Name: 1: /33 Grade: /100 2: /33 3: /33 +1 point. Midterm Exam 2. CHEM 181: Introduction to Chemical Principles October 13, 2015

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1 Name: 1: /33 Grade: /100 2: /33 3: /33 +1 point Directions: Do all three problems. Midterm Exam 2 CHEM 181: Introduction to Chemical Principles October 13, 2015 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

2 1. (Draw Lewis structures, label major and minor resonance structures, show formal charges, and determine hybridization. See problem 1 from exam 2 (2014), 1 from exam 1 (2013) and problem 1 from exam 1 (2012).) Draw Lewis structures for the isomers with the molecular formula XYZ that are shown on the following pages. For each isomer shown, do the following: Label non-zero formal charges. Include all resonance structures that follow the octet rule. Label each resonance structure as either major, minor, very minor, or not valid (i.e., should not be considered). Rank the resonance structures in numerical order (1=best, 2=next best, etc.) If two structures are exactly equivalent, give them the same number. (a) below: is an isomer of XYZ, with the connectivity shown Provide a Lewis structure for. 2

3 (b) below: is an isomer of XYZ, with the connectivity shown Provide a Lewis structure for. 3

4 (c) below: is an isomer of XYZ, with the connectivity shown Provide a Lewis structure for. 4

5 2. Here is the best single electron-dot structure for the molecule XYZ: Consider the following proposed (but not necessarily correct) resonance structures, labeled A through X. Categorize each one according to the following. (You will not receive credit for categorizing a structure if you do not provide an explanation.) (a) Which structures (if any) must be included as resonance structures because they tell you something new about the molecule? Identify each structure by its label, and explain (one sentence each) what is important about its inclusion (i.e., its effect on bond order, formal charge, and/or hybridization). 5

6 (b) Which structures (if any) could be included, but are very minor and their inclusion would not change bond order, formal charge, or hybridization in any significant way. Identify each structure by its label, and explain (one sentence each) what makes the structure unimportant. (c) Which structures (if any), are bad structures that can not be included. Identify each structure by its label, and explain (once sentence each) the problem or problems with it. 6

7 3. (Identical to problem 3 from exam 2 (2014), but with a different formula.) Draw N isomers that have the molecular formula XYZ. 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

8 (b) Label which of the N isomers you expect to be the most stable compound ( most or most stable ) and explain (1 sentence): (c) Label which of the N isomers you expect to be the second most stable compound ( second most or second ) and explain (1 sentence): (d) Label which of the N isomers you expect to be the third most stable compound ( third most or third ) and explain (1 sentence): (e) Label which of the N isomers you expect to be the least stable compound ( least or least stable ) and explain (1 sentence): 8

9 Electronegativity: Atom χ F 3.98 O 3.44 Cl 3.16 N 3.04 Br 2.96 I 2.66 S 2.58 C 2.55 H 2.20 Equations: Bond order = 1 (# bonding electrons # antibonding electrons). 2 Formal charge = # valence electrons ( # lone pair electrons + 1 # bonding electrons) 2 9

10 The Periodic Table of the Elements 1 H Hydrogen He Helium Li Lithium Be Beryllium B Boron C Carbon N Nitrogen O Oxygen F Fluorine Ne Neon Na Sodium Mg Magnesium Al Aluminum Si Silicon P Phosphorus S Sulfur Cl Chlorine Ar Argon K Potassium Ca Calcium Sc Scandium Ti Titanium V Vanadium Cr Chromium Mn Manganese Fe Iron Co Cobalt Ni Nickel Cu Copper Zn Zinc Ga Gallium Ge Germanium As Arsenic Se Selenium Br Bromine Kr Krypton Rb Rubidium Sr Strontium Y Yttrium Zr Zirconium Nb Niobium Mo Molybdenum Tc Technetium (98) 44 Ru Ruthenium Rh Rhodium Pd Palladium Ag Silver Cd Cadmium In Indium Sn Tin Sb Antimony Te Tellurium I Iodine Xe Xenon Cs Cesium Ba Barium La Lanthanum Hf Hafnium Ta Tantalum W Tungsten Re Rhenium Os Osmium Ir Iridium Pt Platinum Au Gold Hg Mercury Tl Thallium Pb Lead Bi Bismuth Po Polonium (209) 85 At Astatine (210) 86 Rn Radon (222) 87 Fr Francium (223) 88 Ra Radium (226) 89 Ac Actinium (227) 104 Rf Rutherfordium (261) 105 Db Dubnium (262) 106 Sg Seaborgium (263) 107 Bh Bohrium (262) 108 Hs Hassium (265) 109 Mt Meitnerium (266) 110 (269) 111 (272) 112 (277) Ce Cerium Pr Praseodymium Nd Neodymium Pm Promethium (145) 62 Sm Samarium Eu Europium Gd Gadolinium Tb Terbium Dy Dysprosium Ho Holmium Er Erbium Tm Thulium Yb Ytterbium Lu Lutetium Th Thorium Pa Protactinium U Uranium Np Neptunium (237) 94 Pu Plutonium (244) 95 Am Americium (243) 96 Cm Curium (247) 97 Bk Berkelium (247) 98 Cf Californium (251) 99 Es Einsteinium (252) 100 Fm Fermium (257) 101 Md Mendelevium (258) 102 No Nobelium (259) 103 Lr Lawrencium (262) 1995 IUPAC masses and Approved Names from masses for from C&EN, March 13, 1995, p from