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Understanding Colligative Properties: Vapor Pressure, Boiling Point Elevation, and Freezing Point Depression

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Colligative properties are influenced by the number of solute particles in a solution, impacting vapor pressure, boiling point elevation, and freezing point depression. Adding a nonvolatile solute lowers vapor pressure proportionally to its concentration. The boiling point of a solution increases as more solute particles are added, requiring a higher temperature to reach 1 atm vapor pressure. Conversely, freezing point decreases with solute addition. Practical applications include using antifreeze in car radiators to prevent freezing. Key calculations involve molality and specific constants.

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Understanding Colligative Properties: Vapor Pressure, Boiling Point Elevation, and Freezing Point Depression

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  1. Colligative Properties: Properties which are affected by the number of particles present in solution.

  2. Lowering the Vapor Pressure (VP) • Addition of a nonvolatile solute to a solution will decrease the vapor pressure of the solution. • Lowering of the VP is proportional to the concentration of the solute added to the solution (ex. Doubling the concentration of solute doubles the effect) • 1 mol of glucose produces essentially the same reduction in VP as does 0.5 mol of NaCl

  3. Boiling Point (BP) Elevation • Addition of a nonvolatile solute lowers the VP of soln. • Boiling Point (BP) = temp at which VP equals 1 atm. • When a solute is added to a solution, it requires higher temp. to attain VP of 1 atm thus raising the BP of the soln.

  4. Boiling Point (BP) Elevation • BP elevation is proportional to the # of solute particles in soln. • 1 mol of NaCl dissolved in water will yield 2 mol of solute particles • 1 mol of FeCl3 dissolved in water will yield 4 mol of solute particles • So, a 1m soln of FeCl3 causes the BP to increase twice as much as a 1m soln of NaCl

  5. Freezing Point (FP) Depression • Freezing Point (FP) = temp at which the first crystals of pure solvent begin to form • Lower VP of the solution results in a lower freezing point as well. • The decrease in FP is directly proportional to the molality of the solute. • Application: adding antifreeze to car radiators to prevent water from freezing.

  6. Calculations • BP Elevation DTb = Kbm • Kb = molal boiling point elevation constant • (See p. 470 in text for list of constants) • m = molality • Ex. At what temperature will a 2m glucose solution boil? DTb = Kbm = (0.52oC/m)(2m) = 1.04oC BP = 100oC + 1.04oC = 101.04oC

  7. Calculations • FP Depression DTf = Kfm • Kf = molal freezing point depression constant • (See p. 470 in text for list of constants) • m = molality • Ex. At what temperature will a 2m glucose solution freeze? DTf = Kfm = (1.86oC/m)(2m) = 3.72oC FP = 0oC - 3.72oC = -3.72oC

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