Intermolecular Forces Lecture Kotz

Intermolecular Forces Lecture Kotz - 2/23/2010 1...

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Unformatted text preview: 2/23/2010 1 Intermolecular Forces, Liquids and Solids Intermolecular Attractions-Remember: We are talking about attractions between molecules (intermolecular forces), or ions; not the bonds between the atoms that make up the molecule.-Ion attractions tend to be the strongest forces (700 to 1100kJ/mol) vs. covalent (100-400kJ/mol) Why are Intermolecular Forces Important-They are directly related to properties such as melting point melting point melting point melting point, boiling point boiling point boiling point boiling point, and the energy needed to convert a solid to a liquid or a liquid to a vapor (enthalpy [latent heat] of fusion and vaporization enthalpy [latent heat] of fusion and vaporization enthalpy [latent heat] of fusion and vaporization enthalpy [latent heat] of fusion and vaporization). MP NaCl = 801 o C MP He = -272.19 o C MP H2O = 0 o C- They are important in determining the solubility solubility solubility solubility of gases, liquids, and solids in various solvents- They are crucial in determining the structures structures structures structures of biologically important molecules such as DNA and proteins (primary, secondary, tertiary and quaternary structures) There are different types of attractions between molecules/ions Ion Ion Ion Ion-Ion; Ion Ion; Ion Ion; Ion Ion; Ion-Dipole; Dipole Dipole; Dipole Dipole; Dipole Dipole; Dipole-Dipole; London Forces Dipole; London Forces Dipole; London Forces Dipole; London Forces, etc. Electrostatic attractions determined by Coulombs Law F = kq 1 q 2 /r 2 k=8.988x10 9 Nm 2 /C 2 Attractive forces depend upon three factors:- The distance between the ion and the dipole. The closer the ion and dipole, the stronger the attraction- The charge on the ion. The higher the ion charge, the stronger the attraction.- The magnitude of the dipole. The greater the magnitude of the dipole, the stronger the attraction Dipole-Dipole Interactions Effects of intermolecular forces Affects solvation energy (enthalpy of hydration): Dissolving substances in one another Exothermicity depends upon 1/r In general: Like substances dissolve like substances Like substances dissolve like substances Like substances dissolve like substances Like substances dissolve like substances (Polar in polar (ionic); nonpolar in nonpolar, etc.) Permanent dipole-attractions: H vap increases with polarity and molar mass 2/23/2010 2 Like Dissolved Like Nonpolar Molecular Interactions *Dipole-Induced Dipole *London (dispersion) Forces (induced dipoles)-Produces momentary dipoles momentary dipoles momentary dipoles momentary dipoles that hold the molecules together-Caused by the temporary shifting of the electron clouds through interactions between molecules-For an homologous series (CH 4 , C 2 H 6 , C 3 H 8 , etc.) polarizability increases with molar mass (greater electron cloud shifting) 2/23/2010 3 Hydrogen Bonding Special case of polar intermolecular attractions Comes from molecules containing one or more of the following:...
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This note was uploaded on 12/07/2011 for the course CHEM 100 taught by Professor Feebeck during the Fall '10 term at Purdue University-West Lafayette.

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Intermolecular Forces Lecture Kotz - 2/23/2010 1...

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