Properties of Solutions AP Chapter 13

The Solution Process Solutions form when one substance disperses uniformly throughout another substance. The ability of substances to form a solution depends on: the types of intermolecular attractions the natural tendency of substances to disperse into larger volumes when not contained. The attractive interaction between solvent molecules and solute is called solvation. When the solvent is water, the interaction is called hydration.

Ionic Substances The dissolution of ionic substances in water is promoted by hydration of the separate ions by the polar water molecules. The overall enthalpy change upon solution formation may be either positive or negative. Solution formation is favored by both a negative enthalpy change (exothermic) and by an increase dispersion in space of the components in the solution (this means a positive entropy change.)

Dissolution of an ionic solid in water

Dispersion Forces Dispersion forces dominate when non-polar substances dissolve in another non-polar substance. Remember, like dissolves like.

Enthalpy Changes Involves: Separation of the solute molecules (∆H1) Separation of the solvent molecules (∆H2) Formation of solute-solvent interactions (∆H3) The overall enthalpy change is : ∆Hsoln = ∆H1 + ∆H2 + ∆H3

Entropy The degree of randomness in a system is called entropy. Processes in which the energy content of the system decreases tend to occur spontaneously. Processes occurring at a constant temperature in which the randomness increases tend to occur spontaneously. In most cases, the formation of solutions is favored by the increase in entropy that accompanies mixing.

Saturated Solutions A saturated solution is present when the number of molecules going into solution (dissolving) equal the number of molecules coming out of solution (crystallizing). Solute + solvent solution This is called dynamic equilibrium. dissolve crystallize

Dynamic Equilibrium

Solubility Solubility is the maximum amount of a solute that will dissolve in a given amount of solvent at a specified temperature, given that excess solute is present. Example: the solubility of NaCl at 0°C 35.7 g per 100 mL of water. This is the maximum amount of NaCl that can dissolve to form a stable equilibrium at that temperature.

Unsaturated and Supersaturated If less solute is dissolved than is needed to make a saturated solution, the solution is unsaturated. Under suitable conditions, it is sometimes possible to form a solution that contains a greater amount of solute than is needed to make a saturated solution. This condition is called supersaturation.

Factors Affecting Solubility The stronger the attraction between solute and solvent molecules, the greater the solubility. Like dissolves like (the substances have similar intermolecular attractive forces.) Polar substances tend to dissolve in polar solvents. Non-polar substances do not dissolve in polar solvents.

Miscibility Polar liquids that mix in polar solvents are miscible. Those that do not dissolve in one another are immiscible.

Pressure Effects The solubilities of liquids and solids are not greatly affected by pressure, but the solubility of a gas is. The solubility of a gas in any solvent is increased as the pressure over the solvent increases. The solubility of the gas increases in direct proportion to its partial pressure above the solution.

The Effect of pressure on gas solubility: The concentration of solute molecules at equilibrium increases in proportion to the pressure.

Henry’s Law The relationship between pressure and solubility of a gas is expressed with Henry’s Law Equation: Sg = kPg Sg is the solubility of the gas in the solution phase and Pg is the partial pressure of the gas over the solution. k is a proportionality constant (Henry’s Law constant) which is different for each solute-solvent pair.

Temperature The solubility of most solid solutes in water increases as the temperature of the solution increases. The solubility of gases in water decreases with increasing temperature.

Concentration Concentration of a solution can be expressed either quantitatively or qualitatively (dilute or concentrated).

Mass Percentage Mass percentage is expressed using the equation: mass % of component = X 100 Mass of component in soln total mass of soln

Parts per Million This calculation is used in dilute solutions and uses 106 (for parts per million – ppm) or 109 (for parts per billion – ppb) as the multiplier instead of 100. ppm of component = X 106 Mass of component in soln total mass of soln

Mole Fraction This is how many moles of substance are in the solution. Mole fraction of component = Moles of component total moles of all components

Molarity Relates the volume of solution to the quantity of solute that it contains. Molarity = Moles solute Liters solution

Molality This is the number of moles of solute per kilogram of solvent: Molality = Moles of solute Kilograms of solvent

Colligative Properties Colligative properties are properties that depend on the quantity of solute particles but not the kind of particles in the solution. Adding solute particles to a solution raises the boiling point and lowers the freezing point of the solution. It also lowers the vapor pressure of the solution.

Raoult’s Law The extent to which a nonvolatile solute lowers the vapor pressure is proportional to its concentration. Raoult’s Law: the partial pressure exerted by the solvent vapor above a solution, PA, equals the product of the mole fraction of the solvent in the solution, XA, times the vapor pressure of the pure solvent, P°A. PA = XA P°A

Raoult’s Law Just like ideal gases follow the ideal gas law, an ideal solution obeys Raoult’s Law. Real solutions best approximate an ideal solution when the solute concentration is low and when the solute and solvent have similar molecular sizes and types of intermolecular attractions.

Boiling Point Elevation Normal boiling point is the temperature at which its vapor pressure equals 1 atm. The boiling point of a solution will be higher than that of the pure liquid. At the surface of a pure liquid, there are only liquid molecules that can evaporate, where as at the surface of a solution, there are a combination of molecules that evaporate.

Boiling Point Elevation ΔTb = Kbm ΔTb is the increase in boiling point relative to that of the pure solvent It is directly proportional to the concentration of the solution expressed by its molality, m. Kb is called the molal boiling-point-elevation constant.

Freezing Point Depression The freezing point of a solution is the point at which the first crystals of pure solvent begin to form in equilibrium with the solution. The freezing point of the solution is lower than that of the pure liquid.

Molal Freezing-Point-Depression Constant ΔTf is the decrease in freezing point, and is a positive quantity. It is directly proportional to the molality of the solute. ΔTf = Kfm Kf is the molal freezing-point-depression constant. Use Table 13.4!

Osmosis Semipermeable materials allow some materials to pass through the network of tiny pores, and not others. Most importantly, semipermeable membranes allow water and other small molecules to pass through, but not larger solute molecules or ions.

Osmosis Osmosis is the movement of solvent molecules through a semipermeable membrane from a less concentrated to a more concentrated solution. This net movement of solvent generates osmotic pressure, Π, which can be measured in units of gas pressure, like atm. Π = RT = MRT n V

The blue arrows represent the net movement of water molecules.

Colloids Colloids are particles that are large on the molecular scale but still small enough to remain suspended indefinitely in a solvent system. (aka colloidal dispersions.) They are intermediate between solutions and heterogeneous mixtures. They have the ability to scatter light; known as the Tyndall effect.

Hydrophilic and Hydrophobic Colloids The most important colloids are those in which water is the solvent, or dispersing agent. Hydrophilic colloids are water loving and are found in the human body and help keep molecules suspended in water. (enzymes, antibodies) Hydrophobic colloids are water fearing and must be stabilized before they can be mixed into water. (droplets of oil.)

How a soap molecule works.