A positive attitude may not solve all your problems, but it will annoy enough people to make it worth the effort. Herm AlbrightHerm Albright ( )

Colligative Properties Properties determined by the number of particles in solution rather than the type of particles. Vapor Pressure Freezing Point Boiling Point Osmotic Pressure

How Vapor Pressure Depression Occurs Nonvolatile solute particles take up space in a solution. Nonvolatile solute particles on surface decrease # of solvent particles on the surface. Less solvent particles can evaporate which lowers the vapor pressure of a liquid.

Vapor Pressures of Pure Water and a Water Solution The vapor pressure of water over pure water is greater than the vapor pressure of water over an aqueous solution containing a nonvolatile solute. Solute particles take up surface area and lower the vapor pressure

Raoult’s Law Vapor pressure of a solution varies directly as the mole fraction of solvent Vapor Pressure Solution = (mole fraction solvent )(Vapor Pressure solvent ) Or P solution = X solvent P o solvent

Raoult’s Law A SOLUTION WITH 0.5 MOLE FRACTION OF THE SOLVENT WOULD HAVE A VAPOR PRESSURE 0.5 TIMES THE VAPOR PRESSURE OF THE SOLVENT.

Ideal Solution All intermolecular attractions are the same. Solute-Solute Solvent- Solvent Solute-Solvent Attractions are the same

Molecular view of Raoult’s law: Boiling point elevation In a solution vapor pressure is reduced compared to pure solvent In a solution vapor pressure is reduced compared to pure solvent Liquid boils when vapor pressure = atmospheric pressure Liquid boils when vapor pressure = atmospheric pressure Must increase T to make vapor pressure = atmospheric pressure Must increase T to make vapor pressure = atmospheric pressure

The molal boiling point constant, K b, is the boiling point elevation of the solvent in a 1 molal solution of a nonvolatile, non electrolyte solute. Boiling point elevation, ∆t b, is the difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent, and it is directly proportional to the molal concentration of the solution. ∆t b =K b *m = boiling point elevation K b = boiling point constant m = molality of solute

MOLAL CONCENTRATION, m, IS EQUAL TO THE NUMBER OF MOLES OF A SOLUTE PER 1000 g OF SOLVENT. m = #moles solute / # kg solvent

SOLVENTK B ( o C/m) Water0.512 Ethanol1.19 Benzene2.53 Cyclohexane2.79 Acetic acid3.07 Nitrobenzene5.24 Camphor5.96

WHAT IS THE BOILING POINT ELEVATION OF A 1.5 m SOLUTION OF SUCROSE (TABLE SUGAR) IN WATER?  T b = K B m = x 1.5 = o C WHAT WOULD IT BE IF YOU HAD: a.1.5 m NaCl b. 1.5 m CaCl 2 c. 1.5 m AlCl 3 WHY THE DIFFERENCE?

What about Electrolytes? Electrolytes break apart into ions. Each ion has an effect on boiling point and freezing points. If a solution has more ions it will change the boiling points and melting points even more.  T b = ik b m where I = number of ions a formula unit can break down into NaCl  Na + + Cl- i = 2 CaCl 2  Ca +2 + Cl - i = 3 AlCl 3  Al Cl - i = 4 i is called the van’t hoff factor

FREEZING POINT DEPRESSION THE FREEZING POINT IS DEPRESSED IN A SIMILAR FASHION AS BOILING POINT ELEVATION. IN THIS CASE, THE SOLUTE PARTICLES INTERFERE WITH THE FORMATION OF THE CRYSTALLINE (SOLID) SOLVENT, AND MORE ENERGY MUST BE REMOVED FROM THE SYSTEM FOR FREEZING TO OCCUR.  T F = -K F m = freezing point depression K F = freezing point depression constant m = molality of the solute Or  T F = -iK F m for an ionic solute

SOLVENTK f ( o C/m) Water1.86 Acetic acid3.90 Benzene5.12 Nitrobenzene7.00 Phenol7.40 Cyclohexane20.2

CALCULATE THE FREEZING POINT OF A 2.0 m SOLUTION OF GLUCOSE. K F FOR WATER = 1.86  T F = -iK F m = -1 x 1.86 x 2 = o C WHAT WOULD BE THE FREEZING POINTS FOR THE FOLLOWING: A.2 m SOLUTION OF NaCl B.2 m SOLUTION OF CaCl 2 C. 2 m SOLUTION OF AlCl 3

THE OCEAN DOES NOT FREEZE AT 0 o C, BUT AT -2.2 o C BECAUSE OF THE SALT CONTENT. THIS IS WHY WE PUT SALT ON ROADS IN WINTER TIME. AND, IT IS WHY WE PUT ETHYLENE GLYCOL IN THE WATER IN OUR CAR RADIATORS IN WINTER TIME. SINCE WATER EXPANDS ON FREEZING, IF WE DID NOT ADD SOMETHING TO DEPRESS FREEZING POINT, THE WATER WOULD EXPAND, AND THE RADIATOR WOULD CRACK.

LET’S CONSIDER OSMOSIS. WHEN WE HAVE A MEMBRANE THAT WILL ALLOW SOLVENT PARTICLES TO MOVE THROUGH, BUT NOT SOLUTE PARTICLES, THE SOLVENT PARTICLES WILL MOVE FROM THE LESS CONCENTRATED SIDE OF THE MEMBRANE TO THE MORE CONCENTRATED SIDE. THIS IS OSMOSIS, AND THE PROCESS IS PICTURED ON THE NEXT SLIDE.

SEMIPERMEABLE MEMBRANE

THINK ABOUT IT LIKE THIS. ON THE SIDE WHERE YOU HAVE MORE SOLVENT MOLECULES, YOU HAVE MORE SOLVENT MOLECULES COLLIDING WITH THE MEMBRANE, SO YOU HAVE MORE SOLVENT MOLECULES MOVING THROUGH. OSMOTIC PRESSURE IS THE PRESSURE YOU WOULD HAVE TO EXERT FROM THE SIDE WITH THE HIGHER CONCENTRATION TO STOP ADDITIONAL SOLVENT MOLECULES FROM MOVING THROUGH.

YOU COULD USE AN APPARATUS LIKE THIS TO DETERMINE OSMOTIC PRESSURE.  gh WHERE  osmotic press.  = density h = height of column g = gravitational constant

OSMOTIC PRESSURE HAS BIOLOGICAL IMPLICATIONS. IF YOU PUT RED BLOOD CELLS IN DISTILLED WATER, THEY WILL EVENTUALLY SWELL UP AND BURST. IF YOU PUT THEM IN SATURATED NaCl, THEY WILL SHRIVEL UP. ANYTHING YOU PUT IN BLOOD IT SHOULD HAVE THE SAME CONCENTRATION OF SALTS AS THE BLOOD. NORMAL SALINE OR 0.16 M NaCl IS USED.