CBIO 2200 1nd Edition Lecture 5 Outline of Last Lecture I Units of Concentration II The Solution Process III Factors Affecting Solubility Pressure and Temperature Outline of Current Lecture I Colligative Properties II Colloids Current Lecture III Colligative Properties a Colligative properties properties that depend on the relative numbers of solute and solvent particles in a solution and not on their identity b Ex if you dissolve salt in water it will lower its freezing point and raise its boiling point c Changes in vapor pressure Raoult s Law i Equilibrium vapor pressure the pressure of vapor when the liquid and vapor are in equilibrium ii Solvent vapor pressure is proportional to the relative number of solvent molecules iii 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 ideal vi In or to use this equation the forces of attraction between solute and solvent must be equal to those between the solvent molecules 1 If solvent solute forces solvent solvent forces its actual vapor pressure will be lower than calculated These 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 vapor pressure will be higher than calculated vii Adding nonvolatile solution to a solvent lowers the vapor pressure of the solvent 1 Raoult s Law can be modified to calculate the lowering of vapor pressure 2 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 1 1 Xsolvent Xsolute 1 ix The decrease in vapor pressure is proportional to the mole fraction of solute d Boiling point elevation i The boiling point elevation change in Tbp is directly proportional to the molality of the solution ii Elevation in boiling point change in Tbp Kbpmsolute 1 Kbp proportionality constant called molal boiling point elevation constant a Units are degrees molal oC m b Kbp corresponds to the elevation in boiling point for a 1 m solution iii Practical consequences of the elevation of boiling point of a solvent on adding a solute 1 Summer protection your car s engine receives from all season antifreeze a Its key ingredient is ethylene glycol nonvolatile substance raises the boiling point of the solution in the radiator e Freezing point depression i Dissolving a solute in a solvent will lower the freezing point of the solution ii Freezing point depression change in Tfp Kfpmsolute iii Kfp is the freezing point depression constant 1 Values are negative quantities giving us a change in temperature iv Antifreeze also lowers freezing temp of solution in radiator so it does not freeze in the winter f Osmotic Pressure i Osmosis the movement of solvent molecules through a semipermeable membrane from a region of lower solute concentration to a region of higher solute concentration IV ii Membrane does not act as a barrier to the solvent such as water so it allows it to pass through iii This continues until there is not net movement of solvent from side to side equilibrium is reached by osmotic pressure iv Osmotic pressure when the pressure created by the column of liquid equals the pressure of the water moving through the membrane 1 Osmotic pressure 2 Osmotic pressure and concentration can be related by an equation a cRT b c is the molar concentration moles L c R is the gas law constant 082057 Latm Kmol g Colligative properties of solutions containing ions i NaCl 1 Most common substance to lower freezing point of water 2 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 well 4 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 three times greater iii The ratio of the observed change in Tfp to the value calculated is called the van t Hoff factor i 1 i Tfp measured Tfp calculated Tfp measured Kfp m 2 Tfp measured Kfp x m x i iv The van t Hoff factor approaches a whole number 2 3 so on only with very dilute solutions v In more concentrated solutions and solvents less polar than water ions are extensively associated in ion pairs Colloids a True solution a solution in which the solute does not settle and solute particles should be in the form of ions or relatively small moelcules i Examples of true solutions NaCl and sugar b In a suspension the solute is added and then settles to the bottom c Colloids aka colloidal dispersions represent a state in between a solution and suspension i Examples of colloids JELL O milk fog and porcelain d Term colloid was coined by chemist Thomas Graham i Found that substances such as starch and glue diffused very slowly in water and they differ in their ability to diffuse through a thin membrane ii This term describes a class of substances that are distinctly different from solutions and suspensions e Two distinguishing characteristics i Generally have high molar masses ii Particles are relatively large 1 Exhibit Tyndall effect they scatter visible light when dispersed in a substance making the mixture appear cloudy iii Even though colloidal particles are large they are not so large that they settle out f Sol colloidal dispersion of a solid substance in a fluid medium g Gel colloidal dispersion that has a structure that prevent it from being mobile h Types of colloids i Classified according to the state of the dispersed phase and the dispersing medium ii Hydrophobic or hydrophilic colloids colloids with water as the dispersing medium iii In hydrophobic colloids there are only weak attractive forces between the water and the surface of the colloid iv Hydrophilic colloids are strongly attracted to water molecules 1 Often have groups such as OH and NH2 on their surfaces 2 Form strong hydrogen bonds to water and stabilize the colloid e g proteins and starch v Emulsions colloidal dispersions of one liquid in another e g oil or fat in water 1 Nonpolar oil molecules do not interact with polar water molecules