lecture 9 - Colligative properties • vapour pressure...

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Colligative properties vapour pressure lowering freezing point depression boiling point elevation osmotic pressure Consider a non-volatile solute B in a volatile sol- vent A where the solute does not dissolve in the solid phase of the solvent. picture Recall G A = G A + RT ln a A For the solvent, recall a A = γ A X A X A = n A n A + n B = mol fraction of A Hence G A < G A (no solute) this being a purely entropic e±ect picture 1
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Vapour pressure lowering G A = G gas = G gas + RT ln( P/P ) if G A is lower (by adding solute), the vapour pressure P must also be lower. boiling point elevation and melting point lowering One can show that Δ T b . p . RT 2 b . p . X B Δ H vap = K b m B Δ T fus ≈ - RT 2 fus X B Δ H fus = - K c m B where Δ T b . p . (the boiling point elevation) and Δ T fus (the freezing point depression) depend on the na- ture of the solvent only K b is the ebullioscopic constant of the solvent K c is the cryoscopic constant of the solvent
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This note was uploaded on 02/05/2010 for the course CHEM 205 taught by Professor Burnell during the Spring '07 term at The University of British Columbia.

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lecture 9 - Colligative properties • vapour pressure...

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