Chapter 13: Properties of Solutions A solution is a homogeneous mixture of a solvent and a solute. The solvent is the substance present in greater amount. Overview The Solution Process What are the driving forces for soln. formation? Relative Concentrations Saturated, Unsaturated, ? Solubility Factors that affect Solubility Quantitative Concentrations Colligative Properties Properties that depend on the concentration of solute particles 13.1 The Solution Process During dissolution, existing forces are broken and new forces are created. The Solution Process The forces involved in solutions are the bonding and intermolecular forces discussed in Ch 11. Dissolve NaCl Dissolved in Water Hydration Hydration (solvation) of cations and anions makes new forces. DHsolvation is the energy for the process: solute(g) + solvent(g) ? solvated solute particles Heat of Solution DHsoln is the energy for the process: solute + solvent ? solution Can analyze as a three-step process. Energy Changes and Solution Formation We define the enthalpy change in the solution process as ?Hsoln = ?H1 + ?H2 + ?H3 ?H1 = energy needed to break up solute (+) ?H2 = energy needed to break up solvent (+) ?H3 = energy given off due to solute-solvent interactions (-) ?Hsoln can either be positive or negative depending on the intermolecular forces. Solution Process Processes tend to occur spontaneously if they lead to a lower energy (enthalpy) or to an increased randomness or disorder (entropy). Processes that increase disorder (entropy) tend to occur spontaneously. Solution Process Why does an exothermic solution process occur? (Hot Pack) ?H < 0 Why does an endothermic solution process occur? (Cold Pack) Entropy Increases Mixtures have more Entropy 13.2 Saturated Solutions and Solubility Concentration: measure of the relative amounts of substances making up a solution Solubility: the maximum amount of solute in a given amount of solution (g/100 g) (The concentration of solute in a saturated solution) Solubility Saturated solution: an equilibrium situation in which the maximum amount of solute is dissolved in a given amount of solvent Solubility Unsaturated solution: less than the maximum amount of solute is dissolved. Solubility Supersaturated solution: more than the maximum amount of solute is dissolved; an unstable state that reverts to a saturated solution if the solution is disturbed in some way 13.3 Factors Affecting Solubility What causes different solubilities? General Rule: ?Like dissolves like? The definition of ?like? involves the nature of the bonding and structure of solute and solvent: ionic or polar covalent vs. non-polar covalent The solution process involves making and breaking ionic bonds and/or intermolecular forces (IMF). Effect of Molecular Structure on Solubility Non-polar liquids, like C6H6 (benzene) and CCl4, dissolve readily in other non-polar liquids, but not very readily in polar liquids. Polar liquids, like HO-CH2-CH2-OH (ethylene glycol) and H2O, dissolve readily in other polar liquids, but not very readily in non-polar liquids. Why is this the case? Hint: Analyze the solution process in terms of forces made and broken. Group Work If you want to dissolve I2 and CH3NH2 in a solvent, which of the following solvents would you choose for each? Benzene, C6H6. Water, H2O. Solubility of Gases Most gases are only slightly soluble in water. (10-3 M) Exceptions are HCl and NH3. Why are these molecules unusually soluble? Demo Effect of Pressure on Solubility Strong effect only for gases dissolved in liquids. Increase in pressure of the gas above the solution increases the solubility of the gas in the solution. Effect of Pressure on Solubility Henry?s Law: C = k P Why does a soft drink fizz when the container is opened? Why are patients with breathing difficulties placed in an oxygen tent? Hyperbaric Chamber Effect of Temperature on Solubility Most, but not all, ionic solids are more soluble at higher temperatures. Effect of Temperature on Solubility When solute is liquid or solid: Endothermic solution process: added heat at higher temperatures helps overcome the intermolecular forces, so solubility increases with increased temperature. Exothermic solution process: added heat overcomes solute-solvent forces, so solubility decreases with increased temperature. Effect of Temperature on Solubility The solubility of gases in water decreases with increasing temperature. The solubility goes to zero at the boiling point of water. Why does the taste of water change if it is boiled? Effect of Temperature on Solubility Thermal Pollution results when hot water is dumped into a small body of water. The increase in temperature causes the solubility of O2 to decrease. 13.4 Ways of Expressing Concentration Qualitative Terms Qualitative terms relating to solubility insoluble, slightly soluble, soluble, very soluble <0.1 g/100g >2 g/100 g Other comparative terms: dilute, concentrated miscible, immiscible, partially miscible 13.4 Ways of Expressing Concentration Quantitatively Quantitative Expressions: Concentration = amount of solute/amount of solvent or solution Variety of units Molarity (M) Molality (m) g/100 g - used to describe solubility Mass % (Percent by mass) Volume % (Percent by volume) Concentration Units Molarity M = moles solute/Liter solution = mol/L Depends on temperature because liquid volumes change with temperature Molality m = moles solute/kilograms solvent = mol/kg Independent of temperature because masses do not change with temperature Concentrations Percent by mass: What mass of NaOH is in 20.0 g of a solution that is 5.0% NaOH by mass? Percent by Volume: What is the percent by volume of a solution that is made by adding 20.0 mL of ethanol to make a solution that has a total volume of 50.0 mL? 13.5 Colligative Properties Colligative properties are those whose value depends only on the concentration of dissolved particles, not on their identity. The particles may be molecules or ions. If they are ions, the important factor is the sum of the concentrations of cations and anions. Colligative Properties: vapor pressure boiling point melting point osmotic pressure Vapor Pressure (of solvent above a solution) Pvap of a solution < Pvap of the pure solvent Evidence: solvent evaporates from a less concentrated solution and condenses into a more concentrated solution in a closed container. Figure 11.16 Vapor Pressure Pvap is lowered because fewer surface positions are occupied by solvent molecules and because intermolecular forces in solutions are usually greater than those in the separated substances. Vapor Pressure The lowering of vapor pressure can also be seen on a phase diagram. Freezing Point Depression Boiling Point Elevation Note on phase diagram that the freezing point decreases and the boiling point increases for a solution, as a result of the lowering of the vapor pressure. Equations that govern these effects: DTf = i Kf m DTb = i Kb m DT is defined to be always positive m = moles solute/kg solvent, independent of T i = number of particles per formula unit Kf and Kb are determined by the identity of the solvent (provided in Table 13.4) Freezing Point Depression Boiling Point Elevation Applications of DT antifreeze ice cream makers CaCl2 on icy roads and in aqueous slurries in winter measure molar masses distinguish between electrolytes and non-electrolytes Group Work Which aqueous solution should have the: Highest boiling point? Lowest melting point? Lowest freezing point? 1 m NaNO3 1 m Na2SO4 1 m Na3PO4 Osmosis Solvent molecules move through a semipermeable membrane from a dilute solution to a concentrated solution; solute cannot move through the membrane. Figure 11.19 Osmotic Pressure Equilibrium is reached when the two solutions have identical concentrations, but solvent continues to move in both directions at equal rates. The movement of solvent creates a pressure that opposes additional net movement Osmotic Pressure Osmotic Pressure Osmotic pressure (p ) follows an equation much like the ideal gas law: p V = n R T or p = M R T R = 0.08206 L atm/mol K M = concentration of particles (molecules or ions) in mol/L Easy to measure small p, so useful for determining molar masses of large molecules like proteins and polymers. Applications of Osmosis Why does lettuce become crispy when soaked in water? Why do prunes expand when soaked in water? Blood cells were soaked in water, 5% glucose, and 25% glucose. Identify the blood cell that received each treatment. Crenation hemolysis Reverse Osmosis Reverse osmosis is a nonspontaneous process. Desalination: Water can be purified by applying pressure on the more concentrated side of the membrane. This forces water to the more dilute (or pure side), decreasing its concentration even more.