C121 11-3 lec33GL

C121 11-3 lec33GL - Lecture 33 November 28, 2011...

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Lecture 33 November 28, 2011 Intermolecular Forces Last Day: Intermolecular Forces (Chapter 16) Vapour Pressure Phase Diagrams Today: Intermolecular Forces (Chapter 16) Phase Diagrams Properties of Solutions (Chapter 17) Solution Types Solution Composition Thermodynamics of Solutions Solubility and Intermolecular Forces
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Vapour Pressure (P vap ) the pressure exerted by a gas in dynamic equilibrium with its liquid (or solid) at a particular temperature in a closed container some molecules have KE high enough to escape IMF in liquid to become gas evaporation some gas molecules hit liquid surface and are recaptured by liquid IMF condensation evaporation rate = condensation rate and the pressure above the liquid remains constant with time at equilibrium as time passes initially P vap
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The vapor pressure of water, ethanol, and diethyl ether as a function of temperature. CH 3 CH 2 CH 2 CH 3 O : Diethyl Ether Dispersion Dipole-dipole CH 3 CH 2 H O Ethanol Dispersion H-Bonding (1 site) H 2 O Dispersion H-bonding (2 sites) H H O
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Claussius-Clapeyron Equation Temperature Dependence of Vapour Pressure ln (P vap ) vs 1/T is linear ln = - H vap R 1 T 2 1 T 1 (P vap ) 1 (P vap ) 2
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Phase Diagram for H 2 O A
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Points of Note for H 2 O: (i) T m (normal melting point) = 0°C • temperature where ice and water are in dynamic equilibrium at P = 1.00 atm (ii) T b (normal boiling point) = 100°C • temperature where liquid and gas are in dynamic equilibrium at P = 1.00 atm (iii) P 3 /T 3 (triple point) P 3 = 0.0060 atm, T 3 = 0.0098°C • temperature and pressure where all three states co-exist in dynamic equilibrium solid liquid gas
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(iv) for H 2 O the solid/liquid boundary line has a negative slope (ie. the f.p. (m.p.) decreases with increasing pressure) why? An increase in pressure will favour the state with the lowest volume (ie. the highest density , d = m/V) Recall for H 2 O d liquid > d solid As the pressure is increased, the more dense liquid state is favoured and thus the f.p. is lowered . (increasing P causes ice to melt) (v) the liquid-vapour boundary line ends abruptly at the critical point P C = 374°C, T C = 218 atm • critical temperature is the temperature above which the vapour cannot be liquefied at any pressure • critical pressure is the pressure that must be applied at T C to liquefy the vapour
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What is happening at the critical point and beyond? • consider pure water in equilibrium with its vapour in a closed
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C121 11-3 lec33GL - Lecture 33 November 28, 2011...

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