NAME: FIRST EXAMINATION - PDF Free Download

1 Chemistry 64 Winter 1994 NAME: FIRST EXAMINATION THIS EXAMINATION IS WORTH 100 POINTS AND CONTAINS 4 (FOUR) QUESTIONS THEY ARE NOT EQUALLY WEIGHTED! YOU SHOULD ATTEMPT ALL QUESTIONS AND ALLOCATE YOUR TIME ACCORDINGLY ALL BOOKS AND PAPERS OTHER THAN TABLES WHICH HAVE BEEN HANDED OUT SHOULD BE PLACED ON THE FLOOR DURING THIS EXAMINATION IF YOU DO WORK ANYWHERE OTHER THAN THE SPACE PROVIDED FOR EACH QUESTION, INDICATE CLEARLY WHERE IT IS LOCATED

2 FOR GRADING USE ONLY Question 1 (36 pts)... Question 2 (18 pts)... Question 3 (16 pts)... Question 4 (30 pts)... TOTAL (100 pts)...===========

3 Question 1 (36 pts) a)(10 pts) State the point group to which the molecule belongs. In some cases you will need to draw the ideal structure, based on VSEPR predictions. (IN EACH CASE THE CENTRAL ATOM IS LISTED FIRST. ALL OTHER ATOMS ARE BONDED DIRECTLY TO THE CENTRAL ATOM.) TeF 5 ICl 4 Cl H C C H Cl IO 2 F 2 IF 7 (pentagonal bipyramid) ICl 2 +

4 b) (6 pts) i. Heating a mixture of B 2 S 3 and sulfur to 570 K in a sealed tube under vacuum gives crystals of the planar molecule B 8 S 16, whose structure is shown below. To which point group does this molecule belong? ii. The compound N 3 S 3 Cl 3 (yellow needles) can be made from S 4 N 4 and Cl 2. Its structure is shown below and features a planar 6-membered ring with equal S-N distances; the S-Cl bonds are also equivalent. To which point group does this molecule belong? iii. Oxidation of Te with AsF 5 in AsF 3 solvent yields brown crystals containing the cation [Te 6 ] 4+, whose trigonal prismatic structure is shown below. To which point group does this molecule belong? = Te

5 f) (6 pts) i. Write the symbols for the following elements: Rhenium, Tungsten, Antimony, Astatine ii. [X] 3+ has the electron configuration [Xe]. Which element is X? iii. Give the symbol for one element which is a liquid at room temperature. iv. Write the name of the element whose atomic number is one less than that of tin. v. Give the name of the element which is the least electronegative halogen. g) (2 pts) Define the terms asymmetric and dissymmetric.

6 (h) (6 pts) Chiral molecules can occur in only a few point groups. List 3 of these groups, and sketch an example of a molecule for each group. DO NOT use C as the central atom. (i) (6 pts) Molecules having dipole moments can occur in only a few point groups. List 3 of these point groups, and sketch an example of a molecule for each group.

7 Question 2 (18 pts) a) (3 pts) Predict whether the Xe-F bond length in XeF 4 will be longer, shorter, or the same length as the I-F one in IF 4. Explain your answer. b) (3 pts) Explain why the first ionization energy of C is less than that of N. c) (3 pts) Predict the relative sizes of the following atoms and ions: N 3, Na +, Ne, O 2. Explain your answer. d) (3 pts) Predict the structure of H 2 F +. Explain your answer. e) (3 pts) Explain why ethylene is a common molecule, but the analogous H 2 Si=SiH 2 cannot be isolated.

8 f) (3 pts) Draw a cartoon picture showing a π-bonding interaction between a p orbital and a d-orbital. (Your choice of orbitals, but say which ones you use). Be sure to label the axes and indicate the nodes. Question 3 (16 pts) An MO energy level diagram for CO is shown below. Use it to answer the following questions, assuming the diagram is also applicable to other heteronuclear diatomic molecules. 4σ 2p 2π 3σ 2p 2s 1π 2σ 2s 1σ C CO O

9 a) (4 pts) Make a cartoon sketch of the HOMO of CO. c) (4 pts) The first ionization energy of CO is less than that of the isoelectronic N 2. Rationalize this experimental observation. [Hint, see question 2, part b] d) (4 pts) Explain why the bond dissociation energy in CF is less than that in CF +. e) (4 pts) Explain why ionization of NO to NO + occurs readily.

10 Question 4 (30 pts) A molecular orbital energy level scheme for homonuclear diatomic molecules is given below. 1σ g < 2σ u < 1π u < 3σ g < 2π g < 4σ u Use this information to rationalize the following experimental observations: a) (3 pts) The bond in Cl 2 is longer and weaker than the bond in Cl 2 +. [Assume that F 2 is a good model for Cl 2.] b) (3 pts) The electron affinity of O 2 is much smaller than that of C 2. c) (3 pts) The molecule Be 2 has not been observed experimentally. d) (3 pts) The internuclear distance in O 2 decreases when an electron is removed, but the internuclear distance in N 2 increases when an electron is removed.

11 A correlation diagram for AH 3 molecules (planar or pyramidal) is shown below. H H A H AH 3 Planar/Pyramidal Correlation Diagram z H A H H x 2e'(*) 2e(*) 3a 1 (*) 2a 1 '(*) 1a 2 '' 2a 1 1e 1e' 1a 1 ' 1a 1

12 (e) (3 pts) Using this diagram and Walsh's rules, predict the structure of the first excited state of NH 3. (f) (3 pts) Discuss the relevance of your answer in part (e) to the dynamic process occurring in ammonia. (g) (3 pts) Predict the structure of CH 3 and make a cartoon sketch of the HOMO of this molecule. Is your description consistent with the behavior of this anion in organic reactions? Explain your answer. (h) (4 pts) Predict the structure of LiH 3.

(i) (5 pts) Another correlation diagram for AH 3 molecules is shown below. It includes another possible geometry, T-shaped (C 2v ). 13

Using this diagram and Walsh's rules, predict which AH 3 molecules will adopt the C 2v geometry. [Suppose A can only be one of the first row elements from Li to Ne]. 14

15 DETACHABLE, HANDY DANDY PERIODIC TABLE OF THE ELEMENTS 1 H 1.0079 Li 3 6.941 11 Na 22.9898 19 K 39.0983 37 Rb 85.4678 55 Cs 132.905 Fr 87 (223) Be 4 9.01218 12 Mg 24.305 20 Ca 40.08 38 Sr 87.62 56 Ba 137.33 88 Ra 226.025 21 Sc 44.9559 Y 39 88.9059 57 La 138.906 89 Ac 227.028 Ti 22 47.88 40 Zr 91.224 72 Hf 178.49 104 Unq (261) V 23 50.9415 Nb 41 92.9064 Ta (262) 73 180.948 105 Unp Cr 24 51.996 95.94 42 Mo W 74 183.85 106 Unh (263) 25 Mn 54.9380 Tc (98) Re 43 75 186.207 107 Uns (262) Fe 26 55.847 Ru 44 101.07 Os 190.2 76 108 Uno (265) 27 Co 58.9332 45 Rh 102.906 Ir 77 192.22 109 Une (266) 28 Ni 58.69 Pd 46 106.42 Pt 78 195.08 Cu 29 63.546 Ag 47 107.868 Au 79 196.967 Zn 65.39 Cd 30 48 112.41 Hg 80 200.59 B 5 10.81 13 Al 26.9815 31 Ga 69.72 In 49 114.82 Tl 81 204.383 C 6 N 7 12.011 14.0067 14 15 Si P 28.0855 30.9738 32 33 Ge As 72.59 74.9216 Sn 50 51 Sb 118.71 121.75 Pb 82 83 Bi 207.2 208.980 O 32.06 34 Se 78.96 Te 52 127.60 Po 84 (209) 8 15.9994 16 S F 9 2 He 4.00260 10 Ne 18.9984 20.179 17 18 Cl Ar 35.453 39.948 35 36 Br 79.904 I 53 126.905 At (210) 85 Kr 83.80 54 Xe 131.29 86 Rn (222) 58 59 60 61 62 63 64 65 66 67 68 69 70 71 Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho 140.12 140.908 144.24 (145) 150.36 151.96 157.25 158.925 162.50 164.930 167.26 168.934 173.04 174.967 90 91 92 93 94 95 96 97 98 99 100 101 102 103 Th Pa U Np Pu Am Cm Bk Cf Es Er Tm Yb Lu Fm Md No Lr 232.038 231.036 238.029 (237) (244) (243) (247) (247) (251) (252) (257) (258) (259) (260)