Chapter11_LEC - Chapter 11 Liquids Solids and Intermolecular Forces Phase Changes Why are molecules attracted to each other Tro Chemistry A

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Chapter 11 Liquids, Solids, and Intermolecular Forces
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Tro, Chemistry: A Molecular Approach 2 Phase Changes Why are molecules attracted to each other?
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Tro, Chemistry: A Molecular Approach 3 Why are molecules attracted to each other? due to attractive forces between opposite charges + ion to – ion ;; + end to - end of polar molecule H-bonding especially strong even non-polar molecules will have temporary charges 2200 the charge density = attraction longer the distance = weaker attraction attractive forces < bonding forces between atoms
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Tro, Chemistry: A Molecular Approach 4 Trends in the Strength of Intermolecular Attraction? 2200 attractions between the atoms or molecules, the energy to separate them boiling a liquid - add energy to overcome the attractions between the molecules or atoms 2200 the intermolecular attractive forces, normal boiling point of the liquid
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Tro, Chemistry: A Molecular Approach 5 Attractive Forces + - + - + - + - + + + + _ _ _ _ + + + + + + + - - - - - - - + + + + + - - - - - Polar mol Non polar mol
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Tro, Chemistry: A Molecular Approach 6 Dispersion Forces fluctuations in the electron distribution - temporary dipole - partial (─) charge - partial (+) charge attractive forces due to temporary dipoles dispersion forces aka London Forces all molecules and atoms will have them a temporary dipole in one molecule, induces a dipole in all the surrounding molecules (induced dipole) He atom
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Tro, Chemistry: A Molecular Approach 7 Magnitude of Induced Dipole depends - 1. polarizability of the electrons (volume of the electron cloud) larger molar mass = more electrons = larger electron cloud = increased polarizability = stronger attractions
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Tro, Chemistry: A Molecular Approach 8 B.pt. of n -Alkanes/Non-polar molecules; long C-chain B.pt 2. shape of the molecule more surface-to-surface contact = larger induced dipole = stronger attraction
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Tro, Chemistry: A Molecular Approach 9
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Tro, Chemistry: A Molecular Approach 10 Practice – Choose the Substance in Each Pair with the Highest Boiling Point a) CH 4 CH 3 CH 2 CH 2 CH 3 b) CH 3 CH 2 CH=CHCH 2 CH 3 cyclohexane C C C C H H H H H H H H H H C H H H H C C C C H H H H H H C C H H H H H H H H H H H H C C H H H C C H C H H C
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Tro, Chemistry: A Molecular Approach 11 Practice – Choose the Substance in Each Pair with the Highest Boiling Point a) CH 4 CH 3 CH 2 CH 2 CH 3 b) CH 3 CH 2 CH=CHCH 2 CH 3 cyclohexane both molecules are nonpolar larger molar mass both molecules are nonpolar flat molecule larger surface-to-surface contact C C C C H H H H H H H H H H C H H H H C C C C H H H H H H C C H H H H H H H H H H H H C C H H H C C H C H H C
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Tro, Chemistry: A Molecular Approach 12 Dipole-Dipole Attractions polar molecules have a permanent dipole the permanent dipole adds to the attractive forces- raise the B.pt & M.pt relative to nonpolar molecules of similar size and shape
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Tro, Chemistry: A Molecular Approach 13 Effect of Dipole-Dipole Attraction on B.pt &M.pt Molar Mass Boiling Point Dipole Size CH 3 CH 2 CH 3 44.09 -42°C 0.08 D CH 3 -O-CH 3 46.07 -24°C 1.30 D CH 3 - CH=O 44.05 20.2°C 2.69 D CH 3 -C N 41.05 81.6°C 3.92 D
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This note was uploaded on 10/07/2010 for the course CHEM 1412 taught by Professor Sophie during the Fall '09 term at Richland Community College.

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Chapter11_LEC - Chapter 11 Liquids Solids and Intermolecular Forces Phase Changes Why are molecules attracted to each other Tro Chemistry A

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