Solutions AP Chemistry

Heat of Solution Substance Heat of Solution (kJ/mol) NaOH -44.51 The Heat of Solution is the amount of heat energy absorbed (endothermic) or released (exothermic) when a specific amount of solute dissolves in a solvent. Substance Heat of Solution (kJ/mol) NaOH -44.51 NH4NO3 +25.69 KNO3 +34.89 HCl -74.84

Steps in Solution Formation H1 Step 1 - Expanding the solute Separating the solute into individual components

Steps in Solution Formation H2 Step 2 - Expanding the solvent Overcoming intermolecular forces of the solvent molecules

Steps in Solution Formation H3 Step 3 - Interaction of solute and solvent to form the solution

Predicting Solution Formation Solvent/ Solute H1 H2 H3 Hsol’n Outcome Polar/ Polar + large - large +/-small Solution forms Nonpolar + small +/- small No solution Nonpolar/ +/- small polar

Solubility Chart

Vapor Pressure - Henry’s Law The concentration of a dissolved gas in a solution is directly proportional to the pressure of the gas above the solution Applies most accurately for dilute solutions of gases that do not dissociate or react with the solvent Yes  CO2, N2, O2 No  HCl, HI

Raoult’s Law Psolution = Observed Vapor pressure of The presence of a nonvolatile solute lowers the vapor pressure of the solvent. Psolution = Observed Vapor pressure of the solution solvent = Mole fraction of the solvent P0solvent = Vapor pressure of the pure solvent

Liquid-liquid solutions in which both components are volatile Modified Raoult's Law: P0 is the vapor pressure of the pure solvent PA and PB are the partial pressures

Raoult’s Law – Ideal Solution A solution that obeys Raoult’s Law is called an ideal solution

Negative Deviations from Raoult’s Law Strong solute-solvent interaction results in a vapor pressure lower than predicted Exothermic mixing = Negative deviation

Positive Deviations from Raoult’s Law Weak solute-solvent interaction results in a vapor pressure higher than predicted Endothermic mixing = Positive deviation

Colligative Properties Colligative properties are those that depend on the concentration of particles in a solution, not upon the identity of those particles. Boiling Point Elevation Freezing Point Depression Osmotic Pressure 14

Freezing Point Depression Each mole of solute particles lowers the freezing point of 1 kilogram of water by 1.86 degrees Celsius. Kf = 1.86 C  kilogram/mol m = molality of the solution i = van’t Hoff factor 15

Boiling Point Elevation Each mole of solute particles raises the boiling point of 1 kilogram of water by 0.51 degrees Celsius. Kb = 0.51 C  kilogram/mol m = molality of the solution i = van’t Hoff factor 16

Freezing Point Depression and Boiling Point Elevation Constants, C/m Solvent Kf Kb Acetic acid 3.90 3.07 Benzene 5.12 2.53 Nitrobenzene 8.1 5.24 Phenol 7.27 3.56 Water 1.86 0.512 17

The van’t Hoff Factor, i Electrolytes may have two, three or more times the effect on boiling point, freezing point, and osmotic pressure, depending on its dissociation. 18

Dissociation Equations and the Determination of i NaCl(s)  Na+(aq) + Cl-(aq) i = 2 AgNO3(s)  Ag+(aq) + NO3-(aq) i = 3 MgCl2(s)  Mg2+(aq) + 2 Cl-(aq) i = 3 Na2SO4(s)  2 Na+(aq) + SO42-(aq) AlCl3(s)  Al3+(aq) + 3 Cl-(aq) i = 4 19

Ideal vs. Real van’t Hoff Factor The ideal van’t Hoff Factor is only achieved in VERY DILUTE solution. 20

Osmotic Pressure The minimum pressure that stops the osmosis is equal to the osmotic pressure of the solution 21

Osmotic Pressure Calculations i = van’t Hoff Factor M = Molarity of the solution R = Gas Constant = 0.08206 Latm/molK 22

Suspensions and Colloids Suspensions and colloids are NOT solutions. Suspensions: The particles are so large that they settle out of the solvent if not constantly stirred. Colloids: The particles intermediate in size between those of a suspension and those of a solution.

Types of Colloids Examples Dispersing Medium Dispersed Substance Colloid Type Fog, aerosol sprays Gas Liquid Aerosol Smoke, airborn germs Solid Whipped cream, soap suds Foam Milk, mayonnaise Emulsion Paint, clays, gelatin Sol Marshmallow, Styrofoam Solid Foam Butter, cheese Solid Emulsion Ruby glass Solid sol

The Tyndall Effect Colloids scatter light, making a beam visible. Solutions do not scatter light. Which glass contains a colloid? colloid solution