Name: Grade: 1: 2: 3: 4: /100 /25 /25 /25 /25 Midterm Exam 2 CHEM 181: Introduction to Chemical Principles October 14, 2014 Directions: Do all four problems. Show all of your work neatly and clearly. Do not skip steps. Partial credit will be awarded for all problems. Correct answers will not receive credit if your work is not shown. If you are not sure exactly what a question means, ask! Not all problems are of equal difficulty, but all are worth the same fraction of the overall grade. 1 1. Ketene has the molecular formula CH2 CO, with the connectivity shown below: H C C O H (a) Provide a Lewis structure for ketene. Include major and minor resonance structures (clearly labeled) with all non-zero formal charges shown. 2 (b) What is the bond order for the C–C bond? Give a short (one sentence) explanation. (c) What is the bond order for the C–O bond? Give a short (one sentence) explanation. (d) Label your Lewis structure with the hybridization of each carbon atom. Give a short (one sentence) explanation. 3 2. Oxazole has the molecular formula C3 H3 NO; it is a cyclic (ring) molecule with the connectivity shown below: H O C C C H N H (a) Provide the best Lewis structure for oxazole. Include any equivalent resonance structures, and label all non-zero formal charges. (b) Provide the second-best Lewis structure for oxazole. Include any equivalent resonance structures, and label all non-zero formal charges. 4 (c) Provide the third-best Lewis structure for oxazole. Include any equivalent resonance structures, and label all non-zero formal charges. (d) Use the major and minor structures you gave in (a)–(c) and estimate bond orders for each of the five bonds that make up the ring. Exact values are not expected; you can either guess numbers or provide qualitative descriptions (i.e., “slightly less than 2,” or “between 2.5 and 3, but closer to 3”). Use the following labels to identify each bond: H O C1 C2 C3 H N H C1 –C2 C1 –O C3 –O C2 –N C3 –N Put down the same answer for both C–O bonds only if the bonds are identical; otherwise, be sure your answer shows which has the higher bond order, even if only slightly. (Do the same for the C–N bonds.) 5 (e) Use the same major and minor structures (and the same labels) to estimate the formal charge on each atom. Also give the hybridization at each atom. H O C1 C2 C3 H N H formal charge hybridization C1 C2 N C3 O Again, only assign two (or more) different atoms the same formal charge if the formal charges are identical; otherwise, be sure your answer shows which formal charge is higher, even if only slightly. 6 3. Draw 4 isomers that have the molecular formula CHNO. Do not use rings. (a) For each isomer, provide a Lewis structure with major and minor resonance structures and non-zero formal charges (if present). 7 (b) Label which of the four isomers you expect to be the most stable compound (“most” or “most stable”) and explain (1 sentence): (c) Label which of the four isomers you expect to be the second most stable compound (“second most” or “second”) and explain (1 sentence): (d) Label which of the four isomers you expect to be the third most stable compound (“third most” or “third”) and explain (1 sentence): (e) Label which of the four isomers you expect to be the least stable compound (“least” or “least stable”) and explain (1 sentence): 8 4. The diagram at the right shows the shapes and energies of the molecular orbitals for CO2 , which can be described as combinations of the C and O 2s and 2p orbitals. O C O Energy Do the following, using the larger diagram on the next page: (a) Label each orbital (b for bonding, a for antibonding, and nb for nonbonding). (b) Place the appropriate number of electrons into the molecular orbitals (draw them on top of the horizontal lines used for energy levels.) (c) What is the bond order for each C–O bond, according to MO theory? Suggestion: think about your above answers, in terms of comparing them to the CO2 Lewis structure. (d) On the same energy scale as the molecular orbitals, indicate (use horizontal lines) the energies of • the 2s and 2p orbitals for atomic carbon and • the 2s and 2p orbitals for atomic oxygen. Explain below how you determined these energies: 9 O C O Energy 10 Electronegativity: Atom F O Cl N Br I S C H χ 3.98 3.44 3.16 3.04 2.96 2.66 2.58 2.55 2.20 Equations: Bond order = 12 (# bonding electrons − # antibonding electrons). Formal charge = # valence electrons − # lone pair electrons + 12 # bonding electrons 11 23 24 25 26 27 28 29 30 7 8 9 39 Y 38 37 57 La 87.62 56 85.4678 55 (227) Actinium 138.9055 (261) Rutherfordium Rf 104 178.49 Hafnium Hf 72 91.224 Zirconium Zr 40 47.867 V Cr Mn 92 U 140.90765 91 Pa Protactinium 140.116 90 Th Thorium Nd 232.0381 231.03588 238.0289 Uranium 144.24 Neodymium Pr Praseodymium Ce Cerium 60 59 58 (262) Bohrium Bh 107 186.207 Rhenium Re 75 (98) Technetium Tc 43 54.938049 Manganese (263) Seaborgium Sg 106 183.84 Tungsten W 74 95.94 Molybdenum Mo 42 51.9961 Chromium (262) Dubnium Db 105 180.9479 Tantalum Ta 73 92.90638 Niobium Nb 41 50.9415 Vanadium 1995 IUPAC masses and Approved Names from http://www.chem.qmw.ac.uk/iupac/AtWt/ masses for 107-111 from C&EN, March 13, 1995, p. 35 112 from http://www.gsi.de/z112e.html (226) (223) Ac Ra Radium Fr 88 87 Francium 89 137.327 132.90545 Lanthanum Ba Barium Cs Cesium 88.90585 Yttrium Sr Strontium Rb Rubidium 44.955910 40.078 39.0983 Titanium Ti Sc Scandium Ca Calcium 22 21 K 20 19 Potassium 24.3050 22.989770 (237) Neptunium Np 93 (145) Promethium Pm 61 (265) Hassium Hs 108 190.23 Osmium Os 76 101.07 Ruthenium Ru 44 55.845 Iron Fe (244) Plutonium Pu 94 150.36 Samarium Sm 62 (266) Meitnerium Mt 109 192.217 Iridium Ir 77 102.90550 Rhodium Rh 45 58.933200 Cobalt Co (243) Americium Am 95 151.964 Europium Eu 63 (269) 110 195.078 Platinum Pt 78 106.42 Palladium Pd 46 58.6934 Nickel Ni (247) Curium Cm 96 157.25 Gadolinium Gd 64 (272) 111 196.96655 Gold Au 79 107.8682 Silver Ag 47 63.546 Copper Cu (247) Berkelium Bk 97 158.92534 Terbium Tb 65 (277) 112 200.59 Mercury Hg 80 112.411 Cadmium Cd 48 65.39 Zinc Zn Al Mg Magnesium Na 12 11 Sodium 13 9.012182 6.941 (251) Californium Cf 98 162.50 Dysprosium Dy 66 113 204.3833 Thallium Tl 81 114.818 Indium In 49 69.723 Gallium Ga 31 26.981538 Aluminum 10.811 Boron Be Beryllium Li Lithium B (252) Einsteinium Es 99 164.93032 Holmium Ho 67 114 207.2 Lead Pb 82 118.710 Tin Sn 50 72.61 Germanium Ge 32 28.0855 Silicon Si 14 12.0107 Carbon C (257) Fermium Fm 100 167.26 Erbium Er 68 208.98038 Bismuth Bi 83 121.760 Antimony Sb 51 74.92160 Arsenic As 33 30.973761 Phosphorus P 15 14.00674 Nitrogen N F Fluorine Neon Ne (258) Mendelevium Md 101 168.93421 Thulium Tm 69 (209) Polonium Po 84 127.60 Tellurium Te 52 78.96 Selenium Se 34 32.066 Sulfur S 16 (259) Nobelium No 102 173.04 Ytterbium Yb 70 (210) Astatine At 85 126.90447 Iodine I 53 79.904 Bromine Br 35 35.4527 Chlorine Cl 17 (262) Lawrencium Lr 103 174.967 Lutetium Lu 71 (222) Radon Rn 86 131.29 Xenon Xe 54 83.80 Krypton Kr 36 39.948 Argon Ar 18 15.9994 18.9984032 20.1797 Oxygen O 10 6 3 5 4.003 1.00794 4 He Helium H 2 Hydrogen 1 The Periodic Table of the Elements