110c-lecture11 - Chapter 23 Phase Equilibria Problems 2 3 6...

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Chapter 23: Phase Equilibria Problems: 2, 3, 6, 17, 19, 27, 44, 45 Gibbs Phase Rule (p 927): p = 3 – f Solids: molecules immobilized in crystal lattice. Can have more than one solid phase (molecules arranged differently in the crystal lattice). Liquids: molecules confined to similar volume as solid, but are free to translate and rotate. Gases: molecules separated from each other and occupy much larger volume. Three Common Chemical Phases (solid liquid gas ) P and T vary independently (p=1, one phase). P or T vary independently (p=2, 2 phases). Neither P or T vary at the triple point (p=3). Triple point: solid, liquid and gas in equilibrium. Critical point: gas and liquid become identical. Benzene Phase Diagram
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Sublimation (s g) Occurs when Triple Point is above Atmospheric Pressure CO 2 sublimes at P = 1 atm (solid-gas equilibrium, no liquid) Normal sublimation temp = -78°C for CO 2 at 1 atm. Tank of CO 2 in the lab contains liquid CO 2 at P=56.5 atm at 293 K. CO 2 Phase Diagram (p929 Fig. 23.4) Sublimation of CO 2 CO 2 (s) CO 2 (g) Carbon dioxide has a vapor pressure of 830 psi at 20 °C. (1psi = 0.068046 atm). Solid CO 2 (dry ice) in a closed container at room temperature can be hazardous: Vapor pressure is 830 psi (56 times normal atmospheric pressure). Sublimation of I 2 I 2 (s) I 2 (g)
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Critical Phase Behaviour Critical point : T and P at which liquid=gas=fluid (72 atm, 31º C for CO 2 ). Supercritical fluids as Dry Solvents (e.g. decaffeination of coffee by CO 2 ) : Dissolve solutes like liquid (dissolving power tuned by pressure). Low viscosity and rapid transport of solutes. Evaporate away and/or compressible (for storage) like a gas. T << T c T ~ T c T > T c
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Note different pressure scales (1 atm = 760 torr, right graph only goes up to 2.6 atm). Benzene mp increases only 1° when P increases from 1 atm to 34 atm Benzene bp decreases from 80.1°C at P = 760 torr to 67°C at P = 500 torr (e.g., sea level vs. 10,000 ft above sea level) Gas phase is much more sensitive to pressure than liquid or solid. Boiling point depends more strongly on Pressure than melting point.
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This note was uploaded on 06/22/2010 for the course CHEM 21360 taught by Professor Ame during the Spring '09 term at East Los Angeles College.

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110c-lecture11 - Chapter 23 Phase Equilibria Problems 2 3 6...

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