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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.

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Presentation on theme: "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."— Presentation transcript:

1 Solutions

2 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.

3 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

4 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

5 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

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

7 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

8 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

9 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.

10 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

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

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

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

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

15 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.

16 Hydrophobic ends dissolve in grease

17 Hydrophilic ends dissolve in water

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

19 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.

20 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.

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

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

23 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.

24 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.

25

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

27 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.

28 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.

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

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

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

32 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?

33 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.

34 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

35 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

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

37 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.

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

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

40 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

41 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

42 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

43 1 atm Vapor Pressure of solution Vapor Pressure of pure water

44 1 atm Freezing and boiling points of solvent

45 1 atm Freezing and boiling points of solvent

46 1 atm T f T b

47 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.

48 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.

49 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

50 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

51 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

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


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