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:
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 solvent 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 (unless you plan on being an Old School Biologist) 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. Molecules can attract each other H 3 is large and negative. Molecules cant attract- H 3 is small and negative. Solutions which form are typically exothermic overall! 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 determines 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 Typically, greater forms of disorder are favored over ordered matter.
Structure and Solubility u Water soluble molecules must have dipole moments -polar bonds. u To be soluble in non polar solvents the molecules must be non polar. u I 2 is very slightly soluble in water, but becomes more soluble in a concentrated KI solution…Explain.
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 effect the amount of solid or liquid that dissolves u They are incompressible. u It does effect gases.
Dissolving Gases u Pressure effects 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 Examples: 1. Soda Can 2. Lake Nyos Tragedy
Lake Nyos Tragedy Suffocated 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.
Degassing Lake Nyos
Temperature Effects u In general: Increased temperature increases the rate at which a solid dissolves. u However, we cant always predict whether it will increase the amount of solid that dissolves. u Consult a graph of experimental data for the best results!
Gases are predictable u As temperature increases, solubility decreases. u Gas molecules can move fast enough to escape the solution. u Thermal pollution of lakes.
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 times the vapor pressure of the pure solvent u Applies only to an ideal solution where the solute doesnt contribute to the vapor pressure.
Raoults Law Example Determine the vapor pressure of a solution made by dissolving 125 g of sucrose into 455 mL of water at 25.0 º C. u The vapor pressure of water at this temp is torr. (Density of water at this temp is g/cm 3 )
Solution (get it?) u 125g sucrose =.365 mol u 155 mL of water = 8.58 mol of water =.959 u Pressure = 22.8 torr (down from torr)
Ionic vs Covalent Solute? u What if we used the same moles of salt rather than sucrose? u Same result? u I dont think so…you get twice the number of solute particles = twice the effective mole fraction of solute! of water drops to.921 from.959…so the vapor pressure drops as well!
Aqueous Solution Pure water u Water has a higher vapor pressure than a solution
Aqueous Solution Pure water u Water evaporates faster from the water than the solution…
u The water condenses faster in the solution so it should all end up there. Aqueous Solution Pure water
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. Nonideal solution: both can evaporate u What are the component vapor pressures? Review Question
u Nonideal Solutions: P total =P A + P B P soln = sol A x P sol A + sol B x P sol B 350 torr = A 511 torr + 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 =.446 Check it in the original equation!
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
Freezing/Boiling Point u Addition of a nonvolatile solute extends the liquid phase! u Decreasing the freezing point of the solution… u Increasing the boiling point of the solution!
1 atm Vapor Pressure of solution Vapor Pressure of pure water
1 atm Freezing and boiling points of water
1 atm Freezing and boiling points of solution
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 greater 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 more significant impact on on melting and freezing points per mole because they generate more particles. 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.
u The 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. u We can change our formulas to T = imK
u Liquid-liquid solutions where both are volatile. u Modify Raoults Law to P total = P A + P B = A P A 0 + A 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. u Solvent and solute are alike. Ideal solutions
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.
Osmotic Pressure = MRT = osmotic pressure u M = molarity (not molality) u R = gas law constant u T = Kelvin temp For electrolyte solutions: = iMRT