Colligative Properties of Solutions (chapter 16)

1. Colligative Properties of Solutions (chapter 16) Colligative properties = physical properties of solutions that depend on the # of particles dissolved, not the kind of particle.

2. Colligative Properties • Lowering vapor pressure • Raising boiling point • Lowering freezing point • Generating an osmotic pressure

3. 2 to focus on… • Lowering vapor pressure • Raising boiling point • Lowering freezing point • Generating an osmotic pressure

4. Boiling Point Elevation • a solution that contains a nonvolatile solute has a higher boiling pt than the pure solvent; the boiling pt elevation is proportional to the # of moles of solute dissolved in a given mass of solvent. Like when adding salt to a pot of boiling water to make pasta 

5. Boiling Point Elevation • Tb = kbm where: Tb = elevation of boiling pt m = molality of solute (mol solute/kg solvent) kb = the molal boiling pt elevation constant • kb values are constants; see table 16.3 pg. 495 • kb for water = 0.52 °C/m

6. Ex: What is the normal boiling pt of a 2.50 m glucose, C6H12O6, solution? • “normal” implies 1 atm of pressure • Tb = kbm • Tb = (0.52 C/m)(2.50 m) • Tb = 1.3 C • Tb = 100.0 C + 1.3 C = 101.3 C

7. Freezing/Melting Point Depression • The freezing point of a solution is always lower than that of the pure solvent. Like when salting roads in snowy places so the roads don’t ice over or when making ice cream 

8. Freezing/Melting Point Depression • Tf = kfm where: Tf = lowering of freezing point m = molality of solute kf = the freezing pt depression constant • kf for water = 1.86 °C/m • kf values are constants; see table 16.2 pg. 494

9. Ex: Calculate the freezing pt of a 2.50 m glucose solution. • Tf = kfm • Tf = (1.86 C/m)(2.50 m) • Tf = 4.65 C • Tf = 0.00C - 4.65 C = -4.65C

10. Electrolytes and Colligative Properties • Colligative properties depend on the # of particles present in solution. • Because ionic solutes dissociate into ions, they have a greater effect on freezing pt and boiling pt than molecular solids of the same molal conc.

11. Electrolytes and Colligative Properties • For example, the freezing pt of water is lowered by 1.86°C with the addition of any molecular solute at a concentration of 1 m. • Such as C6H12O6, or any other covalent compound • However, a 1 m NaCl solution contains 2 molal conc. of IONS. Thus, the freezing pt depression for NaCl is 3.72°C…double that of a molecular solute. • NaCl  Na+ + Cl- (2 particles)

12. Electrolytes - Boiling Point Elevation and Freezing Point Depression The relationships are given by the following equations: • Tf = kf ·m·norTb = kb·m·n Tf/b = f.p. depression/elevation of b.p. m = molality of solute kf/b = b.p. elevation/f.p depression constant n = # particles formed from the dissociation of each formula unit of the solute

13. Ex: What is the freezing pt of a 1.15 m sodium chloride solution? • NaCl  Na+ + Cl- n=2 • Tf = kf·m·n • Tf = (1.86 C/m)(1.15 m)(2) • Tf = 4.28 C • Tf = 0.00C - 4.28 C = -4.28C