Solutions Chapter 16

Solution Formation Properties that effect solution formation include: Stirring – fresh solvent is continuously brought into contact with the solute. (speeds up the rate, not amount) Temperature – increase in kinetic energy leads to increase in frequency and force of collisions between solvent and solute. Particle Size – smaller particles dissolve faster due to more of the surface being exposed to the solvent.

Solubility and Saturation Saturated Solution – maximum amount of solute has been dissolved for a given amount of solvent. The undissolved solute and solvent have reached a state of dynamic equilibrium. The solubility of a substance is the amount of solute that dissolves in a given quantity of a solvent at a specified temperature and pressure to produce a saturated solution.  Unsaturated Solution – solution that contains less solute than a saturated solution at a given temperature and pressure. Supersaturated Solution – contains more solute than it can theoretically hold at a given temperature.

Solubility Curves

Miscible/Immiscible Miscible – two liquids that dissolve in each other in all proportions. (ex: water and ethanol) Partially Miscible – liquids that are slightly soluble in each other (ex: water and diethyl ether) Immiscible – liquids that are insoluble in one another. (ex: oil and vinegar)

Temperature and Pressure Effect on Solubility: Temperature Increase = Increase in solubility of Solids and liquids, while Decrease of solubility of gases. Ex: thermal pollution: less oxygen in heated water Pressure – no major effect on solids and liquids, but the solubility of a gas is strongly effected by the partial pressure of the gas above the solution. Ex: soda This pressure/Gas relationship is illustrated by Henry’s law, which 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. S1/P1 = S2/P2

Crystallization The crystallization of a supersaturated solution can be initiated if a very small crystal, called a seed crystal, of the solute is added or if the inside of the container is scratched. (solute deposits upon the surface of the seed or scratch) ex: rock candy! Supersaturated sugar deposited on the stick

Molarity The concentration of a solution is a measure of the amount of solute that is dissolved in a given quantity of solvent. A dilute solution is one that contains a small amount of solute. A concentrated solution contains a large amount of solute. Molarity (M) = moles of solute/Liters of solution (not just the solvent) M = mol/L When referring to concentration of solutions in a lab, it is called the molar concentration. Ex: 1M HCl would be identified as one molar hydrochloric acid

Dilution Dilution is the process of adding solvent to a solution to make the solution less concentrated. (reduces # of moles per unit of solvent) Moles of solute before dilution = moles of solute after dilution Moles of solute = Molarity of solution x Volume of Solution (rearrangement of Molarity equation) Therefore, M1V1 = M2V2

Concentration in Percent Volume of Solute/Volume of Solution Primarily used when both solute and solvent are liquids 91% alcohol (v/v) = 91 mL of isopropyl alcohol mixed with enough water to make 100 mL of solution OR Grams of Solute/ 100 grams of solution Convenient when solute is a solid 7 g of solute in 100 g of solution = 7% (m/m) Suppose you want to make 2000 g of a solution of glucose in water that has a 2.8% (m/m) concentration of glucose. How much glucose should you use? How much solvent (water) should be used?  (hint: known is that there are 2.8 g glucose per 100 g solution

Colligative Properties Colligative Property: A property that depends only upon the number of solute particles, and not upon their identity. 3 Colligative Properties: Vapor Pressure Lowering Freezing Point Depression Boiling Point Elevation

Vapor Pressure Lowers When a nonvolatile solute is added to a solvent, the vapor pressure of the solution is lower than that of the pure solvent. Ex: Adding NaCl or glucose to a solvent lowers it’s vapor pressure. This is a result of several water molecules being used for solvation of the solute, rather than having enough kinetic energy to escape the surface of the solution. The more ions that dissociate, the lower the vapor pressure. Ex: Compare the effects of adding 1 mole of Glucose, NaCl, or CaCl2 on the vapor pressure of the solution.

Freezing-Point Depression Freezing-Point Depression: The difference in temperature between the freezing point of a solution and the freezing point of the pure solvent. Freezing Point of the solution lowers due to water molecules surrounding (like a shell) the added solute during solvation. This interferes with the ability of the solvent to form an orderly pattern, and therefore takes more kinetic energy in the solvent to freeze. Also proportional to number of solute particles dissolved Ex: adding salt to icy roads or sidewalks

Boiling-Point Elevation Boiling-Point Elevation: The difference in temperature between the boiling point of a solution and the boiling point of the pure solvent. Due to the decrease in vapor pressure when a solute is added. Remember that boiling occurs when vapor pressure = atmospheric pressure.

Molality and Mole Fraction Molality: the number of moles of solute dissolved in 1 kilogram (1000 g) of solvent. (molal concentration) Molality = moles of solute/kilograms of solvent Ex: A 1.00 molal (1m) solution of glucose is prepared by adding 1.00 mol (180 g) of glucose to 1000 g of water. Mole Fraction: the ratio of the moles of the solute to the total number of moles of solvent and solute. X = nsolute/(nsolute +nsolvent)

Molality and FP Depression/BP Elevation For molecular solutes only: the magnitude of the freezing point depression and boiling point elevation is directly proportional to the molal concentration (m) ΔTf = Kf × m Kf : the molal freezing-point depression constant, which is equal to the change in freezing point for a 1-molal solution of a nonvolatile molecular solute. Same equation can be used for boiling-point elevation, by replacing Kf with Kb