CHAPTER 14 SOLUTIONS Introduction 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 lowering 2 freezing point depression 3 boiling point elevation 4 osmotic pressure note that vapor pressure lowering is the key to all four of these properties 9 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 of the 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 P0 where Psolvent the vapor pressure of the solvent in the solution Xsolute the mole fraction of the solute in the solution and P0 solvent the vapor pressure of the pure solvent solvent the lowering of vapor pressure Psolvent is defined as Psolvent P0 solvent Psolvent 10 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 fractional distillation fives increased separations because of the separation increased surface area commonly glass beads or steel wool are inserted into the distilling column 11 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 solvent this 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 m where Tb the boiling point elevation of the solvent Kb the molal boiling point elevation constant for the solvent and m the molality of the solution so 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 m where Tf the freezing point depression of the solvent Kf the molal freezing point depression constant for the solvent and m 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 of 13 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 weight 14 Colligative Properties and Dissociation of Electrolytes Colligative Properties of Solutions electrolytes have larger effects on boiling point elevation and freezing point depression than nonelectrolytes do this 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 molecules one mole of NaCl dissolves in water to produce two moles of aqueous ions 1 mol Na ions 1 mol Cl ions 2 mol ions remember 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 sugar CaCl2 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 mactu meffectiv Tf actual al mideal e mstated Tf if nonelectrolyte meffective accounts for the ions that reassociate a k a how many ions are ACTUALLY 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 is yields 2 ions 1 formula unit so there s less yields 3 ions 1 formula unit so Na Cl Na Cl deviation Ca2 Cl2 there s more deviation Ca2 2Cl remember that absolute temperature means Kelvin 15 Osmotic Pressure Colligative Properties of Solutions osmosis the net flow of a solvent between two solutions separated by a 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 MRT or mRT molarity or molality can be used where the osmotic pressure in atm M the molar concentration of the solution R 0 0821 L atm mol K and T the absolute temperature For 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 on a given graph such as this one the dark blue line water b c its freezing point is around 0 0oC the pink line glucose b c it has the fewest particles when you have equal molar the yellow line sodium chloride b c it has about twice the of particles when you have equal molar the aqua line calcium chloride b c it has about three times the of particles when you have equal molar see