Prof Gilbert LECTURE 28 CHEM 1211 11 22 10 Last time Chapter 10 Intermolecular Forces Intermolecular interactions ion dipole dipole dipole and hydrogen bonds Vapor pressure the stronger the intermolecular forces between the molecules in liquids the lower the vapor pressure and the higher the boiling point Phase diagrams This time Chapter 10 continued Solubility of Gases Henry s law lets us calculate the solubility of a gas Cgas in a liquid where kH is the Henry s law constant for the gas solvent system A few examples at 20 C Note solubility is proportional to partial pressure What does this mean for people living in say Denver where atmospheric pressure is only 83 of that at sea level Vapor pressure of a solution Consider these two beakers in an inverted plastic tub one w distilled water the other w seawater Explanation adding solute to a solvent reduces the vapor pressure of the solvent Raoult s law of the vapor pressure of a solution Psolution Xsolvent Psolvent Inquiry What is the vapor pressure of automobile antifreeze a 50 50 by volume mixture of ethylene glycol d 1 114 g mL M 62 07 g mol in water at 25 C a temperature at which the vapor pressure of pure water is 23 8 torr Colligative properties of Solutions Properties that depend only on the total concentration of solutes not on their chemical properties Thus a 1 M NaCl solution has the same colligative properties as 1 M KCl or 1 M NaNO3 Osmosis the process by which a pure solvent flows through a porous semipermeable membrane separating it from a solution based on the solvent The membrane can be a cell wall such as that surrounding red blood cells suspended in A normal saline B a highly saline solution such as seawater C deionized water To stop the flow requires an opposing back pressure the magnitude of which is the osmotic pressure of the solution R where M is the molar concentration of all particles in solution T is absolute temperature and R is a constant One common value of R 0 0821 L atm mol K yields osmotic pressures in units of atmospheres If more than the required pressure is applied the osmosis process can be forced to run in reverse in a process used to purify water known as reverse osmosis NOTE The pressures involved can be very high Consider the osmotic pressure produced by seawater at 25 C in which the total concentration of dissolved particles is 1 14 M R mol L atm 298 K 28 0 atm L mol K Boiling point elevation and freezing point depression Solutions boil at higher temperatures than pure solvent A molecular view of why Solutions lower temperatures Two handy equations relate boiling point elevation and freezing point depression to the concentration of all solutes in solution expressed in molality m number of moles of solute per kg of solvent Tb Kb m Tf Kf m where the K values are boiling point elevation and freezing point depression constants for the solvent and m is molality NOTE Molality is similar to molarity but note the differences kg of solvent vs L of solution Why is molality used Because pressure and temperature changes can affect the volume a solution occupies but the moles of solute and mass of the solvent are not affected and that is the ratio that defines the boiling and freezing points also freeze at van t Hoff factor i is the number of moles of particles produced by each mole of solute Sometimes it is a little less than you might think Why are the observed values sometimes less than the theoretical values Because sometimes oppositely charged ions aggregate forming ion pairs which reduce the overall number of independent particles Thus the observed elevation in boiling point of a solution is given by the equation Tb i Kb m
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