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Colligative Properties

Colligative Properties. Vapour pressure Boiling point Freezing point Osmotic pressure. Physical vs Chemical. Mixing is physical process; chemical properties don’t change Properties of solutions are similar to those of the pure substances

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Colligative Properties

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  1. Colligative Properties Vapour pressure Boiling point Freezing point Osmotic pressure

  2. Physical vs Chemical • Mixing is physical process; chemical properties don’t change • Properties of solutions are similar to those of the pure substances • Addition of a foreign substance to water alters the properties slightly

  3. Colligative: particles are particles • Colligative comes from colligate – to tie together • Colligative properties have common origin • Colligative properties depend on amount of solute but do not depend on its chemical identity • Solute particles exert their effect merely by being rather than doing • The effect is the same for all solutes

  4. Colligative properties for nonvolatile solutes: Take it to the bank • Vapour pressure is always lower • Boiling point is always higher • Freezing point is always lower • Osmotic pressure drives solvent from lower concentration to higher concentration

  5. Counting sheep (particles) • The influence of the solute depends only on the number of particles • Molecular and ionic compounds will produce different numbers of particles per mole of substance • 1 mole of a molecular solid → 1 mole of particles • 1 mole of NaCl → 2 moles of particles • 1 mole of CaCl2 → 3 moles of particles

  6. Solution Deviants • Like ideal gas law, Raoult’s Law works for an ideal solution • Real solutions deviate from the ideal • Concentration gets larger • Solute – solvent interactions are unequal • Solvent – solvent interactions are stronger than the solute – solvent: Pvap is higher • Solvent – solute interactions are stronger than solvent – solvent interactions: Pvap is lower

  7. CFU 1 • What does non volatile mean? • Which non volatile solute • NaCl or Glucose • Would have the have the most impact on vapor pressure? • Which would have the most impact on Boiling point? • If you add 1 mole of NaCl to 100g grams of water what is the Molality • What is the difference between molality and Molarity

  8. CFU 1 • If you add 1 mole of NaCl to 100g grams of water what is the Molality • What is the difference between molality and Molarity

  9. CFU 2 • What is a nonelectrolyte? • What is meant by aqueous? • What is the value of Kb for water? • What is the value of Kf for water?

  10. CFU 2 • What would be the boiling point and freezing point of a 0.625m aqueous solution of any nonvolatile, nonelectrolyte solute? • non electrolyte means VHF of 1 • ΔTf = VHF x m x Kf • = 1 x 0.625 x -1.86 • = -1.1625Freezing point = 0 -1.1625 • ΔTb = VHF x m x Kb • = 1 x 0.625 x +0.52 • = 0.325 Boiling point = 100 +0.325 = 100.325oC

  11. Liquid

  12. What is the molality of a solution with a boiling temperature of 101.56°C? (The Molal boiling point elevation constant for water is 0.52ºC•m – 1) A) 1m B) 2m C) 3m D) 4m E) 195m • ΔTb = VHF x m x Kb • 1.56 = 1 x m x +0.52 • m = 1.56 = 3 1 x 0.52

  13. Incomplete dissociation • Not all ionic substances dissociate completely • Van’t Hoff factor accounts for this Van’ t Hoff factor: i = moles of particles in soln/moles of solute dissolved

  14. Riding high on a deep depression • Blue curves are phase boundaries for pure solvent • Red curves are phase boundaries for solvent in solution • Freezing point depression • Pure solid separates out at freezing – negative ΔTf • Boiling point elevation • Vapour pressure in solution is lower, so higher temperature is required to reach atmospheric – positive ΔTb

  15. Magnitude of elevation • Depends on the number of particles present • Concentration is measured in molality (independent of T) • Kb is the molal boiling point elevation constant • Note: it is the number of particles

  16. Magnitude of depression • Analagous to boiling point, the freezing point depression is proportional to the molal concentration of solute particles • For solutes which are not completely dissociated, the van’t Hoff factor is applied to modify m:

  17. Osmosis: molecular discrimination • A semi-permeable membrane discriminates on the basis of molecular type • Solvent molecules pass through • Large molecules or ions are blocked • Solvent molecules will pass from a place of lower solute concentration to higher concentration to achieve equilibrium

  18. Osmotic pressure • Solvent passes into more conc solution increasing its volume • The passage of the solvent can be prevented by application of a pressure • The pressure to prevent transport is the osmotic pressure

  19. Calculating osmotic pressure • The ideal gas law states • But n/V = M and so • Where M is the molar concentration of particles and Π is the osmotic pressure • Note: molarity is used not molality

  20. Osmotic pressure and molecular mass • Molar mass can be computed from any of the colligative properties • Osmotic pressure provides the most accurate determination because of the magnitude of Π • 0.0200 M solution of glucose exerts an osmotic pressure of 374.2 mm Hg but a freezing point depression of only 0.02ºC

  21. Determining molar mass • A solution contains 20.0 mg insulin in 5.00 ml develops an osmotic pressure of 12.5 mm Hg at 300 K

  22. Moles insulin = MxV = 3.34x10-6 mol • Molar mass = mass of insulin/moles of insulin = 0.0200 g/3.34x10-6 mol = 5990 g/mol

  23. Volatile solute: two liquids • Total pressure is the sum of the pressures of the two components

  24. Ideal behaviour of liquid mixture • Total pressure in a mixture of toluene (b.p. = 110.6ºC) and benzene (b.p. = 80.1ºC) equals sum of vapor pressures of components

  25. Deviations from ideal • Real solutions can deviate from the ideal: • Positive (Pvap > ideal) solute-solvent interactions weaker • Negative (Pvap < ideal) solute-solvent interactions stronger

  26. Fractional distillation: separation of liquids with different boiling points • The vapour above a liquid is richer in the more volatile component • Boiling the mixture will give a distillate more concentrated in the volatile component • The residue will be richer in the less volatile component

  27. Purification in stages • A 50:50 mixture produces a vapour with a 71:29 composition • That mixture boiled produces a vapour with a 86:14 composition • That mixture boiled produces a vapour with a composition 94:6

  28. The practice of fractional distillation • In practice, it is not necessary to do the distillation in individual steps • The vapour rising up the column condenses and re-evaporates continuously, progressively becoming enriched in the volatile component higher up the tube • If the column is high enough, pure liquid will be collected in the receiver

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