Ch. 14: Mixtures & Solutions Sec. 14.3: Factors Affecting Solvation

Objectives Relate intermolecular forces and the process of solvation. Define solubility and identify factors affecting it.

Solvation Why are some substances insoluble & others are soluble in water? “Like dissolves like” is a rule that usually indicates whether a substance will dissolve in a specific solvent.

Solvation Defined as the process of surrounding solute particles with solvent particles to form a solution. Solvation in water is also called “hydration.”

Solvation To form a solution: solute particles have to separate from one another solvent particles have to separate from on another solute particles have to become surrounded by solvent particles

Example: Solvation of NaCl in Water Hydrogen bonding in water (H2O) solvent solute

Steps in Solvation This means: - attractive forces between solute particles must be broken. (requires energy - is endothermic) - attractive forces between solvent particles must be broken. (requires energy - is endothermic) - attractive forces between solute and solvent particles must form and be stronger than the other attractive forces that exist. (releases energy - is exothermic)

When hydration of ionic compounds (like NaCl) occurs, the solvent-solute attractions have to overcome the solvent-solvent attractions and the solute-solute attractions.

Heat of Solution The breaking of solute to solute and solvent to solvent attractions is endothermic; creating solute to solvent attractions is exothermic. The overall energy change that occurs during the solution formation process is called the heat of solution. ΔHsoln can be exothermic or endothermic (depends on . . . ?) temperature change upon solvation indicates if process is exothermic or endothermic. Energy is required to overcome the attractive forces within the solute and within the solvent, so both steps are endothermicc. When the particles mix, the particles attract each other and energy is released. This step is exothermic.

Aqueous Solutions of Polar Molecular Compounds Solvation occurs in much the same way. Polar sugar molecules are attracted to polar water molecules. However, aqueous solutions contain dissolved polar molecules and do not conduct electricity. Only solutions with hydrated ions are electrolytes. The sugar molecule contains many polar O-H bonds; these interact with water strongly enough for the sugar to dissolve

Oil & Water Aqueous solutions of nonpolar molecules do not form. Oil does not form a solution with water because there is little attraction between polar water molecules and nonpolar oil molecules.

Rate of Solvation To increase collisions between solute & solvent particles (& increase the rate or speed of solvation): 1. Agitate the mixture (stir or shake) - this moves dissolved particles away from undissolved particles, allowing more collisions to occur.

Rate of Solvation 2. Increase the surface area of the solute (break the solute into small pieces) - this allows more area of the solute to be exposed to water molecules, so more collisions will occur. 3. Increase the temperature of the solvent - this increases the KE of the solvent particles so more frequent & energetic collisions occur.

Solubility Solubility is defined as the maximum amount of solute that can dissolve in a given amount of solvent at a particular temperature and pressure. As we have seen, solubility depends on the nature of the solute and solvent. Table 4, pg. 494, shows the solubilities of various solutes in water. Some are slightly soluble, others are very soluble.

Solubility Curves Solubility is usually expressed in grams of solute per 100 g of solvent at a specified temperature. A graph of solubility vs. temperature is called a solubility curve.

Factors Affecting Solubility (Amount NOT Rate) Notice that solubility is affected by temperature. Generally, increasing the temperature of the solvent causes the kinetic energy of the particles to increase, so more collisions (more solvation) occurs.

Factors Affecting Solubility (Amount NOT Rate) Many solid substances are more soluble at high temperatures than at low temperatures. Gases are less soluble at higher temperatures than at lower temperatures. GASES SOLIDS

Factors Affecting Solubility For solids in water, generally solubility ________ with temperature. For gases in water, solubility _________ with temperature.

Saturated Solutions A saturated solution contains the maximum amount of dissolved solute for a given amount of solvent at a specific temperature and pressure. Solubility curves, therefore, relate information about saturated solutions. In a saturated solution, as long as the conditions remain constant, the overall amount of dissolved solute remains constant. If more solute is added to a saturated solution, it will . . . In a saturated solution, if the temperature of the solvent is increased, generally more solute will . . .

Unsaturated Solutions An unsaturated solution is one that contains less dissolved solute for a given temperature and pressure than a saturated solution. If more solute is added to an unsaturated solution, it will . . . More solute can be dissolved in an unsaturated solution.

Formation of a Saturated Solution A solid solute is added to a fixed quantity of water. After a few minutes, the solution is colored due to the dissolved solute, and there is less undissolved solute than in (a). After a longer time, the solution color has deepened, and the quantity of undissolved solute is further diminished from that in (b). The solution in (b) must be unsaturated because more can dissolve. Still later, the solution color and the quantity of undissolved solute appear to be the same as that in (c). Dynamic equilibrium must have been attained in (c) and persists in (d). In both (c) and (d), the solution is saturated. Which solutions are saturated? Which are unsaturated?

Supersaturated Solutions A supersaturated solution contains more dissolved solute than a saturated solution at the same temperature. Such solutions are made by dissolving excess amounts of solid solute at high temperatures and then slowly cooling the solution. If the excess solute remains dissolved, the solution is supersaturated. Such solutions are unstable. If the solution is disturbed or “seeded”, the excess solute will precipitate out.

Example: Glucose

Supersaturated Solutions A supersaturated solution is holding more than the maximum amount of solute. In some cases, such as the sodium acetate solution pictured here, a supersaturated solution may be temporarily stable. Any disturbance however, such as dropping in a small piece of solid sodium acetate (a) will cause the solid to come out of solution (b) and (c). Students might think of the disturbance as a "reminder" to the sodium acetate ions that they would be at lower energy if they were organized in a crystal lattice.

Other Factors Affecting Solubility For gases in liquids, solubility _________ with pressure.

Factors that Affect Solubility A can of soda pop is pressurized with carbon dioxide. When the can is opened the pressure is released, lowering the solubility of carbon dioxide in the solution and causing it to come out of solution as bubbles. The higher the pressure above a liquid, the more soluble the gas is in the liquid. This explains why soda fizzes after the can is opened. The pressure of the open atmosphere is lower than the pressure inside the sealed can.

Henry’s Law Henry’s Law states that at a given temperature, the solubility (S) of a gas in a liquid is directly proportional to the pressure (P) of the gas above the liquid. Mathematically, S1 = S2 P1 P2 Solubility is generally expressed as g/L.

Henry’s Law If 0.85 g of a gas at 405.2 kPa of pressure dissolves in 1.0 L of water at 25 degrees C, how much will dissolve in 1.0 L of water at 101.3 kPa of pressure and the same temperature? A gas has a solubility of 0.66 g/L at 10.0 atm. What is the pressure on a 1.0 L sample that contains 1.5 g of gas? .21 g/L