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Chem 300 - Ch 24/#1 Today’s To Do List

Chem 300 - Ch 24/#1 Today’s To Do List. Raoult’s Law & The Ideal Solution Nonideal Solutions Why do Liquid-Liquid solutions do this?? Gibbs tells all Partial Molar Quantities Gibbs-Duhem Equation. Liquid-Liquid Solutions. Both solution components are liquids Miscible in all proportions

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Chem 300 - Ch 24/#1 Today’s To Do List

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  1. Chem 300 - Ch 24/#1 Today’s To Do List • Raoult’s Law & The Ideal Solution • Nonideal Solutions • Why do Liquid-Liquid solutions do this?? • Gibbs tells all • Partial Molar Quantities • Gibbs-Duhem Equation

  2. Liquid-Liquid Solutions • Both solution components are liquids • Miscible in all proportions • Composition in MF (xi) of liq. mixture • Both have vapor pressures • Dalton’s Law holds: • Ptotal = P1 + P2 • P1 & P2 are partial pressures • Pi = Yi Ptotal Yi = MF of i in vapor

  3. Solution Properties • Vsoln = V1 + V2 ?? Maybe…not • DHmixing = +, - , 0 ?? • Properties proportional to concentration (xi)…not always.

  4. Compare with gas mixtures • Gas: molecules far apart • Liquid: molecules close together • Gas: Attractive/repulsive forces largely ignored • Liquid: Attr/repuls forces significant • Gas: ideal gas model • Liquid: ideal solution model

  5. Raoult’s Law • P*i vapor pressure of pure i. • Pi partial pressure of i in solution of mol fract. Xi. • Raoult’s law: • Pi = P*i Xii = 1 or 2

  6. Ideal Soln: Ptotal vs Xbenz Pi = P*i Xi

  7. Neg. Devia. From RL: Cl3CH/Acetone

  8. Positive Devia. From RL: CS2/(CH3O)2CH2

  9. Pos. Devia. as fcn of Alcohol chain length (alc/H2O mixt.) 1-prop methanol

  10. Dalton’s Law & Ideal Solns • Dalton: Ptotal = P1 + P2 • Apply Raoult’s Law: • Ptotal = P*1 X1 + P*2 X2 = P*1 X1 + P*2 (1 – X1 ) = P*2 + (P*2 - P*1) X1

  11. NonIdeal Solutions • For Nonideal gases: • Introduced fugacity (f) • For Nonideal solutions: • Introduce activity (a) • Generalized Raoult’s Law: • Pi = P*i ai (a = gX)

  12. Limiting Behavior

  13. Ideal Solution

  14. The Model • Ideal Solution: • Strong Intermolecular Attractive forces • But all forces Equal: A-AB-BA-B • NonIdeal Solution: • Attractive forces NOT equal: • A-A, B-B > A-B Positive Devia from RL • A-A, B-B < A-B Negative Devia from RL

  15. Volume Nonadditivity: Partial Molar Volumes • Mixing of Ideal Solutions: • Vtotal = n1 Vm(1) + n2 Vm(2) • Nonideal Solutions: • Define a Partial Molar Volume: • Vj = (Vtotal/nj)P,T,n’ • Vtotal = n1 V1 + n2 V2

  16. Partial Molar Volumes as function of Mol fraction Note opposite effects

  17. PMV of H2O & Ethanol @ 25 ºC

  18. Gibbs-Duhem Equation • Gtotal = n1m1 + n2 m2 • mj = (G/ nj)P,T,n’ • dG = m1 dn1 + n1 dm1 + m2 dn2 + n2 dm2 • dG = -SdT + VdP + m1 dn1 + m2 dn2 • At const T, P  dG = m1 dn1 + m2 dn2 nnn • n1 dm1 + n2 dm2 = 0Gibbs-Duhem Eq.

  19. So… • n1dm1 + n2 dm2 = 0Gibbs-Duhem Eq. • Ties together the behavior of the components in a Solution.

  20. PMV of H2O & Ethanol @ 25 ºC

  21. Next Time More Applications of G-D Eq. Critical behavior in L-L solutions Activity Henry’s Law

  22. Plot of P1 = x1P*1exp[ax22+ bx32]

  23. Solution & Distillation

  24. Fractional Distillation

  25. Azeotropes

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