110c-lecture17

# 110c-lecture17 - Summary of Colligative Properties Melting...

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Summary of Colligative Properties Δ = fus fus fus T T R H x x 1 1 ) 1 ln( * 2 2 Melting temp (T fus ) vs. solute conc (x 2 ) () Δ = vap vap vap T T R H x x 1 1 1 ln * 2 2 Boiling temp (T vap ) vs solute conc (x 2 ) Adding solute (increasing x 2 ) decreases melting temp and increases boiling temp. Melting and Boiling Temperatures Depend on Amount of Dissolved Solute (x 2 ): Osmotic Pressure ( Π ) is proportional to solute concentration: cRT Π c=solute concentration in molar (mol/L) = Π 2 2 1 MW RT C c=C 2 /MW 2 ; C 2 =concentration in g/L * 1 μ ln 1 s 1 + + = Π ... 1 2 2 2 C B MW RT C (B = 0 ideal solution) 2 nd Virial Coeff (B)

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Which Colligative Property Changes the Most? Dissolving one mole of solute in one kg of water will cause the following: Osmotic pressure ( Π ) = 17,000 torr vs. 760 torr (atmospheric pressure). Boiling Point Elevation of 0.52 °C (T vap = 373.78 vs. 373.15 K). Vapor Pressure decrease of 0.3 torr (P 1 = 23.7 vs 24 torr) . Freezing Point Depression of –1.86 °C (T fus = 271.29 vs 273.15 K). 1 mole solute has an Osmolality of 1000 mOsmols per kg of Water. Osmotic Pressure ( Π ) affected most by dissolving a given amount of solute.
Solubility vs Temp 1(sln) + 2(sln) 2(s) ) ( ln 2 T s μ ) ( 2 T solid 2 2 2 ln ) ( ) ( x RT T T liq solid + = μ 2 2 ln 2 2 ln ) ( ) ( ) ( x RT T T T liq s solid + = = μμ RT G RT x fus liq solid Δ = = 2 2 2 ln Δ = Δ = * * 2 2 2 1 1 1 ln fus fus fus fus T fus T fus x T T R H dT RT H x d Δ = * 2 1 1 ln T T R H x fus () 2 2 ln RT H G T x T fus fu s Δ = Δ = δ

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NaCl(s) + H 2 O(l) NaCl(aq) or CaCl 2 (s) + H 2 O(l) CaCl 2 (aq) + = m m dm m 0 2 2 , 2 1 1 ln φ φγ * 1 1 2 ln 5 . 55 P P m = Electrolytes are non-ideal solutes. Vapor pressure for sucrose vs sodium chloride in water: CaCl 2 NaCl Sucrose Molality ln γ molality 00 0 0.2 0.09 0.16 0.4 0.18 0.32 0.6 0.27 0.47 0.8 0.37 0.63 1 0.46 0.79 1.4 0.67 1.12 1.8 0.89 1.46 Slope 0.49 0.81 Molality P 1 * - P NaCl Sucrose Solutions of Electrolytes NaCl(s) + H 2 O(l) Na + (aq) + Cl - (aq) Higher slope caused by: 1. Dissociation into two ions. (Colligative properties) 2. Solute-solute interactions (charge-charge) 3. Electrolytes violate Henry Law. ) ( ) 1 ( ) 1 ( * 1 1 * 1 2 * 1 1 2 1 2 2 2 2 P P P P P x x a = = = = γ γγ sucrose P P 1 * 1 NaCl P P 1 * 1 2 , 2 2 m a m = Ideal in dilute soln (a 2 m 2 ) Non-ideal even in dilute soln (a 2 m 2 ) Henry’s Law
Strong electrolytes in H 2 O dissociate into constituent cations & anions: () ( ) ( ) aq A aq C s A C z z l O H + + + + ⎯→ ν νν 2 where 0 = + + + z z because of electroneutrality .

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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-lecture17 - Summary of Colligative Properties Melting...

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