Solutions and Colligative Properties

1. Solutions and Colligative Properties AP Chemistry Chapter 13

2. Key Vocabulary • Solution – homogeneous mixture composed of a solvent and one of more solutes • Solvent – the dissolving medium; normally present in the greatest amount; often a liquid (water) • Solute – the substance being dissolved; normally present in the smaller amount • Dissociation – part of the dissolving process, where solute particles are separated from each other • Solvation – part of the dissolving process where solvent particles surround each dissociated solute particle (when the solvent is water, this process is called hydration) • Electrolyte – solute that conducts electricity when melted or dissolved (ionic compounds, acids, and bases) • Nonelectrolyte – solute that does not conduct electricity when melted or dissolved • Electrolytes can be strong (ionize completely) or weak (ionize partially)

3. “Like dissolves like” (this general statement can help with answering multiple choice questions, but should not be used as an explanation in the free response section) • Polar solvents will dissolve polar and most ionic solutes. • Nonpolar solvents will dissolve nonpolar solutes. **the reason for this is similar intermolecular forces – if the solvent particles are held together by dipole-dipole forces, then solutes with dipole-dipole forces will also be attracted to those solvent particles. **however, if an ionic compound will not dissolve in water its because the lattice energy within the compound is greater than the ion-dipole force of attraction with the water, thus the water cannot dissociate the salt

4. Concentration Units All are a ratio of solute to solvent (or solution) • Dilute solutions have a low solute to solvent ratio • Concentrated solutions have a high solute to solvent ratio • Percentage (%) • Mass percent = (mass of solute/total mass) *100 • You can also express a mass/volume percent or a volume/ volume percent • Molarity (M) • Moles of solute/liters of solution • If you are diluting a more concentration solution, the final molarity is calculated using MiVi = MfVf

5. More Concentration Units • Mole fraction (no units) • Moles of solute/total moles of solution • To solve for mole fraction, you will have to convert mass of solute to moles, convert mass of solvent to moles, then divide solute moles by the sum of the two • Molality (m) • Moles of solute/kilograms of solvent *You should be able to convert between concentration units (sometimes density will be needed)

6. Colligative Properties • Colligative properties are properties are properties of a solution that depend on the chemical and physical nature of the individual solute. • Vapor pressure lowering – the vapor pressure of a solution will always be lower than that of the pure solvent • Freezing point depression – the f.p. of a solution will also be lower than that of the pure solvent • Boiling point elevation – the b.p. of a solution will always be higher than that of the pure solvent • Osmotic pressure change

7. Vapor Pressure lowering • When a solute is dissolved in a solvent, the intermolecular attractions (solute to solvent) hold the solvent particles in the liquid phase to a greater extent than in the pure solvent alone. Also, at the surface of the liquid, where evaporation takes place, there are fewer solvent particles than in the pure solvent (because the solute and solvent particles are evenly distributed in a solution). • When a nonvolatile, nonelectrolyte solute is dissolved, the vapor pressure of the solvent in solution can be calculated using Raoult’s Law: Psolvent= XsolventPosolvent (where X is the mole fraction of the solvent and Po is the vapor pressure of the pure solvent)

8. Freezing Point Depression • The extent to which the freezing point is lowered can be calculated using the formula below: • ΔT = iKfm • i= the van’t Hoff factor, • Kf= the freezing point constant of the solvent • m = the molality of the solution (More on the van’t Hoff factor later in the slideshow)

9. Boiling point elevation • The extent to which the boiling point is raised can be calculated using a formula similar to the one for freezing point depression: • ΔT = iKbm • i= the van’t Hoff factor, • Kb= the boiling point constant of the solvent • m = the molality of the solution

10. Osmotic Pressure • Osmosis is the movement of solvent particles across a semipermeable membrane from lower to higher concentration (to equalize the two concentrations) • Osmotic Pressure (π) is the pressure needed to prevent the flow of the solvent particles across that membrane; it can be calculated using the equation below: • π = i(nRT/V) • i = the van’t Hoff factor • n = the number of moles of solute • R = 0.0821 L-atm/mol-K • T = Kelvin temperature\ • V = volume of the solution

11. The van’t Hoff factor (i) • The van’t Hoff factor is the number of moles of particles released in the solution per mole of particle dissolved. • For a nonelectrolyte (ex: sugar), there are no ions produced when dissociation occurs, so i = 1 • For an electrolyte, the value of i is determined by writing and balancing the dissociation equation for that electrolyte • NaCl 1Na+ + 1Cl- so i = 2 • MgCl2  1Mg2+ + 2Cl- so i = 3 • AlCl3  1Al3+ + 3Cl- so i = 4