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Ch 11.1-11.2 Kinetic Molecular Theory of Liquids and Solids; Intermolecular Forces • KMT in Solids & Liquids o Condensed States – very little empty space o Less compressible & denser than gases o Stronger, more important intermolecular forces o Table of properties (p. 436): State Volume/Shape Density Compressibility Motion G Indef/Indef Low High Free * L Def/Indef High Nearly Incompressible Flow S Def/Def Highest† Incompressible Vibrate about fixed position • Intermolecular Forces o Forces between two molecules (or atoms) o Strength determines melting and boiling point § Stronger IMF means more energy needed to separate molecules • More Energy ⇒ Higher T ⇒ Higher MP, BP o Generally much weaker than intramolecular Forces (ionic, metallic & covalent bonds) § All substances that are gases at room temperature and most liquids are atomic or molecular o Dipole-Dipole Forces (Sometimes called “van der Waals”) § Electrostatic force between two polar molecules § Can be attractive or repulsive § Moderate IMF: strength depends on dipole moment § Always smaller than full ionic interactions o Hydrogen Bonding § Very strong dipole-dipole interaction § Involves H bonded to N, O, or F on one molecule attracted to N, O, or F on a second molecule • N, O, F small atoms w/ high electronegativity Crystal structure and density of ice: H-Bonding between amino acids in DNA: Ch 11.1-11.2 Kinetic Molecular Theory of Liquids and Solids; Intermolecular Forces p. 2 o Binary Hydrides § H-Bonding makes MP of HF, H2O, and NH3 higher than predicted based on trends: • • IMF generally decreases as mass decreases o Fewer electrons → decreased attraction (see London Dispersion Forces below) Hydrogen bonding causes larger IMF, higher boiling point. o London Dispersion Forces § Induced Dipole Forces § Attraction between non-polar atom or molecule and an ion, polar molecule or other non-polar atom or molecule: § § § § Slight asymmetry in electron charge distribution in one atom induced dipole in neighbor Very weak since dipole is small & temporary Present between all atoms & molecules • Important only in absence of other forces Strength of attraction depends on polarizability: • Ease of separating charges • Depends on # of electrons and shape of species o e.g. He<Ne<Ar, etc., so BP increases down group o Larger molecules: CH4<C6H14 (more electrons) o Linear molecule allows more interactions o Ion-Dipole Forces § Attraction between an ion and a polar molecule § Moderately Strong–Depends on ionic charge and size of dipole § Solvent-solute interactions