2Occur in all phases The solvent does the dissolving. The solute is dissolved.There are examples of all types of solvents dissolving all types of solvent.We will focus on aqueous solutions.
3Ways of Measuring Molarity = moles of solute Liters of solvent % mass = Mass of solute x Mass of solutionMole fraction of component A cA = NA NA + NB
4Ways of Measuring Molality = moles of solute Kilograms of solvent Molality is abbreviated mNormality - read but don’t focus on it (unless you plan on being an “Old School Biologist”)
5Energy of Making Solutions Heat of solution ( DHsoln ) is the energy change for making a solution.Most easily understood if broken into steps.1.Break apart solvent2.Break apart solute3. Mixing solvent and solute
61. Break apart Solvent 2. Break apart Solute. Have to overcome attractive forces. DH1 >02. Break apart Solute.Have to overcome attractive forces. DH2 >0
73. Mixing solvent and solute DH3 depends on what you are mixing.Molecules can attract each other DH3 is large and negative.Molecules can’t attract- DH3 is small and negative.Solutions which form are typically exothermic overall!This explains the rule “Like dissolves Like”
8Size of DH3 determines whether a solution will form Solute and SolventDH3SolutionDH2DH3EnergySolventDH1ReactantsSolution
9Types of Solvent and solutes If DHsoln is small and positive, a solution will still form because of entropy.Typically, greater forms of disorder are favored over ordered matter.
10Structure and Solubility Water soluble molecules must have dipole moments -polar bonds.To be soluble in non polar solvents the molecules must be non polar.I2 is very slightly soluble in water, but becomes more soluble in a concentrated KI solution…Explain.
18Water molecules can surround and dissolve grease. “Helps get grease out of your way.”
19Pressure effectsChanging the pressure doesn’t effect the amount of solid or liquid that dissolvesThey are incompressible.It does effect gases.
20Dissolving GasesPressure effects the amount of gas that can dissolve in a liquid.The dissolved gas is at equilibrium with the gas above the liquid.
21The gas is at equilibrium with the dissolved gas in this solution. The equilibrium is dynamic.
22If you increase the pressure the gas molecules dissolve faster. The equilibrium is disturbed.
23The system reaches a new equilibrium with more gas dissolved. Henry’s Law.P= kCPressure =constant x Concentration of gasExamples:Soda CanLake Nyos Tragedy
24Lake Nyos TragedySuffocated 1,700 people within 25 kilometres (16 mi) of the lake, mostly rural villagers, as well as 3,500 livestock.300 ft fountain of water created an 82 ft wall of water which scoured the lake shore.
26Temperature EffectsIn general: Increased temperature increases the rate at which a solid dissolves.However, we can’t always predict whether it will increase the amount of solid that dissolves.Consult a graph of experimental data for the best results!
28Gases are predictable As temperature increases, solubility decreases. Gas molecules can move fast enough to escape the solution.Thermal pollution of lakes.
29Vapor Pressure of Solutions A nonvolatile solvent lowers the vapor pressure of the solution.The molecules of the solvent must overcome the force of both the other solvent molecules and the solute molecules.
30Psoln = csolvent x Psolvent Raoult’s Law:Psoln = csolvent x PsolventVapor pressure of the solution = mole fraction of solvent times the vapor pressure of the pure solventApplies only to an ideal solution where the solute doesn’t contribute to the vapor pressure.
31Raoult’s Law ExampleDetermine the vapor pressure of a solution made by dissolving 125 g of sucrose into 455 mL of water at 25.0ºC.The vapor pressure of water at this temp is torr. (Density of water at this temp is g/cm3)
32Solution (get it?) 125g sucrose = .365 mol 155 mL of water = 8.58 mol c of water = .959Pressure = 22.8 torr (down from torr)
33Ionic vs Covalent Solute? What if we used the same moles of salt rather than sucrose?Same result?I don’t think so…you get twice the number of solute particles = twice the effective mole fraction of solute!c of water drops to .921 from .959…so the vapor pressure drops as well!
34Water has a higher vapor pressure than a solution Aqueous SolutionPure water
35Water evaporates faster from the water than the solution… Aqueous SolutionPure water
36The water condenses faster in the solution so it should all end up there. Aqueous SolutionPure water
37Review QuestionWhat is the composition of a pentane-hexane solution that has a vapor pressure of 350 torr at 25ºC ?The vapor pressures at 25ºC arepentane 511 torrhexane 150 torr.Nonideal solution: both can evaporateWhat are the component vapor pressures?
38Review Question Nonideal Solutions: Ptotal=PA + PB Psoln = csol A x Psol A + csol B x Psol B350 torr = c A 511 torr + c B 150 torr(A + B = 1…so B = 1-A now you may substitute…)350 torr = 511 A+ 150 (1-A)A = .554 (so B = …) B = .446Check it in the original equation!
39Colligative Properties Because dissolved particles affect vapor pressure - they affect phase changes.Colligative properties depend only on the number - not the kind of solute particles presentUseful for determining molar mass
40Boiling point Elevation Because a non-volatile solute lowers the vapor pressure it raises the boiling point.The equation is: DT = KbmsoluteDT is the change in the boiling pointKb is a constant determined by the solvent.msolute is the molality of the solute
41Freezing point Depression Because a non-volatile solute lowers the vapor pressure of the solution it lowers the freezing point.The equation is: DT = KfmsoluteDT is the change in the freezing pointKf is a constant determined by the solventmsolute is the molality of the solute
42Freezing/Boiling Point Addition of a nonvolatile solute extends the liquid phase!Decreasing the freezing point of the solution…Increasing the boiling point of the solution!
431 atmVapor Pressure of pure waterVapor Pressure of solution
47Electrolytes in solution Since colligative properties only depend on the number of molecules.Ionic compounds should have a greater effect.When they dissolve they dissociate.Individual Na and Cl ions fall apart.1 mole of NaCl makes 2 moles of ions.1mole Al(NO3)3 makes 4 moles ions.
48Electrolytes have a more significant impact on on melting and freezing points per mole because they generate more particles.Relationship is expressed using the van’t Hoff factor: ii = Moles of particles in solutionMoles of solute dissolvedThe expected value can be determined from the formula.
49The actual value is usually less because… …at any given instant some of the ions in solution will be paired.…ion pairing increases with concentration.…i decreases with increasing concentrations.We can change our formulas toDT = imK
50Ideal solutions Liquid-liquid solutions where both are volatile. Modify Raoult’s Law toPtotal = PA + PB = cAPA0 + cAPB0Ptotal = vapor pressure of mixturePA0= vapor pressure of pure AIf this equation works then the solution is ideal.Solvent and solute are alike.
51Deviations If Solvent has a strong affinity for solute (H bonding). Lowers solvents ability to escape.Lower vapor pressure than expected.Negative deviation from Raoult’s law.DHsoln is large and negative exothermic.Endothermic DHsoln indicates positive deviation.
52Osmotic Pressure ∏ = MRT ∏ = osmotic pressure M = molarity (not molality)R = gas law constantT = Kelvin tempFor electrolyte solutions:∏ = iMRT