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Chapter 11 Solutions and Their Properties

Chapter 11 Solutions and Their Properties. I. Introduction. A) Solution - A homogeneous mixture of two or more substances in a single phase (gas, liquid, solid). • Alloy - A solid solution (i.e. brass is a solution of Cu and Zn).

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Chapter 11 Solutions and Their Properties

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  1. Chapter 11Solutions and Their Properties CHM 112 M. Prushan

  2. I. Introduction A) Solution - A homogeneous mixture of two or more substances in a single phase (gas, liquid, solid) •Alloy - A solid solution (i.e. brass is a solution of Cu and Zn) •Solvent - The component that makes up the bulk of the solution. •Solute - The component that dissolves in the solvent. B) Colligatve Properties - Properties of solutions which depend on the number of solute particles in the solution and not the nature of the solute. •Four Colligative Properties 1) Vapor pressure lowering 2) Boiling point elevation 3) Freezing point depression 4) Osmosis CHM 112 M. Prushan

  3. II. Units of Concentration • Not useful in colligative properties because the exact amount of solvent is unknown. • The following concentration units reflect the number of solute particles per solvent molecules and are useful with colligative properties. CHM 112 M. Prushan

  4. II. Units of Concentration • Naturally occurring solutions are often very dilute so environmental chemists, biologists, geologists, etc. often use parts per million (ppm). CHM 112 M. Prushan

  5. IV. Colligative Properties •Colligative properties are independent of the nature of the solvent and depend only on the relative number of solute and solvent particles. • There are four colligative properties of solutions: A. Vapor Pressure Lowering B. Boiling Point Elevation C. Freezing Point Depression D. Osmosis CHM 112 M. Prushan

  6. IV. Colligative Properties A.Vapor Pressure Lowering • The vapor pressure of the solution is lowered because the solute particles at the liquid/vapor boundary block the solvent particle from jumping into the vapor state. Raoult’s Law Psolv = XsolvP°solv or DPsolv = XsoluteP°solv CHM 112 M. Prushan

  7. IV. Colligative Properties B.Boiling Point Elevation • For nonvolatile, nonelectrolyte solvents the change in boiling point (DTbp) is: Kbp=boiling point elevation constant DTbp = Kbpmsolute msolute=molality of solute CHM 112 M. Prushan

  8. IV. Colligative Properties C.Freezing Point Depression • For nonvolatile, nonelectrolyte solvents the change in freezing point (DTfp) is: Kfp=freezing point depression constant DTfp = Kfpmsolute msolute=molality of solute CHM 112 M. Prushan

  9. Measure a change in VP, BP elevation, FP depression, or osmotic pressure. Moles of Solute Use mass of solvent Solution Conc. Molar Mass g solute mol solute IV. Colligative Properties D.Colligative Properties and Molar Mass Determination • Colligative properties can be used to determine the molar mass of a solute when it is dissolved in a solvent of appreciable vapor pressure and a known Kbp or Kfp. CHM 112 M. Prushan

  10. IV. Colligative Properties E.Colligative Properties of Solutions Containing Ions • The change in VP, BP, FP, or osmotic pressure is greater than expected for electrolyte (ionic salt) solutions. Predicted BP elevation of an aqueous 0.100 m NaCl solution DTbp = Kbp • msolute(For H2O Kbp = 0.5121 °C/m) DTbp, calculated = (0.5121 °C/m)(0.100 m) = 0.05121 °C Actual BP elevation of an aqueous 0.100 m NaCl solution DTbp, measured = 0.09470 °C (Almost double the DTbp calculated) •Colligative properties depend on the total number of solute particles in solution. Ionic compounds form ions in solution so the total number of solute particles in solution is equal to the total ions in solution. CHM 112 M. Prushan

  11. IV. Colligative Properties E.Colligative Properties of Solutions Containing Ions NaCl(s)  Na+(aq) + Cl-(aq) 0.100 m 0.100 m 0.100 m 0.200 m total van’t Hoff factor (i) So for ionic solutions: CHM 112 M. Prushan

  12. IV. Colligative Properties E.Colligative Properties of Solutions Containing Ions Predicting van’t Hoff factors NaCl(s)  Na+(aq) + Cl-(aq) 1 particle + 1 particle = 2 particles ipredicted= 2 Na2SO4(s)  2 Na+(aq) + SO42-(aq) 2 particles + 1 particle = 3 particles ipredicted= 3 CHM 112 M. Prushan

  13. IV. Colligative Properties F.Osmosis and Osmotic Pressure Osmosis - The movement of solvent molecules through a semipermeable membrane from a region of low solute concentration to a region of high solute concentration. CHM 112 Summer 2007 M. Prushan

  14. P = osmotic pressure M = molar conc. (mol/L) R = 0.08206 T = Temperature (K) L atm mol K IV. Colligative Properties F.Osmosis and Osmotic Pressure Osmotic Pressure - The pressure created by a column of solution for a system in equilibrium. CHM 112 M. Prushan

  15. IV. Colligative Properties F.Osmosis and Osmotic Pressure Reverse Osmosis is Used for Water Purification CHM 112 M. Prushan

  16. Solutions • Solutions are homogeneous mixtures of two or more pure substances. • In a solution, the solute is dispersed uniformly throughout the solvent. CHM 112 M. Prushan

  17. Solutions How does a solid dissolve into a liquid? What ‘drives’ the dissolution process? What are the energetics of dissolution? CHM 112 M. Prushan

  18. How Does a Solution Form? • Solvent molecules attracted to surface ions. • Each ion is surrounded by solvent molecules. • Enthalpy (DH) changes with each interaction broken or formed. Ionic solid dissolving in water CHM 112 M. Prushan

  19. How Does a Solution Form? • Solvent molecules attracted to surface ions. • Each ion is surrounded by solvent molecules. • Enthalpy (DH) changes with each interaction broken or formed. CHM 112 M. Prushan

  20. How Does a Solution Form The ions are solvated (surrounded by solvent). If the solvent is water, the ions are hydrated. The intermolecular force here is ion-dipole. CHM 112 M. Prushan

  21. dry Dissolution vs reaction • Dissolution is a physical change—you can get back the original solute by evaporating the solvent. • If you can’t, the substance didn’t dissolve, it reacted. NiCl2(s) Ni(s) + HCl(aq) NiCl2(aq) + H2(g) CHM 112 M. Prushan

  22. Degree of saturation • Saturated solution • Solvent holds as much solute as is possible at that temperature. • Undissolved solid remains in flask. • Dissolved solute is in dynamic equilibrium with solid solute particles. CHM 112 M. Prushan

  23. Degree of saturation • Unsaturated Solution • Less than the maximum amount of solute for that temperature is dissolved in the solvent. • No solid remains in flask. CHM 112 M. Prushan

  24. Degree of saturation • Supersaturated • Solvent holds more solute than is normally possible at that temperature. • These solutions are unstable; crystallization can often be stimulated by adding a “seed crystal” or scratching the side of the flask. CHM 112 M. Prushan

  25. Degree of saturation Unsaturated, Saturated or Supersaturated?  How much solute can be dissolved in a solution? CHM 112 M. Prushan

  26. Factors Affecting Solubility • Chemists use the axiom “like dissolves like”: • Polar substances tend to dissolve in polar solvents. • Nonpolar substances tend to dissolve in nonpolar solvents. CHM 112 M. Prushan

  27. Factors Affecting Solubility The stronger the intermolecular attractions between solute and solvent, the more likely the solute will dissolve. Example: ethanol in water Ethanol = CH3CH2OH Intermolecular forces = H-bonds; dipole-dipole; dispersion Ions in water also have ion-dipole forces. CHM 112 M. Prushan

  28. Factors Affecting Solubility Glucose (which has hydrogen bonding) is very soluble in water. Cyclohexane (which only has dispersion forces) is not water-soluble. CHM 112 M. Prushan

  29. Factors Affecting Solubility • Vitamin A is soluble in nonpolar compounds (like fats). • Vitamin C is soluble in water. CHM 112 Summer 2007 M. Prushan

  30. Which vitamin is water-soluble and which is fat-soluble? CHM 112 Summer 2007 M. Prushan

  31. Gases in Solution • In general, the solubility of gases in water increases with increasing mass. Why? • Larger molecules have stronger dispersion forces. CHM 112 M. Prushan

  32. Gases in Solution CHM 112 M. Prushan

  33. Gases in Solution • The solubility of liquids and solids does not change appreciably with pressure. • But, the solubility of a gas in a liquid is directly proportional to its pressure. Increasing pressure above solution forces more gas to dissolve. CHM 112 M. Prushan

  34. Henry’s Law Sg = kPg where • Sg is the solubility of the gas; • k is the Henry’s law constant for that gas in that solvent; • Pg is the partial pressure of the gas above the liquid. CHM 112 M. Prushan

  35. Temperature Generally, the solubility of solid solutes in liquid solvents increases with increasing temperature. CHM 112 M. Prushan

  36. Temperature • The opposite is true of gases. Higher temperature drives gases out of solution. • Carbonated soft drinks are more “bubbly” if stored in the refrigerator. • Warm lakes have less O2 dissolved in them than cool lakes. CHM 112 M. Prushan

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