Chapter 11. Solutions and Their Properties

Solutions Chapter 11 z Homogeneous mixtures:  Solutions – ions or molecules (small particles)  Colloids – larger particles but still uniform (milk, fog) Solutions and Their Properties 1 The Solution Process z The Solution Process During dissolution, some forces are broken and new forces are created z z z z z Break up solvent-solvent attractions (endo.) Break up solute-solute attractions (endo.) Form solvent-solute attractions (exo.) ΔHsoln = ΔH1 + ΔH2 + ΔH3 Exothermic is usually favored 3 Ways of Expressing Concentration z 4 Concentration Units Variety of units z  Most commonly used is M (molarity)  Also molality, mole fraction, ppm, and Normality z  Molarity = moles solute / liter solution = mol/L  Depends on temperature; density of liquids changes with temperature Qualitative terms relating to solubility  Molarity:  insoluble, slightly soluble, soluble, very soluble z Molarity  Ex: 5.00 g NaCl in 0.251 L water Other comparative terms:  Ans: 0.341 M NaCl  dilute, concentrated (solids in liquids)  miscible, immiscible, partially miscible (liquids in z liquids) Mole fraction (X):  Mole Fraction = mole A / (mole A + mole B)  Ex: 5.00 g NaCl in 251 mL water  Ans: 0.00611 NaCl; 0.995 water 5 6 1 Concentration Units z Concentration Units Mass Percent  Mass Percent = (mass of solute / mass of solution) x 100%  Ex: 5.00 g of NaCl in 251 mL H2O (dH2O = 0.9982 g/mL)  Ans: 2.0 % NaCl z Molality  molality = moles solute / kilograms solvent = mol/kg  Independent of temperature because masses do not change with temperature (dH2O = 0.9982 g/mL)  Ex: 5.00 g NaCl in 251 mL water  Ans: 0.340 m NaCl Types of Solutions z z z z z z Worked Ex. 11.4; Problems 11.6, 11.7 Supersaturated Solution 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 Like Dissolves Like z z Acetate Movie Solid Solubility and Temperature Figure 11.6 It takes 2000 mL of H2O to dissolve 1 mL of CCl4 It takes 50 mL of H2O to dissolve 1 mL of CH2Cl2 “Like” refers to the polarity of substances:  Polar substances will dissolve in polar solvents  Nonpolar substances will dissolve in nonpolar solvents  Polar substances WILL NOT readily dissolve in nonpolar solvents (and vice versa) z Solubility of ionic compounds in water generally increases with higher water temps 2 Effect of Temp on Gas Solubility Effect of Pressure on Gas Solubility z z Strong effect only for gases dissolved in liquids. What happens to the amount of dissolved solute when pressure is increased (2nd picture)? Figure 11.8 Henry’s Law z z The solubility of gases in water decreases with increasing temperature. Why? Solubility goes to 0 at boiling point of water. 13 Effect of Pressure on Solubility 14 Colligative Properties z Henry’s Law: concentration (solubility) of a gas in a liquid is proportional to the pressure of the gas over the solution z Wh does a soft drink Why z fizz when opened? z 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: Henrys Law Diet Coke vapor pressure boiling point freezing point osmosis / osmotic pressure 15 Vapor Pressure Lowering z z z Pvap liquid P Adding a solute to a solvent lowers the Pvap Pvap of a solution < Pvap of a pure solvent Raoult’s Law: Psoln = X1Psolvent vap solution 16 B. P. Elevation/F. P. Depression z Phase diagrams can be used to see how vapor pressure changes cause changes in boiling and freezing points. Freezing pure water 17 Figure 11.12 18 3 B. P. Elevation/F. P. Depression z B.P. Elevation/F.P. Depression z Lower vapor pressure (red lines) z Changes in boiling/freezing points are directly proportional to the number of solute particles Equations that govern these effects: ΔTb = Kb m ΔTf = Kf m  Kb and Kf are based on the solvent  m = molality = moles solute / kg solvent z z Calculate molality: 62.345 g of vinyl chloride (CH2CHCl, MM = 62.494 g/mol) is added to 1264 g chloroform. Calculate ΔTb and ΔTf. What’s the solute? What’s the solvent? 20 B.P. Elevation/F.P. Depression Osmotic Pressure Pos molecular animation z Osmosis: selective passage of solvent molecules through a porous membrane 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  M = molarity  R = 0.08206 L atm z z Kb and Kf depend only on the solvent used Worked Ex. 11.11; Problems 11.18, 11.19 mol K 22 Osmosis Reverse Osmosis z 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 through but not dissolved ions. Onion 1M NaCl 4