CHAPTER 14 – SOLUTIONSIntroduction to Colligative Properties colligative properties -- the properties of solutions that depend solely on the number of particles dissolved in the solution; do not depend on the kinds of particles dissolved; physical properties of solutions there are four common types of colligative properties:1. vapor pressure lowering2. freezing point depression3. boiling point elevation4. osmotic pressure*note that vapor pressure lowering is the key to all four of these properties9 Lowering of Vapor Pressure and Raoult’s Law (Colligative Properties of Solutions) the addition of a non-volatile solute to a solution lowers the vapor pressure of the solution; the solute molecules occupy some ofthe spaces that would normally be occupied by the solvent solute lowers the vapor pressure because it keeps some of the solvent particles from escaping; this also raises the boiling point and the freezing point Raoult’s Law models this effect in ideal solutions∆Psolvent = (Xsolute)(P0solvent)where Psolvent = the vapor pressure of the solvent in the solution,Xsolute = the mole fraction of the solute in the solution, andP0solvent = the vapor pressure of the pure solvent the lowering of vapor pressure, ∆Psolvent, is defined as:∆Psolvent = P0solvent – Psolvent10 Fractional Distillation (Colligative Properties of Solutions) distillation -- a technique used to separate solutions that have two or more volatile components with differing boiling points simple distillation -- has a single distilling column; gives a reasonable separation fractional distillation -- fives increased separations because of the increased surface areacommonly, glass beads or steel wool are inserted into the distilling column11 Boiling Point Elevation (Colligative Properties of Solutions) the addition of a non-volatile solute to a solution raises the boiling point of the solution above that of the pure solventthis effect is because the solution’s vapor pressure is lowered (Raoult’s Law) the solution’s temperature must be raised in order to make the solution’s vapor pressure equal to the atmospheric pressure the amount that the temperature is elevated is determined by the number of moles of solute dissolved in the solution the boiling point elevation relationship is:∆Tb = Kb mwhere ∆Tb = the boiling point elevation of the solvent,Kb = the molal boiling point elevation constant for the solvent. andm = the molality of the solutionso, the boiling point (∆Tb) increases () as the molality (m) increases ()12 Freezing Point Depression (Colligative Properties of Solutions) the addition of a non-volatile solute to a solution lowers the freezing point of the solution relative to the pure solvent the freezing point depression relationship is:∆Tf = Kf mwhere ∆Tf = the freezing point depression of the solvent,Kf = the molal freezing point depression constant for the solvent. andm = the molality of the solution fundamentally, freezing point depression and boiling point elevation are the same phenomenon the primary difference between Kf and Kb is the magnitude of the constant, which is evident from a phase diagram for a solution remember that, even when you aren’t told in a problem, the solute is what you have the least of and the solvent is what you have the most of13 Determination of Molecular Weights by Freezing Point Depression or Boiling Point Elevation (Colligative Properties of Solutions) the size of the freezing point depression depends on the size of the Kf for the solvent and the molality concentration of the solution if Kf and kg of solvent are both known (as is often the case in an experiment), then we can determine the # of moles of solute and use that to determine the molecular weight14 Colligative Properties and Dissociation of Electrolytes (Colligative Properties of Solutions) electrolytes have larger effects on boiling point elevation and freezing point depression than nonelectrolytes dothis is because the # of particles released in solution is greater for electrolytes than it is for nonelectrolytes example -- one mole of sugar (C6H12O6) dissolves in water to produce one mole of aqueous sugar moleculesone mole of NaCl dissolves in water to produce two moles of aqueous ions:1 mol Na+ ions + 1 mol Cl- ions = 2 mol ionsremember that colligative properties depend on the # of dissolved particles!NaCl produces twice the # of particles as sugar, so its freezing point depression will be twice that of sugarCaCl2 produces three moles of ions, so its freezing point depression will be three times greater the van’t Hoff factor (i) is essentially the percentage created by the fraction:mactual=meffective=∆Tf (actual)midealmstated∆Tf (ifnonelectrolyte) meffective accounts for the ions that reassociate (a.k.a., how many ions areACTUALLY there) the more ions that you form from one formula unit, the more reassociation there is, the more deviation there is between meffective (the same as meffective) and mideal (the same thing as mstated), and the more different the van’t Hoff factor (i) isNa+Cl-  Na+ + Cl-yields 2 ions / 1 formula unit, so there’s less deviationCa2+Cl2-  Ca2+ + 2Cl-yields 3 ions / 1 formula unit, so there’s more deviation remember that absolute temperature means Kelvin!15 Osmotic Pressure (Colligative Properties of Solutions) osmosis -- the net flow of a solvent between two solutions separated bya semipermeable membrane; the solvent passes from the lower concentration solution to the higher concentration solution; a rate controlled phenomenon because the solvent is passing from the dilute solution into the concentrated solution at a faster rate than in the opposite direction (i.e., establishing equilibrium) the thistle tube thing the formula for osmotic pressure is:π = MRT or π = mRT (molarity or molality can be used)where π = the osmotic pressure in atm,M = the molar concentration of the solution,R = 0.0821 L atm / mol K, andT = the absolute temperatureFor the Test… why would you add ethylene glycol to water? to change the freezing point (see page 207 in the lecture outline for ethylene glycol’s Lewis structure) know how to identify the pure solvent line vs. the solvent + solute line on a phase diagram, such as this one  know how to match the names of different solutions to different plots ona given graph, such as this one the dark blue line =