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Finalexamsolutions - Chemistry 125 December 17, 2008 First...

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Chemistry 125 First Semester Final Examination Answer Key December 17, 2008 1. (18 minutes) Give as specific an example as you can for each of 4 of the following 6 items. Make your answers as brief and clear as possible, and try to make them specific - e.g. real molecules, real numbers, real experiments. a) Pre-19 th Century measurement of a submicron distance. b) A type of isomerism that was predicted long before it was observed. c) Evidence that DNA exists as a double helix with major and minor grooves. d) Evidence for stabilization due to "resonance" e) Early 19th Century observations supporting the radical theory in organic chemistry. f) Determination of the heat of formation of an atom. The following brief summaries give relevant Powerpoint/Wiki links that provide specific ”real molecules, real numbers, real experiments” that make satisfactory answers really good: a) Either Franklin s oil-on-pond experiment (lect 4 frame 29 - Franklin's_Experiment ) or Newton s rings (lect 4 frame 23 - Newton's_Rings ) b) The best example is the van t Hoff / Kohler chiral allenes (lect 26 frames 8-9 - Allene_Chirality ). Paternó s rotational isomers or Sachse s chair are less clear-cut examples. Many answers misunderstood and used examples where observation of isomerism ( e.g. of tartaric acid) preceded the theoretical explanation. c) The unusual intensity distribution of the “reflections” along the branches of Rosalind Frankland s “X” pattern (lect 6 frames 13-14 - Offset_Double_Helix ). d) The best examples are the special stability or the rotational barrier of amide vs . carbonyl-amine (lect 18 frame 4 - Amide_Group_Resonance ) or the unusual acidity of carboxylic acids due to unusual stability of the carboxylate anion (lect 18 frame 7 - Acidity_of_Carboxylic_Acids ). Note that other unusual properties attributable to resonance or HOMO-LUMO mixing ( unusual bond lengths, and even, arguably, reduced reactivity) do not necessarily relate to thermodynamic stabilization. e) Liebig and Wöhler s benzoyl radical (Lect 22 frame 4 - Benzaldehyde_and_Benzoyl ). Note the specification of “organic chemistry”. f) Either spectroscopic determination of the energetic “threshold” for formation of H from H 2 or of O from O 2 (lect 35 frame 27 - Atom_Energy_from_Spectroscopy ) or the Chupka-Inghram oven experiment for C from graphite (lect 36 frames 5-6 - Heat_of_Formation_of_Carbon_Atom ). A number of answers failed to discuss heat of atomization (substituting heat of formation of a molecule or heat of combustion of an element).
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Chem 125 First Semester Final Exam Answer Key 12/17/08 Page 2 2. (3 min) Draw an accurate figure showing the conformation of chair cyclohexane . [Axial bonds should be parallel to the three-
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This note was uploaded on 01/30/2011 for the course CHEM 2OA3 taught by Professor Stover during the Spring '10 term at McMaster University.

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Finalexamsolutions - Chemistry 125 December 17, 2008 First...

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