Chapter 11. Solutions. The Solution Process. The Solution Process. Hot and Cold Packs. The Solution Process

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1 Solutions Chapter 11 Homogeneous mixtures: Solutions ions or molecules (small particles) Colloids larger particles but still uniform (milk, fog) Solutions and Their Properties 1 During dissolution, some forces are broken and new forces are created Solvent-solvent interactions Solute-solute interactions Solvent-solute interactions H soln = H 1 + H 2 + H 3 3 Figure Generally, if the solute is more strongly attracted to the solvent attraction (than to itself weak IMF), then dissolution is favorable; exothermic ( H soln < 0) if the solute is more strongly attracted to itself (than to the solvent strong IMF), then dissolution is not favorable; endothermic ( H soln > 0) Hot and Cold Packs Hot packs dissolving solid releases heat CaCl 2 (s) H soln = kj/mol MgSO 4 (s) H soln = kj/mol Cold packs dissolving solid absorbs heat NH 4 NO 3 (s) H soln = kj/mol

2 Ways of Expressing Concentration Variety of units Most commonly used is M (molarity) Also ppm, mole fraction, molality, and Normality Qualitative terms relating to solubility insoluble, slightly soluble, soluble, very soluble <0.1 g/100g >2 g/100 g Other comparative terms: dilute, concentrated (solids in liquids) miscible, immiscible, partially miscible (liquids in liquids) 7 Concentration Units Molarity Molarity = moles solute / liter solution = mol/l Depends on temperature; density of liquids changes with temperature (d H2O at 20 o C = g/cm 3 ) Molarity: Ex: 5.0 g NaCl in water that gives a volume of 251mL Ans: 0.34 M NaCl Mole fraction (X): Mole Fraction = mole A / (mole A + mole B) Ex: 5.0 g NaCl in 251 ml water Ans: NaCl; water 8 Concentration Units Mass Percent Mass Percent = (mass of solute / mass of solution) x 100% Ex: 5.0 g of NaCl in 251 ml water Ans: 2.0 % NaCl Molality molality = moles solute / kilograms solvent = mol/kg Independent of temperature because masses do not change with temperature Ex: 5.0 g NaCl in 251 ml water Ans: 0.34 m NaCl Types of Solutions Unsaturated solution: contains less solute than it has the capacity to dissolve Saturated solution: contains the maximum amount of solute that will dissolve in a given solvent at a specific temperature Supersaturated solution: contains more solute than is present in a saturated solution Crystallization: process in which a dissolved solute comes out of solution and forms crystals Supersaturated Solution Supersaturated Solution Acetate

3 Solid Solubility and Temperature Solubility of ionic compounds in water generally increases with higher water temps Effect of Temperature on Solubility The solubility of gases in water decreases with increasing temperature. Why? Solubility goes to 0 at boiling point of water. Figure Effect of Pressure on Solubility Strong effect only for gases dissolved in liquids. What happens to the amount of dissolved solute when pressure is increased (2 nd picture)? Figure 11.8 Effect of Pressure on Solubility Henry s Law: concentration (solubility) of a gas in a liquid is proportional to the pressure of the gas over the solution Why does a soft drink fizz when the container is opened? More dilute solution More concentrated solution Molarity to molality Remember the distinction between molarity (M) and molality (m) Moles of solute M = L of solution m = Moles of solute kg of solvent To do these calculations, you must have one extra piece of information: density What is the molality of a M HCl(aq) solution? The density of the solution is g/ml. Colligative Properties Colligative properties: depend only on the number of solute particles in solution and not on the nature of the solute particles. The particles may be covalent molecules or ionic compounds. (We ll only deal with covalent molecules.) Colligative means depending on the collection Examples: vapor pressure boiling point freezing point osmosis / osmotic pressure What is the molarity of a 3.21 m KOH(aq) solution? (density of solution = g/ml) 19

4 Vapor Pressure Lowering Adding a solute to a solvent lowers the P vap P vap of a solution < P vap of a pure solvent Vapor Pressure Lowering P vap 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. Raoult s Law: partial pressure of a solvent vapor over a solution is the product of the mole fraction of the solvent (X 1 ) and the vapor pressure of the solvent (P o 1). P 1 = X 1 P o 1 20 B. P. Elevation/F. P. Depression Phase diagrams can be used to see how vapor pressure changes cause changes in boiling and freezing points. B. P. Elevation/F. P. Depression Lower vapor pressure (red lines) Freezing pure water Freezing solution Figure Freezing Point Depression Changes in freezing point are directly proportional to the number of solute particles Equations that govern these effects: Boiling Point Elevation Changes in boiling point are directly proportional to the number of solute particles Equations that govern these effects: T f = K f m T b = K b m T f is the change in normal freezing point K f is based on the identity of the solvent m is molality = moles solute/kg solvent T b is the change in normal boiling point K b is based on the identity of the solvent m is molality = moles solute/kg solvent Calculate the freezing point of a solution when g of vinyl chloride (CH 2 CHCl) is added to 1264 g cyclohexane. Calculate the boiling point of a solution when g of vinyl chloride (CH 2 CHCl) is added to 1264 g cyclohexane. 25

5 B.P. Elevation/F.P. Depression Group Work Calculate the molar mass of an unknown solute when the freezing point of water is lowered by 7.77 C after 651 g of the solute is dissolved in 2505 g of water. K b and K f depend only on the solvent being used Osmosis pickling of cucumbers Osmotic Pressure Osmosis: selective passage of solvent molecules through a porous membrane P os anim from a dilute solution to a more concentrated one. Semipermeable membrane: allows the passage of solvent molecules but blocks the passage of solute molecules. Π = MRT 30 Reverse Osmosis Uses high pressure to force water from a more concentrated solution to a less concentrated one through a semipermeable membrane. The membrane allows water molecules to pass through but not dissolved ions.