exam3answers - Last Name: First Name: Circle which section...

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Last Name: First Name: Circle which section you are enrolled in : 8:00 section 9:00 section CH 310M/318M Fall 2004 Dr. Brian M. Bocknack Midterm Exam #3 Thursday, November 18, 2004 7:00–10:00 p.m. Do not open this exam booklet until you are told to do so! This exam is closed book, meaning no books, notes, calculators, or molecular models are allowed. PLEASE WRITE YOUR NAME AT THE TOP OF EACH PAGE!!! Exams written in pencil are not eligible for regrades. Answers written in pencil with ink overlay will not be graded. If you use a pen, only blue or black ink is acceptable. Be prepared to show your valid UT ID card to a proctor when you turn in your exam. Page Page Page 2 (20) 7 (18) 12 (20) 3 (20) 8 (24) 13 (20) 4 (20) 9 (18) 14 (25) 5 (20) 10 (10) 15 (25) 6 (40) 11 (20) Total (300) Including this cover page, 4 blank pages (scratch paper), and the last page, which contains a periodic table, this exam contains 20 pages. DO ALL QUESTIONS BEST OF LUCK!!!
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Last Name: First Name: 2 1. (Problem worth 40 points) Consider reactions A and B shown below. Assume that both reactions follow the S N 2 reaction mechanism. H 3 CC l +O H H 3 CO H +C l H 3 CB r H H 3 H +B r A : B : (a) Based on relative leaving group ability, which reaction, A or B, would you expect to be faster? Circle your choice below, and then explain why the leaving group you chose is the better leaving group. A B Explanation : Br is a better leaving group than Cl , because as a weaker base, it is better able to stabilize a negative charge. We have discussed in lecture on several occasions how bond dissociation energies can be used to estimate H, the enthalpy change, associated with a chemical reaction. Additionally, several of the assigned textbook problems in Chapter 7 required you to practice this skill. The bond dissociation energies that are relevant for reactions A and B are listed below: Bond Bond Energy (kcal/mol) H 3 C–Cl 85 H 3 C–Br 70 H 3 C–OH 92 (b) Use bond dissociation energies to estimate H for reactions A and B. Yes, this requires you to do some math, but simple arithmetic should not require the use of a calculator! Make sure you indicate clearly the algebraic sign (positive or negative) of your answers! H Σ Bond energies(bonds broken) – Σ Bond energies(bonds formed) H for Reaction A : H = (85 – 92) kcal/mol = –7 kcal/mol H for Reaction B: H = (70 – 92) kcal/mol = –22 kcal/mol Page Total
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Last Name: First Name: 3 (c) Draw a reaction energy diagram for each reaction ( A and B ), using the template provided below to guide you. You should superimpose both curves on the same diagram, being sure to indicate clearly which curve corresponds to reaction A , and which curve corresponds to reaction B . Since we are interested in H, assume that the reactants in
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exam3answers - Last Name: First Name: Circle which section...

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