Solutions

Occur in all phases u The solvent does the dissolving. u The solute is dissolved. u There are examples of all types of solvents dissolving all types of solvent. u We will focus on aqueous solutions.

Ways of Measuring u Molarity = moles of solute Liters of solute u % mass = Mass of solute x 100 Mass of solution Mole fraction of component A A = N A N A + N B

u Molality = moles of solute Kilograms of solvent Molality is abbreviated m u Normality - read but dont focus on it. u It is molarity x number of active pieces Ways of Measuring

Energy of Making Solutions Heat of solution ( H soln ) is the energy change for making a solution. u Most easily understood if broken into steps. u 1.Break apart solvent u 2.Break apart solute u 3. Mixing solvent and solute

1. Break apart Solvent Have to overcome attractive forces. H 1 >0 2. Break apart Solute. Have to overcome attractive forces. H 2 >0

3. Mixing solvent and solute H 3 depends on what you are mixing. If molecules can attract each other H 3 is large and negative. Molecules cant attract- H 3 is small and negative. u This explains the rule Like dissolves Like

EnergyEnergy Reactants Solution H 1 H 2 H 3 Solvent Solute and Solvent Size of H 3 helps determine whether a solution will form H 3 Solution

Types of Solvent and solutes If H soln is small and positive, a solution will still form because of entropy. u There are many more ways for them to become mixed than there is for them to stay separate.

Structure and Solubility u Water soluble molecules must have dipole moments -polar bonds. u To be soluble in nonpolar solvents the molecules must be non polar. u Read Vitamin A - Vitamin C discussion pg. 509

Soap PO-O- CH 3 CH 2 O-O- O-O-

Soap u Hydrophobic non- polar end PO-O- CH 3 CH 2 O-O- O-O-

Soap u Hydrophilic polar end PO-O- CH 3 CH 2 O-O- O-O-

PO-O- CH 3 CH 2 O-O- O-O- _

u A drop of grease in water u Grease is non-polar u Water is polar u Soap lets you dissolve the non-polar in the polar.

Hydrophobic ends dissolve in grease

Hydrophilic ends dissolve in water

u Water molecules can surround and dissolve grease. u Helps get grease out of your way.

Pressure effects u Changing the pressure doesnt affect the amount of solid or liquid that dissolves u They are incompressible. u It does affect gases.

Dissolving Gases u Pressure affects the amount of gas that can dissolve in a liquid. u The dissolved gas is at equilibrium with the gas above the liquid.

u The gas is at equilibrium with the dissolved gas in this solution. u The equilibrium is dynamic.

u If you increase the pressure the gas molecules dissolve faster. u The equilibrium is disturbed.

u The system reaches a new equilibrium with more gas dissolved. u Henrys Law. P= kC Pressure = constant x Concentration of gas The stronger the attraction of the two, the higher the constant.

Temperature Effects u Increased temperature increases the rate at which a solid dissolves. u We cant predict whether it will increase the amount of solid that dissolves. u We must read it from a graph of experimental data.

Gases are predictable u As temperature increases, solubility decreases. u Gas molecules can move fast enough to escape. u Thermal pollution.

Vapor Pressure of Solutions u A nonvolatile solvent lowers the vapor pressure of the solution. u The molecules of the solvent must overcome the force of both the other solvent molecules and the solute molecules.

Raoults Law: P soln = solvent x P solvent u Vapor pressure of the solution = mole fraction of solvent x vapor pressure of the pure solvent u Applies only to an ideal solution where the solute doesnt contribute to the vapor pressure.

Aqueous Solution Pure water u Water has a higher vapor pressure than a solution

Aqueous Solution Pure water u Water evaporates faster from for water than solution

u The water condenses faster in the solution so it should all end up there. Aqueous Solution Pure water

Practice Problem u A solution of cyclopentane with a nonvolatile compound has vapor pressure of 211 torr. If vapor pressure of the pure liquid is 313 torr, what is the mole fraction of the cyclopentane?

Please enter your answer u Determine the vapor pressure of a solution at 25 C that has 45 grams of C 6 H 12 O 6, glucose, dissolved in 72 grams of H 2 O. The vapor pressure of pure water at 25 C is 23.8 torr.

u What is the composition of a pentane-hexane solution that has a vapor pressure of 350 torr at 25ºC ? u The vapor pressures at 25ºC are pentane 511 torr hexane 150 torr. u What is the composition of the vapor? Practice Question

u Liquid-liquid solutions where both are volatile. u Modify Raoults Law to P total = P A + P B = A P A 0 + B P B 0 u P total = vapor pressure of mixture u P A 0 = vapor pressure of pure A u If this equation works then the solution is ideal. Ideal solutions

χbχb χAχA Vapor Pressure P of pure A P of pure B Vapor Pressure of solution

Deviations u If solvent has a strong affinity for solute (H bonding). u Lowers solvents ability to escape. u Lower vapor pressure than expected. u Negative deviation from Raoults law. H soln is large and negative exothermic. Endothermic H soln indicates positive deviation.

χbχb χAχA Vapor Pressure Positive deviations- Weak attraction between solute and solvent Positive ΔH soln

χbχb χAχA Vapor Pressure Negative deviations- Strong attraction between solute and solvent Negative ΔH soln

Colligative Properties u Because dissolved particles affect vapor pressure - they affect phase changes. u Colligative properties depend only on the number - not the kind of solute particles present u Useful for determining molar mass

Boiling point Elevation u Because a non-volatile solute lowers the vapor pressure it raises the boiling point. The equation is: T = K b m solute T is the change in the boiling point u K b is a constant determined by the solvent. m solute is the molality of the solute

Freezing point Depression u Because a non-volatile solute lowers the vapor pressure of the solution it lowers the freezing point. The equation is: T = -K f m solute T is the change in the freezing point u K f is a constant determined by the solvent m solute is the molality of the solute

1 atm Vapor Pressure of solution Vapor Pressure of pure water

1 atm Freezing and boiling points of solvent

1 atm Freezing and boiling points of solvent

1 atm T f T b

Electrolytes in solution u Since colligative properties only depend on the number of molecules. u Ionic compounds should have a bigger effect. u When they dissolve they dissociate. u Individual Na and Cl ions fall apart. u 1 mole of NaCl makes 2 moles of ions. u 1mole Al(NO 3 ) 3 makes 4 moles ions.

u Electrolytes have a bigger impact on on melting and freezing points per mole because they make more pieces. Relationship is expressed using the vant Hoff factor i i = Moles of particles in solution Moles of solute dissolved u The expected value can be determined from the formula of the compound.

u The actual value is usually less because u At any given instant some of the ions in solution will be paired up. u Ion pairing increases with concentration. i decreases with increasing concentration. u We can change our formulas to = iKm

LAB u Purpose: to experimentally determine the vant Hoff factor for sodium chloride u Materials and equipment Sodium chloride Water Food coloring BeakersThermometer Graduated cylinderIce cube tray Foam cup

Lab u 1. Make approximately 0.50 m, 1.0 m, and 1.5 m NaCl solutions u 2. Add a different color of food coloring for each u 3. Put in labeled ice tray u 4. Freeze overnight u 5. Melt the ice cubes in their own solutions and determine the freezing point depression

Lab u Calculations u 1. Determine the vant Hoff factor for sodium chloride in each solution. u Error analysis and conclusion