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Ch 12. Properties of Solutions; Mixtures of Substances at the Molecular Level

Ch 12. Properties of Solutions; Mixtures of Substances at the Molecular Level. Brady & Senese, 5th Ed. Index. 12.1. Substances mix spontaneously when there is no energy barrier to mixing 12.2. H eats of solution come from unbalanced intermolecular attractions

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Ch 12. Properties of Solutions; Mixtures of Substances at the Molecular Level

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  1. Ch 12. Properties of Solutions; Mixtures of Substances at the Molecular Level Brady & Senese, 5th Ed.

  2. Index 12.1. Substances mix spontaneously when there is no energy barrier to mixing 12.2. Heats of solution come from unbalanced intermolecular attractions 12.3. A substance's solubility changes with temperature 12.4. Gases become more soluble at higher pressures 12.5. Molarity changes with temperature; molality, weight percentages, and mole fractions do not 12.6. Solutes lower the vapor pressure of a solvent 12.7. Solutions have lower freezing points and higher boiling points than pure solvents 12.8. Osmosis is flow of solvent through a semipermeable membrane due to unequal concentrations 12.9. Ionic solutes affect colligative properties differently than nonionic solutes

  3. Mixing Processes • Mixing occurs due to interaction between molecules “like dissolves like” • As partition is removed, molecules are able to move freely and interact • Mixed state is statistically more probable 12.1. Substances mix spontaneously when there is no energy barrier to mixing

  4. The Process Of Dissolution • Polar solutes dissolve in polar solvents • Non-polar solutes dissolve in non-polar solvents • Dipoles of solvent may induce dipoles in solute, effecting dissolution 12.1. Substances mix spontaneously when there is no energy barrier to mixing

  5. Miscibility of Liquids • Liquids that can dissolve in one another are miscible, while insoluble liquids are immiscible • Ethanol and water are miscible, while benzene and water are not 12.1. Substances mix spontaneously when there is no energy barrier to mixing

  6. Learning Check Which of the following are miscible in water? water ammonia carbon disulfide acetic acid 12.1. Substances mix spontaneously when there is no energy barrier to mixing

  7. Your Turn! Which of the following are likely to be miscible with water? • CH3CH2CH2CH3 • C6H6 • CH3CO2H • All are expected to be miscible 12.1. Substances mix spontaneously when there is no energy barrier to mixing

  8. Dissolution Of An Ionic Compound In Water • Positive end of the dipole of the water surrounds the anions of the ionic solid, extracting them from the lattice • Negative end of the dipole orients toward the cations, surrounding and extracting them from the lattice 12.1. Substances mix spontaneously when there is no energy barrier to mixing

  9. Dissolution Of A Polar Compound In Water Dipole of the water interacts with the oppositely charged dipoles of the solid, extracting them from the crystal 12.1. Substances mix spontaneously when there is no energy barrier to mixing

  10. Enthalpy (Heat) Of Solution • Heat of solution (Ηsoln ) is the energy exchanged when a solute dissolves in a solvent at constant pressure • Enthalpy is a state function: the pathway can be written in any way and the result will be the same • When Ηsoln=0, solution is called an ideal solution 12.2. Enthalpy of solution comes from unbalanced intermolecular attractions

  11. Dissolution Of An Ionic Solid • Visualized in steps: • step1: ionic solid breaks apart into vapor phase lattice energy (U) • step 2: vapor phase interacts with solvent solvation energy (ΔHsolv); if solvent is water, (Ηhydration) Ηsoln (ion in water)= U + Ηsolvation 12.2. Heats of solution come from unbalanced intermolecular attractions

  12. Dissolution: Liquid In Liquid • Step1: solute expands • Step2: solvent expands • Step 3 solute & solvent mix • If the Ηsoln=0, we have an ideal solution Ηsoln = Η1 + Η2 + Η3 12.2. Heats of solution come from unbalanced intermolecular attractions

  13. Dissolution: Liquid in Liquid (Ideal) 12.2. Heats of solution come from unbalanced intermolecular attractions

  14. Dissolution: Gas In Liquid • step1: expansion of solvent • step2: mixing • Ηsoln = Η1 + Η2 12.2. Heats of solution come from unbalanced intermolecular attractions

  15. Your Turn! What factor does not affect the value of ΔHsoln ? • The polarities of solute and solvent • The size of the solute • The charge on the solute • The temperature of the solution • All affect the value 12.2. Heats of solution come from unbalanced intermolecular attractions

  16. Saturated Solutions • Solute is at equilibrium with the dissolved solute • Addition of more dissolved solute results in supersaturation and precipitation of excess solid • The presence of less solute than the solubility results in an unsaturated solution 12.3. A substance's solubility changes with temperature

  17. Solubility Varies With Temperature • Solubility may increase or decrease with increasing temperature • The extent to which temperature has an effect is specific to the solute and solvent • Most gases are less soluble in water at high temperature, while most solids are more soluble 12.3. A substance's solubility changes with temperature

  18. Case Study: Dead Zones During the industrial revolution, factories were built on rivers so that the river water could be used as a coolant for the machinery. The hot water was dumped back into the river and cool water recirculated. After some time, the rivers began to darken and many fish died. The water was not found to be contaminated by the machinery. What was the cause of the mysterious fish kills? increased temperature lowered amounts of dissolved oxygen 12.3. A substance's solubility changes with temperature

  19. Effects Of Temperature On Solubility • Solubility varies with temperature according to the enthalpy of solvation • The efficiency of a solvation process (K) depends on the enthalpy (ΔH) in Joules, the ideal gas constant (R), and the temperature (T) in Kelvin • If the dissolution process is endothermic (ΔΗ is +), increasing temperature results in greater efficiency 12.3. A substance's solubility changes with temperature

  20. Your Turn! The solubility of a substances increases with increased temperature if: • ΔHsolution >0 • ΔHsolution <0 • ΔHsolution =0 12.3. A substance's solubility changes with temperature

  21. Pressure Effects On Solubility Of Gases • Cg=kHPg • C = concentration of dissolved gas (M) • kH = Henry’s Constant • P = pressure applied to system (mm Hg) • kH (M/mm Hg) • N2 8.42×10 -7 • O2 1.66×10-4 • CO2 4.48×10-5 • Gases are all more soluble at higher pressures (the cause of “the bends”) 12.4. Gases become more soluble at higher pressures

  22. Learning Check What is the concentration of dissolved nitrogen in a solution that is saturated in N2 at 2.0 atm kH= 8.42×10 -7 (M / mm Hg) • Cg=kHPg • Cg= 8.42×10 -7 (M / mm Hg) × 2.0 atm × 760 mmHg/atm • Cg=1.3 ×10-3M 12.4. Gases become more soluble at higher pressures

  23. Your Turn! When you open a bottle of seltzer, it fizzes. How should you store it to increase the time before it goes flat? • Heat it and pressurize it • Cool it and pressurize it • Heat it and reduce the pressure • Cool it and reduce the pressure 12.4. Gases become more soluble at higher pressures

  24. Units Of Concentration • Molarity (M) = moles solute / L solution • changes with Temperature • Molality (m) = moles solute/kg solvent • mole fraction (X) • X = moles component/ total moles • Percent by mass (%) • (mass solute / mass solution)*100 12.5. Molarity changes with temperature; molality, weight percentages, and mole fractions do not

  25. Units Of Very Low Concentrations • Parts per million (ppm) • μg solute/mL soln • Parts per billion (ppb) • ng solute/ mL soln • In extremely dilute solutions mostly solvent is present • When the solvent is water (d≈1g/mL) thus for ppm ≈μg solute/g soln • 1/106 magnitude difference leads to the name 1 part per 1 billion 12.5. Molarity changes with temperature; molality, weight percentages, and mole fractions do not

  26. Organize Your Thoughts! • All concentration units are a ratio of information • Develop a sense of the data that you have available 12.5. Molarity changes with temperature; molality, weight percentages, and mole fractions do not

  27. 1.0 1 L Learning Check: What Does Molarity Tell Us? M=moles solute/L solution. What are the m, X, % and ppm concentration of a 1.0M solution of KCl with a density of 0.99 g/mL m = 1.1 X = 0.019 % =7.5 ppm=7.5(104) 74.55 990 915.44 51.815 50.815 12.5. Molarity changes with temperature; molality, weight percentages, and mole fractions do not

  28. 1000 1.0 Learning Check: What Does Molality Tell Us? m=moles solute/kg solvent.What are the M, X, % and ppm concentration of 1.0 m KCl with a density of 0.98 g/mL M = 0.91 X = 0.018 % = 6.9 ppm =6.9×104 74.55 1074.55 1096 mL =1.096 L 55.51 56.51 12.5. Molarity changes with temperature; molality, weight percentages, and mole fractions do not

  29. 0.060 1 Learning Check: What Does Mole Fraction Mean? Xsolute= moles solute/moles total. What are the M, m, % and ppm concentration of a solution that has XKCl = 0.060 with a density of 0.87 g/mL M =2.4 m = 3.5 % = 21 ppm =1.8×105 4.473 16.93 21.403 24.601 mL =.024601 L 0.94 12.5. Molarity changes with temperature; molality, weight percentages, and mole fractions do not

  30. 1.05 100 Learning Check: What Does % Mass Tell Us %=(mass solute/mass solution) x 100. What are the M, m, X and ppm concentration of a 1.05 % KCl solution with a density of 1.15 g/mL M =0.162 m = 0.142 X = 2.26×10-4 ppm =1.21×104 98.95 86.957 mL =.086957 L .0140843 54.9255 55.0664 12.5. Molarity changes with temperature; molality, weight percentages, and mole fractions do not

  31. Your turn! Which of the following corresponds to a 3.5M solution of NaCl with a density of 0.997 g/mL? MM H2O: 18.0153; NaCl: 58.443 12.5. Molarity changes with temperature; molality, weight percentages, and mole fractions do not

  32. Raoult’s Law • Vapor pressure of a liquid varies as a function of purity • X= mole fraction of solvent • P0= vapor pressure of pure solvent • Psolution=XsolventP0solvent • Psolution=XAP0A+XBPB0 • Where A and B are both volatile components. 12.6. Solutes lower the vapor pressure of a solvent

  33. Learning Check The vapor pressure of 2-methylheptane is 233.95 torr at 55°C. 3-ethylpentane has a vapor pressure of 207.68 at the same temperature. What would be the pressure of the mixture of 78.0g 2-methylheptane and 15 g 3-ethylpentane? • Psolution=XAP0A+XBP0B • mole 2-methylheptane : 78.0g/114.23 g/mol = 0.68283 mol • mole 3-ethylpentane: 15g/100.2 g/mol = 0.1497 mol • X2-methylheptane=0.8202 ; X3-ethylpentane =1-0.8202 = 0.1798 P = 230 torr 12.6. Solutes lower the vapor pressure of a solvent

  34. Learning Check The vapor pressure of 2-methyl hexane is 37.986 torr at 15°C. What would be the pressure of the mixture of 78.0g 2-methylhexane and 15 g naphthalene which is nearly non-volatile at this temperature? • Psolution=XsolventP0solvent • mol 2-methylhexane: 78.0g/100.2 g/mol = 0.778443 mol • mol naphthalene: 15 g/128.17 g/mol = 0.11703 • X2-methylhexane = 0.869309 • Psolution = 0.869309×37.986 torr • P=33.02 torr 12.6. Solutes lower the vapor pressure of a solvent

  35. Your Turn! n-hexane and n-heptane are miscible in a large degree and both volatile. If the vapor pressure of pure hexane is 151.28 mm Hg and heptane is 45.67 at 25º, which equation can be used to determine the mole fraction of hexane in the mixture if the mixture’s vapor pressure is 145.5 mm Hg? • X(151.28 mmHg)=145.5 mmHg • X(151.28 mmHg) + (X)(45.67 mm Hg) = 145.5 mmHg • X(151.28 mmHg)+(1-X)(45.67 mm Hg)=145.5 mm Hg • None of these 12.6. Solutes lower the vapor pressure of a solvent

  36. Solute Effects On Phase Changes: • Regardless of the identity of the dissolved particles, the presence of an impurity will result in a change in the boiling point and freezing point. • The effect is solely dependent on the nature of the solvent, a factor labeled K, and the concentration of particles present (m) • ΔT=mK • boiling point elevation ΔT=Tmix-Tpure • freezing Point Depression ΔT=Tpure-Tmix 12.7. Solutions have lower freezing points and higher boiling points than pure solvents

  37. Effects Of Impurities On Phase Changes 12.7. Solutions have lower freezing points and higher boiling points than pure solvents

  38. Some BP/FP Constants 12.7. Solutions have lower freezing points and higher boiling points than pure solvents

  39. Learning Check According to the Sierra™ Antifreeze literature, the freezing point of a 40/60 solution of sierra antifreeze and water is -4 °F. What is the molality of the solution? -4°F = 1.8 (°C) + 32 -20. °C 11=m 12.7. Solutions have lower freezing points and higher boiling points than pure solvents

  40. from before: -4°F = 1.8 (°C) + 32 =-20. °C 10.75=m Learning Check: In the previous sample of a Sierra™ antifreeze mixture, 100 mL is known to contain 42 g of the antifreeze and 60. g of water, what is the molar mass of the compound found in this antifreeze if it has a freezing point of -4°F? 0.6452 mol solute 650 g/mol solute 12.7. Solutions have lower freezing points and higher boiling points than pure solvents

  41. Learning Check: In the previous sample of a Sierra™ antifreeze mixture, the freezing point is -4°F?What will be its boiling point? from before: -4°F = 1.8 (°C) + 32 =-20. °C T=105 °C 12.7. Solutions have lower freezing points and higher boiling points than pure solvents

  42. Your Turn! Beer is known to be around a 5% ethanol (C2H5OH) solution with a density of 1.05 g/mL. What is its expected boiling point?( Kf=0.51°/m) • 100ºC • 101ºC • 102ºC • 103ºC • Not enough information given MM: H2O=18.0153; C2H5OH=46.069 12.7. Solutions have lower freezing points and higher boiling points than pure solvents

  43. Osmosis • When two solutions are separated by a semi-permeable membrane, solvent molecules flow from areas of low concentration to areas of high concentration • As this occurs, the height of liquid rises in the higher concentration solution, building up “Osmotic pressure” (π) 12.8. Osmosis is flow of solvent through a semipermeable membrane due to unequal concentrations

  44. Relative Concentration Terms In Osmosis • Hypotonicsolutions have lower ion concentrations than the cells. • Hypertonicsolutions have higher ion concentrations than the cells • Isotonicsolutions have the same ion concentration as the cells 12.8. Osmosis is flow of solvent through a semipermeable membrane due to unequal concentrations

  45. Osmosis • π=MRT • the concentration, is in molarity, M • T=Temperature, in Kelvin • R=Ideal Gas Constant, 0.082057 L·atm/mol·K • The basis for kidney function, rising sap, and dialysis 12.8. Osmosis is flow of solvent through a semipermeable membrane due to unequal concentrations

  46. Learning Check: Osmosis A solution of D5W, 5% dextrose (C6H1206) in water is placed into the osmometer shown at right. It has a density of 1.0 g/mL. The surroundings are filled with distilled water. What is the expected osmotic pressure at 25°C? 12.8. Osmosis is flow of solvent through a semipermeable membrane due to unequal concentrations

  47. Learning Check For a typical blood plasma, the osmotic pressure at body temperature (37°C) is 5409 mm Hg. If the dominant solute is serum protein, what is the concentration of serum protein? 12.8. Osmosis is flow of solvent through a semipermeable membrane due to unequal concentrations

  48. Dialysis • Pores on the semi-permeable membrane may be of varied size • In dialysis, the pores are fairly large, allowing transfer of solvent, ions, and small proteins • Larger cells, such as red blood cells are prevented from passing through the pores • The dialysis bath may be enriched in substances lacking in the blood, and is hypotonic in waste products in the blood • Exchange of vital components may be made 12.8. Osmosis is flow of solvent through a semipermeable membrane due to unequal concentrations

  49. Your Turn! Suppose that your tap water has 250 ppb of dissolved H2S, and that its density is about 1.0 g/mL. What is its osmotic pressure at 25°C? • 0.00058 atm • 0.064 atm • 0.059 atm • None of these 0.21 atm MM: H2S=34.076 12.8. Osmosis is flow of solvent through a semipermeable membrane due to unequal concentrations

  50. Ionic Solutes Affect Colligative Properties Differently Than Non-ionic Solutes • substances that ionize make more particles in a solution than their own concentration suggests • i is a factor that demonstrates how many ions are formed per formula unit or molecule • the apparent molality of particles is then im. 12.9. Ionic solutes affect colligative properties differently than nonionic solutes

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