Chapter 13 Section 2 Colligative Properties of Solutions Colligative Properties of Solutions • Colligative properties - properties that depend on the concentration of solute particles but not on their identity • Vapor-Pressure Lowering • Freezing-Point Depression • Boiling-Point Elevation • Osmotic Pressure Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 13 Section 2 Colligative Properties of Solutions Vapor-Pressure Lowering • Nonvolatile substance - has little tendency to become a gas under existing conditions. • The boiling point and freezing point of a solution differ from those of the pure solvent. • A nonvolatile solute raises the boiling point and lowers the freezing point. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 13 Visual Concepts Comparing Volatile and Nonvolatile Liquids Click below to watch the Visual Concept. Visual Concept Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 13 Section 2 Colligative Properties of Solutions Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 13 Section 2 Colligative Properties of Solutions Vapor Pressures of Pure Water and a Water Solution Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 13 Section 2 Colligative Properties of Solutions Freezing-Point Depression • Freezing-point depression, ∆tf - difference between the freezing points of the pure solvent and a solution of a nonelectrolyte in that solvent. • Molal freezing-point constant (Kf ) - freezing-point depression of the solvent in a 1-molal solution of a nonvolatile, nonelectrolyte solute. ∆tf = Kfm Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 13 Section 2 Colligative Properties of Solutions Molal Freezing-Point and Boiling-Point Constants Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 13 Visual Concepts Freezing-Point Depression Click below to watch the Visual Concept. Visual Concept Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 13 Section 2 Colligative Properties of Solutions Freezing-Point Depression Sample Problem C What is the freezing-point depression of water in a solution of 17.1 g of sucrose, C12H22O11, in 200. g of water? What is the actual freezing point of the solution? Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 13 Section 2 Colligative Properties of Solutions Boiling-Point Elevation • Boiling-point elevation, ∆tb - difference between the boiling points of the pure solvent and a nonelectrolyte solution of that solvent. • Molal boiling-point constant (Kb) - boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile, nonelectrolyte solute. ∆tb = Kbm Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 13 Visual Concepts Boiling-Point Elevation and the Presence of Solutes Click below to watch the Visual Concept. Visual Concept Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 13 Section 2 Colligative Properties of Solutions Boiling-Point Elevation Sample Problem E What is the boiling-point elevation of a solution made from 20.1 g of a nonelectrolyte solute and 400.0 g of water? The molar mass of the solute is 62.0 g. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 13 Section 2 Colligative Properties of Solutions Osmotic Pressure • Semipermeable membrane - allows the passage of some particles while blocking the passage of others. • Osmosis - movement of solvent through a semipermeable membrane from the side of lower solute concentration to the side of higher solute concentration • Osmotic pressure - external pressure that must be applied to stop osmosis. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 13 Section 2 Colligative Properties of Solutions Osmotic Pressure Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 13 Section 2 Colligative Properties of Solutions Electrolytes and Colligative Properties • Electrolytes depress the freezing point and elevate the boiling point of a solvent more than expected. • Electrolytes produce more than 1 mol of solute particles for each mole of compound dissolved. mol of solute particles 2 C12H22O11 C12H22O11(aq) H O 1 2 NaCl(s) Na (aq) + ClŠ(aq) H O CaCl2 (s) Ca2 (aq) + 2ClŠ(aq) H2 O Chapter menu 2 3 Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 13 Section 2 Colligative Properties of Solutions Calculated Values for Electrolyte Solutions • Colligative properties depend on the total concentration of solute particles. • The changes in colligative properties caused by electrolytes will be proportional to the total molality of all dissolved particles, not to formula units. • For the same molal concentrations of sucrose and sodium chloride, you would expect the effect on colligative properties to be twice as large for sodium chloride as for sucrose. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 13 Section 2 Colligative Properties of Solutions Electrolytes and Colligative Properties Sample Problem F What is the expected change in the freezing point of water in a solution of 62.5 g of barium nitrate, Ba(NO3)2, in 1.00 kg of water? Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 13 Section 2 Colligative Properties of Solutions Electrolytes and Colligative Properties Sample Problem F Solution Given: solute mass and formula = 62.5 g Ba(NO3)2 solvent mass and identity = 1.00 kg water ∆tf = Kfm Unknown: expected freezing-point depression Solution: mass of solute (g) 1 mol solute mass of solvent (kg) molar mass solute (g) mol molality of solute kg Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Section 2 Colligative Properties of Solutions Chapter 13 Electrolytes and Colligative Properties Sample Problem F Solution Solution: mol mol ions molality of solute molality conversion kg mol C g kg H2O Kf mol ions 62.5 g Ba(NO3 )2 1.00 kg H2O exp ected freezing - po int depression (C) mol Ba(NO3 )2 261.35 g Ba(NO3 )2 Chapter menu 0.239 mol Ba(NO3 )2 kg H2O Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 13 Section 2 Colligative Properties of Solutions Electrolytes and Colligative Properties Sample Problem F Solution Solution: 2 Ba(NO3 )2 (s) Ba2 (aq) + 2NO3Š(aq) H O Each formula unit of barium nitrate yields three ions in solution. 0.239 mol Ba(NO3 )2 kg H2O -1.86C g kg H2O 3 mol ions mol Ba(NO3 )2 mol ions - 1.33C Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 13 Section 2 Colligative Properties of Solutions Actual Values for Electrolyte Solutions • The actual values of the colligative properties for all strong electrolytes are almost what would be expected based on the number of particles they produce in solution. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 13 Section 2 Colligative Properties of Solutions Actual Values for Electrolyte Solutions • The differences between the expected and calculated values are caused by the attractive forces that exist between dissociated ions in aqueous solution. • According to Debye and Hückel a cluster of hydrated ions can act as a single unit rather than as individual ions, causing the effective total concentration to be less than expected. • Ions of higher charge have lower effective concentrations than ions with smaller charge. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved.
