chapter11a.forBB

chapter11a.forBB - Chapter 11: Liquids, Solids and...

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1 1 Chapter 11: Liquids, Solids and Materials 2 The Liquid State A liquid forms when a gas condenses. •This occurs at low T (and/or high P ). Average intermolecular attraction > avg E k . Molecules are close together and in constant motion ± They have a distribution of speeds (and E k ) ± A few have enough E k to slide past each other.
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2 3 Liquids are very hard to compress • Molecules are close together and resist moving closer. • Used as hydraulic fluids to transmit pressure. The Liquid State 4 The Liquid State Viscosity = Resistance to flow. • High viscosity = slow flow. ± low viscosity: water, ethanol, gasoline… ± high viscosity: honey, corn syrup …
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3 5 High viscosity is caused by: Large intermolecular attractions. Large entanglement of molecules. Higher T = lower viscosity E k > E barrier required to move past another molecule. • Increase T = increase average E k . ± More molecules have E k > E barrier The Liquid State 6 Liquid molecules attract each other. Surface molecules experience unbalanced forces. They are not as well stabilized. Smaller surface area higher stability. The Liquid State: surface properties
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4 7 Raindrops are spherical (surface/ V ratio minimized). Surface tension = E required to form a flat surface. Substance Formula Surface Tension J/m 2 at 20°C Octane C 8 H 18 2.16 x 10 -2 Ethanol C 2 H 5 OH 2.23 x 10 -2 Chloroform CHC l 3 2.68 x 10 -2 Benzene C 6 H 6 2.85 x 10 -2 Water H 2 O 7.29 x 10 -2 Mercury Hg 46. x 10 -2 The Liquid State 8 The Liquid State Surface tension = E required to form a flat surface.
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5 9 The “winner” depends on the amount of liquid and any surface/liquid attractions . A drop of water: ± puddles on a dirty surface ± “beads” on a waxy surface Why don’t all liquids form spheres in all cases? It’s a balancing act … Gravity pulls down (giving a puddle). Surface tension contracts (into a sphere). The Liquid State 10 Capillary action A competition between: • Adhesion: forces between a surface and a liquid. • Cohesion: forces between liquid molecules. The shape of the meniscus illustrates the relative strength of the adhesive and cohesive forces The Liquid State
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11 The Liquid State 12 Volatility = tendency of a liquid to vaporize. low T velocity or energy number of molecules Threshold for escape high T Volatility increases with increased T . Increase T = increase E k = more can escape. Vapor Pressure
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This note was uploaded on 07/15/2009 for the course CHE 131 taught by Professor Kerber during the Fall '08 term at SUNY Stony Brook.

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chapter11a.forBB - Chapter 11: Liquids, Solids and...

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