class4-2 - Class 4.2 M o r e C o llig a tiv e P r o p e r...

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Class 4.2 More Colligative Properties, CHEM 102H T. Hughbanks Boiling point elevation and freezing point depression When nonvolatile solutes are present in a solvent, the solution’s boiling point rises and freezing point declines. For very dilute solutions, these effects depend only on what the solvent is and the the solute concentration. The origin of this behavior is basically the same as the origin of osmotic pressure. Entropy of the solute gas decreases when volume decreases in vaporization n moles of solute molecules are forced into the decreased solvent volume when a mole of solvent leaves (V m = volume of a mole of solvent). S solute = n Rln[(V-V m )/V] = n Rln[1 - (V m /V)] Since V >> V m , ln[1 - (V m /V)] - V m /V S solute = - n R(V m /V) = - ( n /V)RV m V–V m V
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Total Entropy of vaporization is less for the solution than for the pure solvent S vap,soln = S vap,pure + S solute ( S solute < 0) T b,soln = H vap,pure / S vap,soln - for dilute solution = H vap,pure /[ S vap,pure + S solute ] [ H vap,pure / S vap,pure ][1 - ( S solute / S vap,pure )] = T b,pure - T b,pure ( S solute / S vap,pure ) Therefore, T b,soln = T b,pure + T b,pure [( n /V)RV m / S vap,pure ] Boiling point elevation T b,pure [( n /V)RV m / S vap,pure ] V m = L solvent mol solvent
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class4-2 - Class 4.2 M o r e C o llig a tiv e P r o p e r...

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