240_l17 - Simple Mixtures Atkins 7th: Sections 7.4-7.5;...

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Simple Mixtures The Properties of Solutions Atkins 7th: Sections 7.4-7.5; Atkins 8th: 5.4-5.5 Liquid Mixtures Colligative Properties Boiling point elevation Freezing point depression Solubility Osmosis Activities Solvent Activity Solute Activity Last updated: Nov. 22, 2006
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Liquid Mixtures Gibbs energy of mixing for two liquids is calculated the same as for gases. For two liquids A and B ( note that * signifies pure substance ): G i ' n A μ ( A % n B μ ( B Upon mixing, the chemical potentials are given by μ J = μ J * + RT ln x J G f ' n A ( A % RT ln x A } % n B ( B % RT ln x B } Thus, the Gibbs energy of mixing is ) mix G ' nRT ( x A ln x A % x B ln x B ) and the entropy of the mixing of two liquids is ) mix S '& nR ( x A ln x A % x B ln x B ) meaning that the ideal enthalpy of mixing is zero. ) mix H ' 0 Same expressions as for mixing of two perfect gases. But. ..
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Liquid Mixtures: Miscibility Solution ideality is quite different from perfect gas behaviour: # Perfect gases : no interactions between the molecules # Ideal solutions : interactions between molecules, but average of interactions between A and B molecules is same as average interactions A-A and B-B in pure liquids # Real solutions : Composed of molecules for which A-A, A-B, B-B interactions are all different from one another Somtimes, molecules of one type might cluster together (additional contribution to entropy), so: • Enthalpy change might be large and +ve • Negative entropy change • Positive Gibbs energy for mixing In this case, separation is spontaneous, and liquids are immiscible (example, CCl 4 and H 2 O, which are non-polar and polar, respectively) Liquids can also be partially miscible , which means that they are miscible only in certain compositions (partially dissolve in one another)
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Real Solutions & Excess Functions Thermodynamic properties of real solutions are described with excess functions , X E S E ' ) mix S & ) mix S ideal For example, the excess entropy , S E , is the difference between the observed thermodynamic function and the function for an ideal solution: So, the further excess energies are from zero indicates the degree to which a solution is non- ideal - a regular solution has H E 0, S E = 0, with two kinds of molecules distributed randomly (like ideal solution), but having different interactions with one another (a) The benzene/cyclohexane mixing , shows that mixing is endothermic (b) Tetrachloroethane/cyclohexane mixing shows a volume contraction at low C 2 Cl 4 fractions, and expansion at high C 2 Cl 4 fractions V E H E
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Colligative Properties Colligative properties literally denotes properties which depend on a collection of particles # Boiling point elevation # Freezing point depression # Solubility of liquids and solids # Osmotic pressure Each of these properties is going to depend on the composition of the mixture (i.e., the relative mole fractions of the components) Important Assumptions : Solute is not volatile, does not contribute to the chemical
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240_l17 - Simple Mixtures Atkins 7th: Sections 7.4-7.5;...

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