Lecture 23: Review of Colligative Properties and Intro to Equilibrium Colligative properties: freezing point depression solid  solution boiling point elevation solution  gas Fractional distillation Introduction to Chemical Equilibrium vapor pressure

Freezing Point Depression Solute must be excluded from solid Consider equilibration of vapor pressure of solvent and solute. liquid solid add solute Vapor Pressure freezing point drops Temperature

Boiling point increases Boiling Point Elevation Solute must be excluded from gas There is a blanket of gas (an “atmosphere”) at a fixed pressure (typically 1 atm) Boiling is when vapor pressure reaches blanket pressure. liquid add solute Vapor Pressure blanket gas pressure typically 1 atm Boiling point increases Temperature

slope 0.05 atm/K Predict the boiling point of a solution of 8 mol of water and 1 mol of NaCl: NaCl(s)  Na+(aq) + Cl-(aq) (1) 101°C (2) 102°C (3) 104°C (4) 108°C

Fractional Distillation Solvent and solute are both volatile Vapor is enriched in more volatile component Can be quite complicated for non-ideal solutions Step 1 X2soln = 0.75 X2vap = 0.66 Step 2 X2soln = 0.66 X2vap = 0.50 1 2 Step 3 X2soln = 0.50 X2vap = etc.