H10- IMFs-solutions - galloway (jlg4528) H10: IMFs mccord...

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galloway (jlg4528) – H10: IMFs – mccord – (50960) 1 This print-out should have 25 questions. Multiple-choice questions may continue on the next column or page – fnd all choices be±ore answering. IMFs are intermolecular ±orces. Temporary dipoles are also called instanta- neous dipoles. Exam 3 Info: All students ±rom Dr. Mc- Cord’s MWF 11am class (50960) must report to the correct room ±or Exam 3 on Tuesday, November 1st (7-9PM). There are 2 di²er- ent rooms and the class is split according to the frst letter o± your last name. Go to the right room or you WILL receive a zero on the exams. The room assignments are: A-L go to UTC 2.112A M-Z go to UTC 2.102A 001 10.0 points What would be the most signifcant type o± intermolecular ±orces in a liquid sample o± ³uoro±orm (CHF 3 )? 1. dipole-dipole correct 2. covalent 3. hydrogen bonding 4. dispersion 5. ionic Explanation: London ±orces, dispersion ±orces, van der Waals or induced dipoles all describe the same intermolecular ±orce. London ±orces are in- duced, short-lived, and very weak. Molecules and atoms can experience London ±orces be- cause they have electron clouds. London ±orces result ±rom the distortion o± the elec- tron cloud o± an atom or molecule by the presence o± nearby atoms or molecules. Permanent dipole-dipole interactions are stronger than London ±orces and occur be- tween polar covalent molecules due to charge separation. H-bonds are a special case o± very strong dipole-dipole interactions. They only occur when H is bonded to small, highly electroneg- ative atoms – F, O or N only. Ion-ion interactions are the strongest due to extreme charge separation and occur between ionic molecules. They can be thought o± as both inter- and intramolecular bonding. CHF 3 is a polar molecule that does not con- tain H bonds; there±ore, dipole-dipole ±orces will be the most signifcant type o± intermolec- ular ±orces present. 002 10.0 points What is the predominant intermolecular ±orce between IBr molecules in liquid IBr? 1. ionic ±orces 2. dipole ±orces 3. hydrogen bonds 4. dispersion ±orces correct 5. covalent bonds Explanation: You might have thought that dipole ±orces was right. The ±act is that the dipole be- tween iodine and bromine is incredibly weak - too weak to account ±or the ±act that this compound is a liquid. Another FACT is that dispersion ±orces are the dominant ±orces here due to the SIZE o± these atoms and hence molecule. Just know that not ALL dipole ±orces are bigger than any set o± dispersion ±orces. Size matters, and dispersion ±orces win out here. 003 10.0 points Which o± the ±ollowing structures represents a possible hydrogen bond? 1. Cl H ··· Cl 2. C H ··· O 3. F H ··· F correct
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galloway (jlg4528) – H10: IMFs – mccord – (50960) 2 4. Br H ··· Br Explanation: H-bonds are a special case of very strong dipole-dipole interactions. They only occur when H is bonded to small, highly electroneg- ative atoms – F, O or N only. 004
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This note was uploaded on 11/14/2011 for the course CHEMISTRY 50960 taught by Professor Dr.mccord during the Fall '11 term at University of Texas at Austin.

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H10- IMFs-solutions - galloway (jlg4528) H10: IMFs mccord...

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