Mole fraction, Molarity Moles of solute Molarity = Liters of solution What is the molarity of an ascorbic acid solution (C6H8O6) prepared by dissolving 1.80 grams in enough water to make 125 mL of solution. How many milliliters of this solution contain 0.0100 mol ascorbic acid. 1 mol C6H8O6 = 0.0102 mol C6H8O6 1.80 g C6H8O6 176 g C6H8O6 0.0102 mol C6H8O6 Molarity = = 0.0818 M .125 L soln

Solvation or hydration Properties of Solutions: The Solution Process Solvation or hydration Cl- Na+ Na+ - Na+ + Cl- - Cl- - Na+ + Na+ + Cl- - Na+ + Cl- Cl- Cl-

Properties of Solutions: The Solution Process Energy Changes and Solution Formation Solute-solute interactions Solvent-solvent interactions Solute-solvent interactions  Hsoln =  H1 +  H2 +  H3

Properties of Solutions: The Solution Process Energy Changes and Solution Formation NaOH:  Hsoln = -44.48 NH4OH:  Hsoln = 26.4 The overall change in enthalpy can be exo- or endothermic This explains why “like dissolves like”

Properties of Solutions: Solution Formation, Spontaneity and Disorder…WHY DOES THIS STUFF DISSOLVE? London dispersion forces bp = 77 bp = 69 Little energy is exchanged

Properties of Solutions: Solution Formation, Spontaneity and Disorder…WHY DOES THIS STUFF DISSOLVE? Processes in which the energy content of the system decreases tend to occur spontaneously. Processes in which the disorder of the system increases tend to occur spontaneously When molecules of different types are brought together, an increase in disorder occurs spontaneously unless the molecules are restrained sufficiently by strong intermolecular forces. Salt does not dissolve in gasoline because strong forces are holding the Na+ and Cl- ions together and the intermolecular forces associated with nonpolar substances is not sufficient to dislodge them.

Properties of Solutions: Saturated Solutions and Solubilityc As solid solute begins to dissolve in a solvent, the concentrations of solute particles in solution increases, so the chances of their colliding with the surface of the solid increases. This may lead to crystallization. Saturated: a solution with undissolved solute dissolve Solute + Solvent  Solution crystallize Supersaturated: a solution which contains a greater amount of solute than needed to form a saturated solution

Properties of Solutions: Factors Affecting Solubility As a rule, solubility increases with increasing molecular mass Polar liquids tend to dissolve in polar solvents. If the dissolve they are miscible. If they do not dissolve they are immiscible

Properties of Solutions: Factors Affecting Solubility Hydrogen-boning interactions between solute and solvent may lead to high solubility Because of H-bonding, solute-solute, solvent-solvent, and solute-solvent interactions are not appreciably different. Ethanol and ethanol Ethanol and water There is no significant change in the environment

Properties of Solutions: Factors Affecting Solubility However, the numbers carbon atoms in an alcohol does effect it solubility in water. As the length of the chain increases, the OH groups decrease leading to a decrease in solubility

Properties of Solutions: Factors Affecting Solubility Substances with similar intermolecular attractive forces tend to be soluble in one another….LIKE DISSOLVES LIKE Glucose has 5-OH groups on a six carbon framework which makes the molecule fairly soluble in water

Properties of Solutions: Factors Affecting Solubility Substances with similar intermolecular attractive forces tend to be soluble in one another….LIKE DISSOLVES LIKE Predict whether each of the following substances is more likely to dissolve in carbon tetrachloride or water: C7H16. NaHCO3, HCl

Pressure Effects The solubility of a gas in any solvent is increased as the pressure of the gas over the solvent increases By contrast the solubility of solids and liquids are not appreciable affected by pressure.

The relationship between pressure and solubility is expressed by Henry’s Law: Cg = kPg where Cg is the solubility of the gas in the solution phase, Pg is the partial pressure of the gas over the solution, and k is a Henry’s law constant which differs from one solute-solvent pair to another. For example the solubility of N2 gas in water at 25 ° C and 0.78 atm. is 5.3 x 10-4 M k = 5.3 x 10-4 M/0.78 = 6.8 x 10-4 mol/l-atm. Assume that the partial pressure of the N2 is doubled, Henry’s law, Cg = kPg predicts that the solubility will also double. Cg = kPg Cg = (6.8 x 10-4 mol/l-atm)(1.56 atm) Cg = 0.0011

In general, the solubility of gases decreases as temperature increases Temperature Effects In general, the solubility of gases decreases as temperature increases

Temperature Effects In general, the solubility of of ionic compounds, solids, normally increases as the temperature increases.

Lowering the Vapor Pressure: Explaining How it is Done!! A non-volatile solute added to a solvent reduces the capacity of the solvent molecules in the liquid phase to move into the gas phase The extent to which a nonvolatile solute lowers the vapor pressure is proportional to the concentration

Colligative Properties are the first evidence that salts break up into ions. Xa = Mole of solvent Moles of solvent + moles of solute x i If 10g of MgCl2 is added to 100ml of H2O at 23ºC, what would be the vapor pressure. The vapor pressure for water at this temperature is 21.07 torr.

Properties of Solutions:Colloids Suspensions that are the dividing Line Between Solutions and Heterogeneous Mixtures Tyndell effect: a scattering of light by colloidal particles

Properties of Solutions:Colloids The most important colloids are those in which the dispersing medium is water Hydrophilic and Hydrophobic Colloids Hydrophilic colloids are kept in suspension by interaction with surrounding water molecules

Properties of Solutions:Colloids The most important colloids are those in which the dispersing medium is water Hydrophilic and Hydrophobic Colloids Hydrophobic colloids can be stabilized in water by the adsorption of ions onto their surface

Properties of Solutions:Colloids The most important colloids are those in which the dispersing medium is water Hydrophilic and Hydrophobic Colloids