Colligative Properties

Colligative Properties  Colligative properties depend on quantity of solute molecules.  Vapor pressure lowering  Boiling point elevation  Freezing point depression  Osmotic pressure

Colligative Properties of Solutions Definition: Properties, that depend on the NUMBER of solute particles present in solution Concentration: Osmol per liter or osmolarity An OSMOL is a mole of solute particle

2 x (6 x ) C 6 H 12 O 6 does not dissociate C 6 H 12 O 6 1 M (6 x ) 1 osmol/L NaCl does dissociate Na + + Cl - 1 M 1 osmol/L 2 osmol/L

Lowering the Vapor Pressure  Non-volatile solvents reduce the ability of the surface solvent molecules to escape the liquid.  Therefore, vapor pressure is lowered.  The amount of vapor pressure lowering depends on the amount of solute. solventsolution

Vapor pressure lowering  Raoult’s Law – a nonvolatile solute will lower the vapor pressure of a solvent  Pure water will have a higher vapor pressure than salt water.  Ideal solution: one that obeys Raoult’s law.  Raoult’s law breaks down when the solvent-solvent and solute-solute intermolecular forces are greater than solute-solvent intermolecular forces.

Phase Diagram Analysis Pressure 760 torr Temperature Gas Liquid Solid ΔT m ΔT b

Boiling-Point Elevation Interpret the phase diagram for a solution. Non-volatile solute lowers the vapor pressure. At 1 atm (normal bp of pure liquid) there is a lower vapor pressure of the solution.  a higher bp is required to reach a vapor pressure of 1 atm for the solution. ΔTb = Kbmi

Solvent Bp (°C) Kb (°C m–1) Mp (°C) Km (°C m–1) Water Acetic acid Benzene Chloroform613.63–– Camphor–– Cyclohexane Freezing-Point Depression

Freezing-Point Depression  At 1 atm (normal boiling point of pure liquid) there is no depression by definition  The solution freezes at a lower temperature (ΔT f ) than the pure solvent.  Decrease in freezing point (ΔT f ) is directly proportional to molality (K f is the molal freezing-point- depression constant):  ΔT f = K f mi  Colligative properties can be used to determine the MW of an unknown compound.

1. Vapor pressure lowering 2. Freezing point depression Raoult’s law P soln = X solv P 0 P soln = vapor pressure of solution X solv = mole fraction of solvent P 0 = vapor pressure of pure solvent  T f = K f m  T f = freezing point depression m= molality K f = cryoscopic constant

3. Boiling point elevation  T b = K b m  T= boiling point elevation K b = ebullioscopic constant m= molality Cryoscopic and ebullioscopic constants Solvent KfKfKfKf KbKbKbKb Water Benzene Phenol °C kg solvent (mol solute) -1

4. Osmotic pressure  = M  RT  = osmotic pressure M = molarity R = JK -1 mol -1

Osmotic Pressure  Semipermeable membrane: permits passage of some components of a solution. Example: cell membranes and cellophane.  Osmosis: the movement of a solvent from low solute concentration to high solute concentration.  There is movement in both directions across a semipermeable membrane.  As solvent moves across the membrane, the fluid levels in the arms becomes uneven.  Eventually the pressure difference between the arms stops osmosis. Concentrated solution Membrane Dilute solution

Osmotic Pressure  Osmotic pressure, P, is the pressure required to stop osmosis.  Isotonic solutions: two solutions with the same P separated by a semipermeable membrane.  Hypotonic solutions: a solution of lower P than a hypertonic solution.  Osmosis is spontaneous.

Osmotic Pressure & Biology  Red blood cells are surrounded by semipermeable membranes.  Crenation:  red blood cells placed in hypertonic solution (a lower solute concentration exists in the cell)  osmosis occurs and water passes out of the cell causing the cell to shrivel up.  Hemolysis is opposite  red blood cells placed in a hypotonic solution (a higher solute concentration in the cell)  water moves into the cell causing the cell to burst.  To prevent crenation or hemolysis, IV (intravenous) solutions must be isotonic.

Osmotic Pressure Applications  Pickling food by placing in salty solutions.  Water moves into plants through osmosis.  Salt added to meat or sugar to fruit prevents bacterial infection (a bacterium placed on the salt will lose water through osmosis and die).  Dialysis machines work by osmosis.  Reverse osmosis is used in desert countries to produce drinking water from the sea. Salty sea water would normally draw in more water to this side Drinking water is produced by applying a pressure that exceeds P. P > P

Electrolytes ‘Anomalous’ behavior: 1.Ability to conduct electric current 2.Greater effect on colligative properties  T f values: (k f = 1.86°C for H 2 O) m 0.01 m 0.1 m Sucrose NaCl K2SO K3[Fe(CN)6]

 T f = K f m measured value expected value i =  T f K f m K f m i = van’t Hoff factor: i

 T f = i K f m  T b = i K b m  = i MRT i values: m m Infinite dilution Sucrose HCl KCl ,00 MgSO ,00 K 2 SO