lec6_whc - Last Lecture-Intermolecular Forces; ch. 12 Ionic...

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Last Lecture-Intermolecular Forces; ch. 12 • Ionic Bonds • Dipole - Dipole Bonds • Hydrogen Bonding • London Forces
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Covalently Bonded Solids Carbon Allotropes m.p=3500°C Fullerenes/Buckyballs C 60 Nanotubes Network solid
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Ionic bonds are electrostatic; Opposite charges attract Na + + Cl - NaCl Na + 0 valence electrons Cl - 8 valence electrons Coulomb-like forces!
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Dipole - Dipole Interactions • Electronegativity based – electron/nucleus attraction • Polar molecules alignment C O H H δ - δ + O H H δ + δ -
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Hydrogen Bonding • Electrostatic interaction between H and either O, N, or F. – Size and charge density •N H 3 , H 2 O, and HF • Boiling point increase with mol. wt. – Heavy anions with H bonding boil higher
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Hydrogen Bonding • When an H atom is bonded to a highly electronegative atom O, N or F • Electrons in H-F bond spend much time near F • Lone pair on neighboring F sees a proton and comes close! • Very strong! In particular: NH 3 , H 2 O, and HF
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Hydrogen Bonding in Water around a molecule in the solid in the liquid all waters H-bonded some waters H-bonded bent hexagonal rings (open) closely packed (dense) H 2 O expands upon freezing - order
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Other examples of H-Bonds e.g. acetic acid: pairing in liquid and vapor reduces Æ low H vap Also: Intramolecular H-Bonds e.g. Proteins:
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Butane or acetone? Example 12C C. A. Trans-dichloroethylene or cis-dichloroethylene ? Butane or formaldehyde? B. Which has a higher boiling point? Consider both the molecular masses and the dipole moments.
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cis-dichloroethylene trans-dichloroethylene 48° 60° -21°C Formaldehyde (M=30g/mol) Solution 12C δ + + + + Here the mass is more important that dipole! -0.5°C Butane (M=58g/mol) +56.2°C Acetone (M=58g/mol) Here same mass but one has large dipole! C A B
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Example 12D Order these substances by boiling point: Benzene Toluene Phenol Chlorobenzene
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Solution 12D Benzene 80 o C; No dipole moment; 0.0 Debye Toluene 110 o C , Dipole Moment= 0.37 Debye Phenol 182 o C , Dipole moment= 1.2 Debye. Note H-bond potential!
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This note was uploaded on 06/13/2010 for the course CHEM 995940767 taught by Professor Topadakis during the Spring '10 term at UC Davis.

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lec6_whc - Last Lecture-Intermolecular Forces; ch. 12 Ionic...

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