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21-Liquids and Solids - 5.111 Lecture 21 LIQUIDS AND...

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Unformatted text preview: 5.111 Lecture # 21. LIQUIDS AND SOLIDS [Pages 185-191 and 203—205 from the “Chemical Principles” textbook, 4th edition, by Peter Atkins & Loretta Jones, Freeman, New York, 2008] Fundamental molecular characteristics of gases, liquids, and solids: Phase Motion Average distance ---------------------------------------------------------- between molecules Translational Rotational Vibrational at 1 atm Gas free free free ~10 molecular diameters Liquid hindered hindered free <1 molecular diameter Solid none none about fixed positions <1 molecular diameter Liguids: kinetic energy versus intermolecular interactions. Short-range versus long-range order. Liquid water versus ice. Viscosigv Resistance to flow; consequence of intermolecular interactions. The range of values: Water Glycerol Ethyl ether Mercury 0 1.787 12,110 0.284 1.685 25 0.890 954 0.222 1.525 100 0.282 — 0.118 1.240 Dependence of viscosity on the temperature. Sur ace tension The net inward pull — [Slide 21.1] Surface tension and intermolecular interactions. [Slide 21.2] Spherical shape of liquid (e. g., water) droplets — [Slide 21.3]. Capillary action — [Slide 21.4]. Wetting. The meniscus [Slide 21.4]. Uooofi 3w 0. m m, mhdh 7 H885 mind 3:913: NDV 5582: V. w H 23me w .NN #9350 CNN owEoEumHBp 95.28 m w . m N ocoNGB ATEFZEE r 235 £383 oowwSm. 003. a @2st MO mficmmfivrfl ouwwuam ”on udm<h When does a liquid become a solid? Classification of solids on the basis of the bonds that hold their atoms, ions, or molecules in place: Metallic solids — cations held together by a sea of electrons. Ionic solids — cations and anions held in place by electrostatic attraction. Molecular solids ~— molecules held in place by intermolecular forces. Network solids —— atoms held in place by covalent bonding to their neighbors. [Slide 21.5]. Given our focus on intermolecular interactions, consider molecular solids in more detail. Crystalline versus amomhous solids — [Slide 21.6]. If intermolecular interactions are very strong, crystalline molecular solids are hard, brittle, and high-melting. Crystals of table sugar (sucrose) as an example. 85m a minis ocofimfiammu mmufiom wcfion wan mauve 32 39528 h383w “mom «NH ad nmm «Naom— $38—08 “83> E @3302: £30m mat—m8 :wE 50> 61:3 “Emu ”Em: NOE 75 nm xqu nU am €036: 30338 $52458 03w 5ng E @338 805 mmufiom mam—BL Ea $32: 3: Megan. "Emu “EB offload “6sz .52 Dee mucuswcg 312985 98 \Emomboflo £50me 6:833 633sz 3583a #81:» 93 -w 8:82: motmtouoamav moan—«fi— $20 mum—om mo muumfiouufimav 1‘29ka #6 HES. Consider the structure of i£e_. [Slide 21.7]. Each oxygen is surrounded by 4 hydrogens in a tetrahedral arrangement (2 linked through o-bonds, the other two through H—bonds). Hence an open network of H20 molecules held together by hydrogen bonds. What happened when ice melts? Collapse of H-bonds on melting. Molecules in liquid water are packed less uniformly but more densely than in ice. Hence the density of ice is lower than that of water (0.92 g-cm‘3 and 1.00 g-cm'3, respectively, at 0°C). (Ice floats on water.) In this respect, water is highly unusual. Potholes. Liquid crystals as a mesophase. Flow like liquids, ordered like crystalline solids. Nematic, smectic (cell membranes), and cholesteric phases of liquid crystals —— [Slide 21.8]. Anisotropic and isotropic materials. Thermotropic and lyotropic liquid crystals. ...
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