CBIO 2200 1nd Edition Lecture 5 Outline of Last Lecture I. Units of ConcentrationII. The Solution ProcessIII. Factors Affecting Solubility: Pressure and TemperatureOutline of Current Lecture I. Colligative PropertiesII. ColloidsCurrent LectureIII. Colligative Propertiesa. Colligative properties: properties that depend on the relative numbers of solute and solvent particles in a solution and not on their identityb. Ex: if you dissolve salt in water, it will lower its freezing point and raise its boiling pointc. Changes in vapor pressure: Raoult’s Lawi. Equilibrium vapor pressure: the pressure of vapor when the liquid and vapor are in equilibriumii. Solvent vapor pressure is proportional to the relative number of solvent moleculesiii. Psolvent = (Xsolvent)(Posolvent)iv. This equation states that the vapor pressure of solvent over a solution (Psolvent) is a fraction of the pure solvent equilibrium pressure (Posolvent)v. An ideal solution is one that obeys Raoult’s Law, but no solution is truly idealvi. In or to use this equation, the forces of attraction between solute and solvent must be equal to those between the solvent molecules1. If solvent – solute forces > solvent – solvent forces, its actual vaporpressure will be lower than calculatedThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.2. If solvent – solute forces < solvent – solvent forces, its actual vaporpressure will be higher than calculatedvii. Adding nonvolatile solution to a solvent lowers the vapor pressure of the solvent1. Raoult’s Law can be modified to calculate the lowering of vapor pressure2. Change in Psolvent = Psolvent – Posolvent viii. If a solution that has only volatile solvent and nonvolatile solute, the sum of the mole fractions must be 11. Xsolvent + Xsolute = 1ix. The decrease in vapor pressure is proportional to the mole fraction of soluted. Boiling point elevationi. The boiling point elevation (change in Tbp) is directly proportional to the molality of the solutionii. Elevation in boiling point = change in Tbp = Kbpmsolute1. Kbp = proportionality constant called molal boiling point elevation constanta. Units are degrees/molal (oC/m)b. Kbp corresponds to the elevation in boiling point for a 1 m solutioniii. Practical consequences of the elevation of boiling point of a solvent on adding a solute1. Summer protection your car’s engine receives from “all-season” antifreezea. Its key ingredient is ethylene glycol (nonvolatile substance) raises the boiling point of the solution in the radiatore. Freezing point depressioni. Dissolving a solute in a solvent will lower the freezing point of the solutionii. Freezing point depression = change in Tfp = Kfpmsoluteiii. Kfp is the freezing point depression constant1. Values are negative quantities, giving us a – change in temperatureiv. Antifreeze also lowers freezing temp of solution in radiator so it does not freeze in the winterf. Osmotic Pressurei. Osmosis: the movement of solvent molecules through a semipermeable membrane from a region of lower solute concentration to a region of higher solute concentrationii. Membrane does not act as a barrier to the solvent (such as water) so it allows it to pass throughiii. This continues until there is not net movement of solvent from side to side; equilibrium is reached by osmotic pressureiv. Osmotic pressure: when the pressure created by the column of liquid equals the pressure of the water moving through the membrane1. Osmotic pressure (Π)2. Osmotic pressure and concentration can be related by an equationa. Π = cRTb. c is the molar concentration (moles/L)c. R is the gas law constant - .082057 Latm/Kmolg. Colligative properties of solutions containing ionsi. NaCl1. Most common substance to lower freezing point of water2. Effective because it is an electrolyte (dissolve into ions)3. Because colligative properties depend on the number of particles of solute per solvent particle, NaCl works well4. 1 mol of NaCL dissolves into 2 moles of ions, which means the effect on the freezing point of water should be twice as large as molecules only containing 1 mole (e.g. sugar)ii. Other ionic compounds act in the same way (e.g. Na2SO4 is three times larger); they approach, but do not actually reach the value of two or threetimes greateriii. The ratio of the observed change in Tfp to the value calculated is called thevan’t Hoff factor (i)1. i = ΔTfp, measured/ΔTfp calculated = ΔTfp measured/Kfp m2. ΔTfp measured = Kfp x m x iiv. The van’t Hoff factor approaches a whole number (2,3, so on…) only with very dilute solutionsv. In more concentrated solutions, and solvents less polar than water, ions are extensively associated in ion pairsIV. Colloidsa. True solution: a solution in which the solute does not settle, and solute particles should be in the form of ions or relatively small moelculesi. Examples of true solutions: NaCl and sugarb. In a suspension, the solute is added and then settles to the bottomc. Colloids (aka colloidal dispersions) represent a state in between a solution and suspension i. Examples of colloids: JELL-O, milk, fog, and porcelaind. Term colloid was coined by chemist Thomas Grahami. Found that substances such as starch and glue diffused very slowly in water, and they differ in their ability to diffuse through a thin membraneii. This term describes a class of substances that are distinctly different from solutions and suspensionse. Two distinguishing characteristics:i. Generally have high molar massesii. Particles are relatively large1. Exhibit Tyndall effect: they scatter visible light when dispersed in a substance making the mixture appear cloudyiii. Even though colloidal particles are large, they are not so large that they settle outf. Sol: colloidal dispersion of a solid substance in a fluid mediumg. Gel: colloidal dispersion that has a structure that prevent it from being mobileh. Types of colloidsi. Classified according to the state of the dispersed phase and the dispersingmediumii. Hydrophobic or hydrophilic colloids: colloids with water as the dispersing mediumiii. In hydrophobic colloids, there are only weak attractive forces between the water and the surface of the colloidiv. Hydrophilic colloids are strongly attracted to water molecules1. Often have groups such as – OH and – NH2 on their surfaces2. Form strong hydrogen bonds to water and stabilize the