CHM 1172 Test 1 study guide.docx - Chapter 11...

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Chapter 11. Intermolecular Forces and Liquids and Solids - Differences between gases and condensed phases related to the distance between molecules and how it affects properties such as: volume/shape; density; compressibility; molecular motion (11.1) 11.2 Intermolecular Forces - Inter molecular forces, the attractive forces between molecules, vs intra molecular forces (those which hold a molecule together). Intermolecular forces generally weaker than intramolecular forces (including the electrostatic forces which hold ions together). - Polar molecules are dipoles (i.e. they have a dipole moment arising from an uneven distribution of electrons in the molecule). - Intermolecular forces involving polar molecules include o Dipole-dipole forces between similar or different polar molecules. o Ion-dipole interactions between either an anion or cation and a polar molecule o Dipole-induced dipole interactions in which a dipole can induce a temporary dipole in a nonpolar molecule - Nonpolar molecules can only engage in weak intermolecular forces involving induced (temporary) dipoles called d ispersion forces. The strength of these interactions increases with the number of electrons (i.e. molar mass). - Van der waals forces refers to dipole-dipole, dipole-induced dipole and dispersion forces. - Hydrogen bonding is a special case of dipole-dipole interaction between the hydrogen in an N-H, O-H or F-H group and an electronegative O, N or F atom. It gives rise to unusually high boiling points for NH 3 (ammonia), H 2 O (water) and HF (hydrogen fluoride) compared to analogous compounds of higher molar mass. 11.3 Liquids - Surface tension and viscosity are properties unique to liquids - At a liquid surface, intermolecular forces between the liquid molecules pull the liquid down (into the bulk) and laterally, but not upward, causing the surface to “tighten.” Surface tension is a measure of the energy required to stretch the surface of the liquid (units are energy/area). - Capillary action is rise of liquid in a narrow tube, and occurs when adhesive forces – the attractive forces between the liquid and the tube – are stronger than the cohesive forces that hold the liquid molecules together. - Viscosity is a measure of a liquid’s resistance to flow. Liquids with strong intermolecular forces (such as hydrogen bonding) have higher viscosities than liquids with weak intermolecular interactions. - Water has a number of unusual properties due to the high degree of hydrogen bonding: Higher heat capacity because energy input to increase molecular motion must also overcome the hydrogen bond interactions
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Higher density in the liquid state than the solid state, due to the somewhat “open” crystal structure of ice. This leads to a maximum in density for water at 4C as the first molecules to melt lead to a slight increase in density (filling up the extra space that was present in the crytals).
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