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Chapter 10 Properties of Solutions

GAS. Condensation. Sublimation. Deposition. Evaporation. Melting. SOLID. LIQUID. Freezing. Chapter 10 Properties of Solutions. 17.1 Solution Composition 17.2 The Thermodynamics of Solution Formation (skip) 17.3 Factors Affecting Solubility 17.4 The Vapor Pressures of Solutions

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Chapter 10 Properties of Solutions

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  1. GAS Condensation Sublimation Deposition Evaporation Melting SOLID LIQUID Freezing Chapter 10Properties of Solutions 17.1 Solution Composition 17.2 The Thermodynamics of Solution Formation (skip) 17.3 Factors Affecting Solubility 17.4 The Vapor Pressures of Solutions 17.5 Boiling-Point Elevation and Freezing-Point Depression 17.6 Osmotic Pressure 17.7 Colligative Properties of Electrolyte Solutions 17.8 Colloids

  2. Colligative Properties of Solutions • For Colligative properties, the difference between a pure solvent and dilute solution depends only on the number of solute particles present and not on their chemical identity. • Examples • Vapor Pressure Depression • Boiling Point Elevation • Melting Point Depression • Osmotic Pressure

  3. Lowering of Vapor Pressure • Vapor Pressure of a solvent above a dilute solution is always less than the vapor pressure above the pure solvent. Elevation of Boiling Point • The boiling point of a solution of a non-volatile solute in a volatile solvent always exceeds the boiling point of a pure solvent • Boiling • liquid in equilibrium with its vapor at the external pressure. • Boiling Point • Vapor press = external pressure • Normal boiling point • Vap press. = 1 atm

  4. Elevation of Boiling Point & Vapor Pressure Depression Phase diagrams for pure water (red lines) and for an aqueous solution containing a nonvolatile solution (blue lines).

  5. The solute and solvent can be any combination of solid (s), liquid (l), and gaseous (g) phases. Solution Composition Dissolution: Two (or more) substances mix at the level of individual atoms, molecules, or ions. Solution: A homogeneous mixture (mixed at level of atoms molecules or ions Solvent: The major component Solute: The minor component

  6. mass of component total mass of mixture X 100% Solution Composition Mass Fraction, Mole Fraction, Molality and Molarity Mass percentage (weight percentage): mass percentage of the component = Mole fraction: The amount of a given component (in moles) divided by the total amount (in moles) X1 = n1/(n1 + n2) for a two component system X2 = n2/(n1 + n2) = 1 – X1 orX1+X2=1

  7. Molality msolute= moles solute per kilogram solvent = moles per kg or (mol kg-1) Molarity (biochemists pay attention) csolute = moles solute per volume solution = moles per liter of solution (mol L-1)

  8. Factors Affecting Solubility • Molecular Interactions • Review chapter 4 • Polar molecules, water soluble, hydrophilic (water loving) • E.g., Vitamins B and C; water-soluble • Non-polar molecules, soluble in non-polar molecules, hydrophobic (water fearing) • E.g., Vitamins A, D, K and E; fat-soluble

  9. Factors Affecting Solubility of Gases • Structure Effects • Pressure Effects

  10. Henry’s Law (for dilute solutions) The mole fraction of volatile solute is proportional to the vapor pressure of the solute. P= kHX kH = Henry’s Law constant, X = mole fraction. Increasing the partial pressure of a gas over a liquid increases the amount of gas disolved in the liquid. kH depends on temperature.

  11. When the partial pressure of nitrogen over a sample of water at 19.4°C is 9.20 atm, the concentration of nitrogen in the water is 5.76 x 10-3 mol L-1. Compute Henry’s law constant for nitrogen in water at this temperature.

  12. When the partial pressure of nitrogen over a sample of water at 19.4°C is 9.20 atm, then the concentration of nitrogen in the water is 5.76 x 10-3 mol L-1. Compute Henry’s law constant for nitrogen in water at this temperature.

  13. When the partial pressure of nitrogen over a sample of water at 19.4°C is 9.20 atm, then the concentration of nitrogen in the water is 5.76 x 10-3 mol L-1. Compute Henry’s law constant for nitrogen in water at this temperature.

  14. Factors Affecting Solubility • Structure Effects • Pressure Effects • Temperature Effects for Aqueous Solutions The solubility of somesolids as a function of temperature. The aqueous solubilities of most solids increase with increasing temperature, some decrease with temp. Endothermic – heat is absorbed by the system (think evaporation of water, or melting of ice) Exothermic – heat is evolved by the system (think fire, or freezing of water).

  15. Factors Affecting Solubility • Structure Effects • Pressure Effects • Temperature Effects for Aqueous Solutions The solubility of some gases in water as a function of temperature at a constant pressure of 1 atm.

  16. The Person Behind the Science Francois-Marie Raoult (1830-1901) Highlights • 1886 Raoult's law , the partial pressure of a solvent vapor in equilibrium with a solution is proportional to the ratio of the number of solvent molecules to non-volatile solute molecules. • allows molecular weights to be determined, and provides the explanation for freezing point depression and boiling point elevation. Moments in a Life • Raoult was a prominent member of the group which created physical chemistry, including Arrhenius, Nernst, van t'Hoff, Planck. For ideal solutions Psoln = XsolventP°solvent

  17. Raoult’s Law, non-volatile solute • Consider a non-volatile solute (component 2) dissolved in a volatile solvent (component 1). • X1 = the mole fraction of solvent Raoult’s Law P1=X1 P°1 P°1 = the vapor pressure of pure component 1

  18. Raoult’s Law, volatile solute • Volatile solute (component 1) • Volatile solvent (component 2) P1 = X1 P°1 P2 = X2 P°2 Ptot = P1+P2

  19. Vapor pressure for a solution of two volatile liquids. Positive deviation = solute-solvent attractions < solvent-solvent attractions For non-ideal Solutions Negative deviation = solute-solvent attractions > solvent-solvent attractions

  20. Osmotic Pressure Fourth Colligative Property • Important for transport of molecules across cell membranes, called semipermeable membranes • Osmotic Pressure = Π Π = M RT ΠV = n RT PV = nRT Molarity (M) = moles/L or n/V

  21. Osmotic Pressure The normal flow of solvent into the solution (osmosis) can be prevented by applying an external pressure to the solution. • Osmotic Pressure useful for • Determining the Molar Mass of protein and other macromolecules • small concentrations cause large osmotic pressures • Can prevent transfer of all solute particles • Dialysis at the wall of most plant and animal cells

  22. Dialysis: Representation of the functioning of an artificial kidney • A cellophane (polymeric) tube acts as the semi-permeable membrane • Purifies blood by washing impurities (solutes) into the dialyzing solution.

  23. A dilute aqueous solution of a non-dissociating compound contains 1.19 g of the compound per liter of solution and has an osmotic pressure of 0.0288 atm at a temperature of 37°C. Compute the molar mass of the compound.

  24. A dilute aqueous solution of a non-dissociating compound contains 1.19 g of the compound per liter of solution and has an osmotic pressure of 0.0288 atm at a temperature of 37°C. Compute the molar mass of the compound

  25. The Person Behind the Science J.H. van’t Hoff (1852-1901) Highlights • Discovery of the laws of chemical dynamics and osmotic pressure in solutions • Mathematical laws that closely resemble the laws describing the behavior of gases. • his work led to Arrhenius's theory of electrolytic dissociation or ionization • Studies in molecular structure laid the foundation of stereochemistry. Moments in a Life • 1901 awarded first Noble Prize in Chemistry van’t Hoff Factor (i) • ΔT= − i m K

  26. Colligative Properties of Electrolyte Solutions • Elevation of Boiling Point • ΔTb = m Kb • Where m = molality • (Molality is moles of solute per kilogram of solvent) • The Effect of Dissociation • ΔTb = i m Kb • i = the number of particles released into the solution per formula unit of solute • e.g., NaCl dissociates into i = 2 • e.g., Na2SO4 dissociates into i = 3 • (2 Na+ + 1 SO4-2) • e.g., acetic acid (a weak acid and weak electrolyte) does not dissociate i = 1 • also • Depression of Freezing Point • ΔTf = − m Kf • ΔTf = − i m Kf

  27. Elevation of Boiling Point The Effect of Dissociation ΔTb = i m Kb

  28. Colloids: Colloidal Dispersions • Colloids are large particles dispersed in solution • 1nm to 1000 nm in size • E.g., Globular proteins 500 nm • Examples • Opal (water in solid SiO2) • Aerosols (liquids in Gas) • Smoke (solids in Air) • Milk (fat droplets & solids in water) • Mayonnaise (water droplets in oil) • Paint (solid pigments in liquid) • Biological fluids (proteins & fats in water) • Characteristics • Large particle size colloids: translucent, cloudy, milky) • Small particle size colloids: can be clear

  29. Colloidal Dispersions • Tyndall Effect • Light Scattering

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