Chapter 11 (Solutions and Their Properties)

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  • Figure: 13-32-03UNE13.06 Title: Exercise 13.6 Caption: Vitamins E and B 6 .
  • Chapter 11 (Solutions and Their Properties)

    1. 1. Chapter 11 Solutions and Their Properties CHM 112 Summer 2007 M. Prushan
    2. 2. I. Introduction CHM 112 Summer 2007 M. Prushan • Alloy - A solid solution (i.e. brass is a solution of Cu and Zn) A) Solution - A homogeneous mixture of two or more substances in a single phase (gas, liquid, solid) • 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
    3. 3. II. Units of Concentration CHM 112 Summer 2007 M. Prushan • 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.
    4. 4. II. Units of Concentration CHM 112 Summer 2007 M. Prushan • Naturally occurring solutions are often very dilute so environmental chemists, biologists, geologists, etc. often use parts per million (ppm) .
    5. 5. IV. Colligative Properties CHM 112 Summer 2007 M. Prushan A. Vapor Pressure Lowering B. Boiling Point Elevation C. Freezing Point Depression D. Osmosis • There are four colligative properties of solutions: • Colligative properties are independent of the nature of the solvent and depend only on the relative number of solute and solvent particles.
    6. 6. IV. Colligative Properties CHM 112 Summer 2007 M. Prushan • 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. A. Vapor Pressure Lowering Raoult’s Law P solv = X solv P° solv or  P solv = X solute P° solv
    7. 7. IV. Colligative Properties CHM 112 Summer 2007 M. Prushan • For nonvolatile, nonelectrolyte solvents the change in boiling point (  T bp ) is: B. Boiling Point Elevation  T bp = K bp m solute K bp = boiling point elevation constant m solute = molality of solute
    8. 8. IV. Colligative Properties CHM 112 Summer 2007 M. Prushan • For nonvolatile, nonelectrolyte solvents the change in freezing point (  T fp ) is: C. Freezing Point Depression  T fp = K fp m solute K fp = freezing point depression constant m solute = molality of solute
    9. 9. IV. Colligative Properties CHM 112 Summer 2007 M. Prushan • 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 K bp or K fp . D. Colligative Properties and Molar Mass Determination Measure a change in VP, BP elevation, FP depression, or osmotic pressure. Solution Conc. Moles of Solute Molar Mass Use mass of solvent g solute mol solute
    10. 10. IV. Colligative Properties CHM 112 Summer 2007 M. Prushan • The change in VP, BP, FP, or osmotic pressure is greater than expected for electrolyte (ionic salt) solutions. E. Colligative Properties of Solutions Containing Ions Predicted BP elevation of an aqueous 0.100 m NaCl solution  T bp = K bp • m solute (For H 2 O K bp = 0.5121 °C/m)  T bp, calculated = (0.5121 °C/m)(0.100 m) = 0.05121 °C Actual BP elevation of an aqueous 0.100 m NaCl solution  T bp , measured = 0.09470 °C (Almost double the  T bp 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.
    11. 11. IV. Colligative Properties CHM 112 Summer 2007 M. Prushan E. Colligative Properties of Solutions Containing Ions NaCl(s)  Na + (aq) + Cl - (aq) 0.100 m van’t Hoff factor ( i ) So for ionic solutions: 0.100 m 0.100 m 0.200 m total
    12. 12. IV. Colligative Properties CHM 112 Summer 2007 M. Prushan E. Colligative Properties of Solutions Containing Ions NaCl(s)  Na + (aq) + Cl - (aq) Predicting van’t Hoff factors 1 particle + 1 particle = 2 particles i predicted = 2 Na 2 SO 4 (s)  2 Na + (aq) + SO 4 2- (aq) 2 particles + 1 particle = 3 particles i predicted = 3
    13. 13. IV. Colligative Properties CHM 112 Summer 2007 M. Prushan 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.
    14. 14. IV. Colligative Properties CHM 112 Summer 2007 M. Prushan F. Osmosis and Osmotic Pressure Osmotic Pressure - The pressure created by a column of solution for a system in equilibrium.   osmotic pressure M = molar conc. (mol/L) R = 0.08206 T = Temperature (K) L atm mol K
    15. 15. IV. Colligative Properties CHM 112 Summer 2007 M. Prushan F. Osmosis and Osmotic Pressure Reverse Osmosis is Used for Water Purification
    16. 16. Solutions <ul><li>Solutions are homogeneous mixtures of two or more pure substances. </li></ul><ul><li>In a solution, the solute is dispersed uniformly throughout the solvent . </li></ul>CHM 112 Summer 2007 M. Prushan
    17. 17. Solutions <ul><li>How does a solid dissolve into a liquid? </li></ul><ul><li>What ‘drives’ the dissolution process? </li></ul><ul><li>What are the energetics of dissolution? </li></ul>CHM 112 Summer 2007 M. Prushan
    18. 18. How Does a Solution Form? <ul><li>Solvent molecules attracted to surface ions. </li></ul><ul><li>Each ion is surrounded by solvent molecules. </li></ul><ul><li>Enthalpy (  H) changes with each interaction broken or formed. </li></ul>CHM 112 Summer 2007 M. Prushan Ionic solid dissolving in water
    19. 19. How Does a Solution Form? <ul><li>Solvent molecules attracted to surface ions. </li></ul><ul><li>Each ion is surrounded by solvent molecules. </li></ul><ul><li>Enthalpy (  H) changes with each interaction broken or formed. </li></ul>CHM 112 Summer 2007 M. Prushan
    20. 20. How Does a Solution Form <ul><li>The ions are solvated (surrounded by solvent). </li></ul><ul><li>If the solvent is water, the ions are hydrated . </li></ul><ul><li>The intermolecular force here is ion-dipole. </li></ul>CHM 112 Summer 2007 M. Prushan
    21. 21. Dissolution vs reaction <ul><li>Dissolution is a physical change — you can get back the original solute by evaporating the solvent. </li></ul><ul><li>If you can’t, the substance didn’t dissolve, it reacted. </li></ul>CHM 112 Summer 2007 M. Prushan Ni(s) + HCl(aq) NiCl 2 (aq) + H 2 (g) NiCl 2 (s) dry
    22. 22. Degree of saturation <ul><li>Saturated solution </li></ul><ul><ul><li>Solvent holds as much solute as is possible at that temperature. </li></ul></ul><ul><ul><li>Undissolved solid remains in flask. </li></ul></ul><ul><ul><li>Dissolved solute is in dynamic equilibrium with solid solute particles. </li></ul></ul>CHM 112 Summer 2007 M. Prushan
    23. 23. Degree of saturation <ul><li>Unsaturated Solution </li></ul><ul><ul><li>Less than the maximum amount of solute for that temperature is dissolved in the solvent. </li></ul></ul><ul><ul><li>No solid remains in flask. </li></ul></ul>CHM 112 Summer 2007 M. Prushan
    24. 24. Degree of saturation <ul><li>Supersaturated </li></ul><ul><ul><li>Solvent holds more solute than is normally possible at that temperature. </li></ul></ul><ul><ul><li>These solutions are unstable; crystallization can often be stimulated by adding a “seed crystal” or scratching the side of the flask. </li></ul></ul>CHM 112 Summer 2007 M. Prushan
    25. 25. Degree of saturation <ul><li>Unsaturated, Saturated or Supersaturated? </li></ul><ul><li> How much solute can be dissolved in a solution? </li></ul>CHM 112 Summer 2007 M. Prushan
    26. 26. Factors Affecting Solubility <ul><li>Chemists use the axiom “like dissolves like”: </li></ul><ul><ul><li>Polar substances tend to dissolve in polar solvents. </li></ul></ul><ul><ul><li>Nonpolar substances tend to dissolve in nonpolar solvents. </li></ul></ul>CHM 112 Summer 2007 M. Prushan
    27. 27. Factors Affecting Solubility <ul><li>The stronger the intermolecular attractions between solute and solvent, the more likely the solute will dissolve. </li></ul>CHM 112 Summer 2007 M. Prushan Example: ethanol in water Ethanol = CH 3 CH 2 OH Intermolecular forces = H-bonds; dipole-dipole; dispersion Ions in water also have ion-dipole forces.
    28. 28. Factors Affecting Solubility <ul><li>Glucose (which has hydrogen bonding) is very soluble in water. </li></ul><ul><li>Cyclohexane (which only has dispersion forces) is not water-soluble. </li></ul>CHM 112 Summer 2007 M. Prushan
    29. 29. Factors Affecting Solubility <ul><li>Vitamin A is soluble in nonpolar compounds (like fats). </li></ul><ul><li>Vitamin C is soluble in water. </li></ul>CHM 112 Summer 2007 M. Prushan
    30. 30. CHM 112 Summer 2007 M. Prushan Which vitamin is water-soluble and which is fat-soluble?
    31. 31. Gases in Solution <ul><li>In general, the solubility of gases in water increases with increasing mass. </li></ul><ul><li>Why? </li></ul><ul><li>Larger molecules have stronger dispersion forces. </li></ul>CHM 112 Summer 2007 M. Prushan
    32. 32. Gases in Solution CHM 112 Summer 2007 M. Prushan
    33. 33. Gases in Solution <ul><li>The solubility of liquids and solids does not change appreciably with pressure. </li></ul><ul><li>But, the solubility of a gas in a liquid is directly proportional to its pressure. </li></ul>CHM 112 Summer 2007 M. Prushan Increasing pressure above solution forces more gas to dissolve.
    34. 34. Henry’s Law <ul><li>S g = kP g </li></ul><ul><li>where </li></ul><ul><li>S g is the solubility of the gas; </li></ul><ul><li>k is the Henry’s law constant for that gas in that solvent; </li></ul><ul><li>P g is the partial pressure of the gas above the liquid. </li></ul>CHM 112 Summer 2007 M. Prushan
    35. 35. Temperature <ul><li>Generally, the solubility of solid solutes in liquid solvents increases with increasing temperature. </li></ul>CHM 112 Summer 2007 M. Prushan
    36. 36. Temperature <ul><li>The opposite is true of gases. Higher temperature drives gases out of solution. </li></ul><ul><ul><li>Carbonated soft drinks are more “bubbly” if stored in the refrigerator. </li></ul></ul><ul><ul><li>Warm lakes have less O 2 dissolved in them than cool lakes. </li></ul></ul>CHM 112 Summer 2007 M. Prushan

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