likely to form insoluble salts with OH - ion? Mg 2+ Ag + Fe 3+ F - a) only Fe 3+ b) only Ag + c) Mg 2+ and Fe 3+ d) only F The unit cell of the rulite lattice is illustrated to the right. The compound is composed of titanium and oxygen. Based on the unit cell shown here (Ti ions are located at the corners, and in the body center of the cube and oxide ions are located as follows: two in opposite quadrants of the bottom face, two at directly above the first two in the top face and the final two ions are in the plane of with the body centered titanium ion). What is the formula of rulite? a) Ti 2 O 4 b) Ti 2 O 3 c) TiO 2 d) TiO Part II. Reactions & Chemistry of the Main Group elements (40 pts) 1. Reactions (10 pts: 2 pts each) Predict the composition of the product(s), its state and balance the reactions: a) Ga (s) + S 8 (s) Heat b) OF 2 (s) + H 2 O (l) Heat c) Ca (s) + P 4 (s) d) SO 2 (g) + H 2 O (l) e) PBr 3 (l) + H 2 O (l) Heat 2

3 2. (30 pts: 5 pts each): Answer ONLY 6 of the following: a) Explain why BF 3 is a weaker Lewis acid than BCl 3? b) Suggest an explanation for the fact that the mobilities of the alkaline metal ions in aqueous solution follow the sequence: Li + < Na + < K + < Rb + < Cs +. c) Briefly discuss the extraction of aluminum from its ores (for full credit, chemical reactions must be included in your answer). 3

4 d) Write a brief account of the bonding and reactivity of Borazine, N 3 B 3 H 6 which emphasizes the ways in which this compound is similar or dissimilar to benzene, C 6 H 6. e) Stability constants for the formation of [M(18-crown-6)] + complexes in acetone are given below. Comment critically on these data. M + : Li + Na + K + Rb + Cs + log K: f) Write a brief account of the chemistry of the xenon fluorides and their decomposition reactions by water. g) Write down, in order, the names and symbols of the elements in group 14. Classify them in terms of metallic, semi-metallic or non-metallic and then give the general notation showing the ground state electronic configuration of each element. 4

5 Part III: Bonding (15 pts: 5 pts each) a) The azide ion, N 3 - is linear with equal N-N bond distances whereas the N-N bond distances in HN 3 are different; the terminal N-N bond (113 pm) is shorter than the central N-N distance (124 pm). Explain this observation. b) Suggest a likely structure for the dimmer of BeCl 2, present in the vapor phase below 1020 K. What is the appropriate hybridization scheme for the Be center? When BeCl 2 dissolves in diethyl ether to form monomeric BeCl 2.2Et 2 O; suggest a structure for each of the two compounds and give a description of the bonding. c) Construct an MO diagram for the formation of O 2 ; show only the participation of the valence orbitals of the oxygen atoms. (b) Use the diagram to rationalize the following trend in O-O bond distances: O 2, 121 pm; O 2 +, 112 pm; O 2 -, 134 pm; O 2 2-, 149 pm. (c) Which of these species are paramagnetic? 5

6 Part III (20 pts) Show all work for full credit. Please express all answers with proper units and correct number of significant figures. 1. (7 pts) Given that E for: VO 2+ (aq) + 2 H + (aq) + e V 3+ (aq) + H 2 O (l) is V, calculate the reduction potential, E in a solution when ph = 2.2 and the ratio of the concentrations of [VO 2+ ]:[V 3+ ] = 1:2. Comment on this result. [H + ] = 6.31 x 10-3 E = V 2. (6 pts) Use the following data to a) estimate the bond dissociation enthalpy of ClF: D(F-F) = 158 kj/mol; D(Cl-Cl) = 242 kj/mol; χ P (F) = 4.0; χ P (Cl) = 3.2. b) the charge distribution in ClF if the dipole moment is 0.86 D and the bond length is 163 pm. Which atom carries a negative charge?.??? kj/mol

7 3. (7 pts) Construct Born-Haber cycle for MgO. Use the following thermochemical data to calculate the lattice energy of MgO: standard enthalpy of formation at 298 K ( f H ) of MgO = -602 kj/mol, a H (Mg, s) = 148 kj/mol, IE 1 (Mg,g) = 738 kj/mol, IE 2 (Mg,g) = 1451 kj/mol, D(O 2, g) = 498 kj/mol, EA H 1 (O, g) = -141 kj/mol, and EA H 2 (O, g) = 880 kj/mol kj/mol (3927) 7

8 PREFERENCE SHEET FOR CHEM 251 Final Exam Fall 2005 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 = 96,485 C/mol R = x 10-3 kj/mol 1 D = x Cm e = 1.60 x C 1pm = m Useful Equations: G = - RT ln K = - zfe D = χ P (F) χ P (Cl) Nernst Eqn: E = E - (RT/zF) ln [reduced form]/[oxidized form] µ = q x e x d H Li Na K Rb Cs Fr (223) 4 Be Mg Ca Sr Ba Ra Periodic Table of the Elements 21 Sc Y La Ac Ti Zr Hf Rf (261) 23 V Nb Ta Db (262) 24 Cr Mo W Sg (263) 25 Mn Tc (98) 75 Re Bh (262) 26 Fe Ru Os Hs (265) 27 Co Rh Ir Mt (266) 28 Ni Pd Pt Uun (269) 29 Cu Ag Au Uuu (272) 30 Zn Cd Hg Uub (277) 5 B Al Ga In Tl C Si Ge Sn Pb N P As Sb Bi O S Se Te Po (209) 9 F Cl Br I At (210) 2 He Ne Ar Kr Xe Rn (222) 58 Ce Th Pr Pa Nd U Pm (145) 93 Np Sm Pu (244) 63 Eu Am (243) 64 Gd Cm (247) 65 Tb Bk (247) 66 Dy Cf (251) 67 Ho Es (252) 68 Er Fm (257) 69 Tm Md (258) 70 Yb No (259) 71 Lu Lr (260) 8

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CHEM 251 (Fall-2003) Final Exam (100 pts)

CHEM 251 (Fall-2003) Final Exam (100 pts) CEM 251 (Fall-2003) Final Exam (100 pts) Name: -------------------------------------------------------------------------------, SSN -------------------------------- LAST NAME, First (Circle the alphabet