CH11 MSJ jlm - Chapter 11 Liquids, Solids, and Materials...

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1 Chapter 11 Chapter 11 Fritz London 1900-1954. Studied intermolecular induced-dipole interactions. Johannes D. van der Waals 1837-1923.* Studied intermolecular forces in VPT relationships in liquids and gases. Liquids, Solids, and Materials Liquids, Solids, and Materials
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2 Kinetic Energy Attractive Intermolecular Forces High temperatures Low temperatures Gases, Liquids and Solids Gases, Liquids and Solids
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3 Ionic Forces Ionic Forces + + - - - - + + + Ion-Ion e.g. NaCl(s) Ion-Dipole e.g. NaCl(aq) Ions form strong intermolecular forces with the polar molecule water. Ion-ion forces are very strong and produce high boiling points and melting points. NaCl dissolved in water From now on we will concentrate on covalent molecules
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4 Intermolecular Interactions Intermolecular Interactions Non-Polar Molecules: Polar Molecules: Molecules with F-H, O-H, N-H Bonds: 1. Dispersion (Disp) Forces 1. Dispersion (Disp) Forces 2. Dipole-Dipole (DD) Forces 1. Dispersion (Disp) Forces 2. Dipole-Dipole (DD) Forces 3. Hydrogen Bonding (HB) Forces
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5 London Dispersion Forces London Dispersion Forces (aka van der Waals Forces) (aka van der Waals Forces) The electrons on one atom are attracted to the nucleus on a neighboring atom. This creates an “instantaneous” (i.e. temporary) dipole on the first atom. The instantaneous dipole on the first atom then induces an instantaneous dipole on the second atom. The two induced dipoles attract each other. London Dispersion Forces are proportional to a molecule’s polarizability, which is the ease with which the electron cloud can be deformed. The polarizability, is approximately proportional to the number of electrons in the molecule.
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6 Element MW # e - T bp [amu] [K] He 4.0 2 4 Ne 20.2 10 27 Ar 39.9 18 87 Kr 83.9 36 121 Xe 131.3 54 166 The noble gases are spherical, non-polar atoms. As the molecule (atom) increases in size, the boiling point increases (vapor pressure decreases). Noble Gases Noble Gases
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7 London Dispersion Forces London Dispersion Forces London Dispersion Force # e - London Dispersion Force MW T bp is a good measure of the strength of intermolecular forces. A higher T bp indicates stronger intermolecular forces. Compd. MW # e - T bp [amu] [ o C ] F 2 38 18 -188 Cl 2 71 34 -34 Br 2 160 70 +59 I 2 254 106 +184 Compd. MW # e - T bp [amu] [ o C ] CH 4 16 10 -161 C 2 H 6 30 18 -88 C 3 H 8 44 26 -42 n-C 4 H 10 58 34 0 Dispersion Forces Increasing
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8 Dispersion forces also depend upon the molecular shape. Pentane: C 5 H 12 - MW = 72 amu H 3 C C H 2 H 2 C C H 2 CH 3 n-pentane T bp = 36 o C neopentane T bp = 9 o C H 3 C C CH 3 CH 3 H 3 C Large Contact Area (Surface Area) Small Contact Area (Surface Area)
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9 Dipole-Dipole Forces Dipole-Dipole Forces In liquids of polar molecules, oppositely charged ends of the molecules tend to attract each other, causing partial alignment.
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This note was uploaded on 04/12/2011 for the course CHEM 1420 taught by Professor Acree during the Spring '08 term at North Texas.

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CH11 MSJ jlm - Chapter 11 Liquids, Solids, and Materials...

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