Chpt 17 - Chapter 17 Objectives 17.1 Define solutions and...

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Unformatted text preview: Chapter 17 Objectives 17.1 Define solutions and the different mathematical methods for describing solution composition. 17.2 Provide a qualitative and pictorial description of vapor pressure. 17.3 Discuss Raoult's Law in a quantitative and qualitative method for describing vapor pressure. 17.4 Outline the role of colligative properties and provide mathematical and qualitative descriptions for colligative properties. 05/12/09 Zumdahl Chapter 17 1 Solution Composition The solute and solvent can be any combination of solid (s), liquid (l), and gaseous (g) phases. Dissolution: Two (or more) substances spread out, or disperse, into each other at the level of individual atoms, molecules, or ions. Solution: A homogenous mixture Solvent: The major component Solute: Minor component(s) 05/12/09 Zumdahl Chapter 17 2 Measures of Solution Composition Mass Fraction, Mole Fraction, Molality, Molarity Mass percentage (weight percentage): mass percentage of a component = mass of component total mass of mixture 100% Mole fraction: The chemical amount (in moles) divided by the total amount (in moles) X1 = n1 / (n1 + n2 + n3 + ...) X1 + X2 + X3 + ... = 1 and n1 + n2 + n3 +... = n 05/12/09 Zumdahl Chapter 17 3 Molality m = (temperature independent) Moles of solute per kg of solvent Molarity solute c = Moles of solute per liter of solution Units: moles per liter or mol L-1 05/12/09 Zumdahl Chapter 17 4 Section 17.2 The Thermodynamics of Solution Formation (skip) skip will cover Thermodynamics later in course, Chapters 9 & 10. 05/12/09 Zumdahl Chapter 17 5 Factors Affecting Solubility 1. Structure Effects Review chapter 4 "Like Dissolves Like" concept Polar molecules, water soluble, hydrophilic (water loving) E.g., Vitamins B and C; water-soluble Non-polar molecules, soluble in non-polar molecules, hydrophobic (water fearing) E.g., Vitamins A, D, K and E; fat-soluble Vitamin C Vitamin A 05/12/09 Zumdahl Chapter 17 6 Factors Affecting Solubility (con't) 1. Structure Effects 1. Pressure Effects Henry's Law (for dilute solutions) The vapor pressure of a volatile solute in a sufficiently dilute solution is proportional to the mole fraction of the solute in the solution. H k is the H Henry's Law Constant k depends on temperature and on strength of solute-solvent interaction 05/12/09 Zumdahl Chapter 17 7 When the partial pressure of nitrogen over a sample of water at 19.4C is 9.20 atm, then the concentration of nitrogen in the water is 5.76 x 10-3 mol L-1. Compute the Henry's law constant for nitrogen in water at this temperature. Given PN = 9.20 atm 2 c N = [N 2 ] = 5.76x10-3 mol/l 2 Henry' s Law PN = k N X N 2 2 2 XN = 2 05/12/09 nN 2 nN + nH 2 O 2 nN nH 2 O 8 2 Zumdahl Chapter 17 Given PN = 9.20 atm 2 XN2 nN2 / nH2O = 2 O c N =[N 2 ] = 5.76x10-3 mol/l 2 Henry' s Law PN = k N X N 2 2 _________________________ 5.76 x 10-3 mol/L 2 XN = 2 nN 2 nN + nH 2 nN nH (1000 g/L) / (18.0 g/mol) 2 O 2 Therefore, = 9.20 atm / 1.04 x 10-4 kN2 = 8.86 x 10-4 atm 05/12/09 Zumdahl Chapter 17 9 Factors Affecting Solubility (con't) 1. 2. 1. Structure Effects Pressure Effects Temperature Effects for Aqueous Solutions (a) The aqueous solubilities of a majority of solids increase with increasing temperature, but some decrease with increasing temperature 05/12/09 Zumdahl Chapter 17 10 Factors Affecting Solubility (con't) 1. 2. 1. Structure Effects Pressure Effects Temperature Effects for Aqueous Solutions (b) The aqueous solubilities of most gases decrease with increasing temperature. In solvents other than water, gas solubilities often increase with increasing temperature. 05/12/09 Zumdahl Chapter 17 11 The Person Behind the Science Francois-Marie Raoult (1830-1901) Highlights 1886 Raoult's law , the partial pressure of a solvent vapor in equilibrium with a solution is proportional to the ratio of the number of solvent molecules to nonvolatile solute molecules. allows molecular weights to be determined, and provides the explanation for freezing point depression and boiling point elevation. For ideal solutions Moments in a Life Raoult was a prominent member of the group which created physical chemistry, including Arrhenius, Nernst, van t'Hoff, Planck. 05/12/09 Zumdahl Chapter 17 12 Raoult's Law Consider a non-volatile solute (component 2) in a solvent (component 1) that has a measurable vapor pressure X1 = mole fraction of solvent P1=X1P1O Raoult's Law Solvent always obeys Raoult's Law in the limit where X1 1 Ideal solution obeys Raoult's Law at all compositions Solute(s) always obey Henry's solute-solvent attractions < (>) Law in the limit of solvent-solvent attractions 05/12/09 Zumdahl Chapter 17 infinite dilution 13 Positive (Negative) Deviation from Ideality Ideal Solution P1 = X1 P1 P2 = X2 P2 Ptot = P1 + P2 05/12/09 Zumdahl Chapter 17 14 Vapor Pressure of a Solution of Two Volatile Liquids 05/12/09 Zumdahl Chapter 17 15 Colligative Properties of Solutions For some physical properties, the difference between a pure solvent and a dilute solution depends only on the number of solute particles present and not on their chemical identify. Such properties are called Colligative Properties. Colligative Properties include V apor pressure lowering B oiling point elevation F reesing point depression O smotic pressure 05/12/09 Zumdahl Chapter 17 16 Lowering of Vapor Pressure The vapor pressure of a solvent above a dilute solution is always less than the vapor pressure above the pure solvent. Elevation of Boiling Point The boiling point of a solution of a non-volatile solute in a volatile solvent always exceeds the boiling point of a pure solvent Boiling: Liquid in equilibrium with vapor Boiling Point: Temp. at which Psolvent = Pext lim ati on GAS Co nd Normal boiling point: Psolvent = 1 atm en n tio sa o ap Ev Su b os itio n i rat De p on SOLID Melting Freezing LIQUID 05/12/09 Zumdahl Chapter 17 17 Elevation of Boiling Point and Lowering of Vapor Pressure T = K m b solute T is the boiling point elevation Kb is molal boiling - point elevation constant m is the molality of the solute in solution solute Kb is solvent dependent but solute independent Phase diagrams for pure water (red lines) and for an aqueous solution containing a nonvolatile solution (blue lines) 05/12/09 Zumdahl Chapter 17 18 T-Kbmsolute T = -Kfmsolute 05/12/09 Zumdahl Chapter 17 19 Osmotic Pressure Important for transport of molecules across cell membranes, called semipermeable membranes Osmotic Pressure = = g d h PV = nRT = c RT V = n RT c = Molarity or moles/L = n / V 05/12/09 Zumdahl Chapter 17 20 Osmotic Pressure (con't) The normal flow of solvent into the solution (osmosis) can be prevented by applying an external pressure to the solution. Osmotic Pressure useful for Determining the Molar Mass of protein and other macromolecules Small concentrations cause large osmotic pressures Can prevent transfer of all solute particles Dialysis at the wall of most plant and animal cells 05/12/09 Zumdahl Chapter 17 21 Dialysis A cellophane (polymeric) tube acts as the semipermeable membrane Purifies blood by washing impurities (solutes) into the dialyzing solution. Representation of the functioning of an artificial kidney 05/12/09 Zumdahl Chapter 17 22 Sample Problem A dilute aqueous solution of a non-dissociating compound contains 1.19 g of the compound per liter of solution and has an osmotic pressure of 0.0288 atm at a temperature of 37C. Compute the molar mass of the compound. Strategy 1.) use = cRT to find the c in mol/L g 2.) Recall that mole = Molar mass g g Given l 3.) Rearrange M = = = mole mole c l 05/12/09 Zumdahl Chapter 17 23 A dilute aqueous solution of a non-dissociating compound contains 1.19 g of the compound per liter of solution and has an osmotic pressure of 0.0288 atm at a temperature of 37C. Compute the molar mass of the compound Solution c = = 0.0288 atm / (0.0821 L atm mol-1 K-1) (310. K) c = 1.13 x 10-3 mol L-1 M = M = 1050 g mol-1 05/12/09 Zumdahl Chapter 17 24 The Person Behind the Science J.H. van't Hoff (1852-1901) Highlights Discovery of the laws of chemical dynamics and osmotic pressure in solutions Mathematical laws that closely resemble the laws describing the behavior of gases. his work led to Arrhenius's theory of electrolytic dissociation or ionization Studies in molecular structure laid the foundation of stereochemistry. van't Hoff Factor (i) Moments in a Life 05/12/09 1901 awarded first Noble Prize in Chemistry Zumdahl Chapter 17 T = - i m K 25 Colligative Properties of Electrolyte Solutions Elevation of Boiling Point Tb = m Kb (m = molality) The Effect of Dissociation b T = i m K (i = the number of particles released into the solution per formula unit of solute) e.g., NaCl dissociates into i = 2 (Na + Cl ) Analogously, Depression of Freezing Pt. 05/12/09 Zumdahl Chapter 17 26 Colloids: Colloidal Dispersions 1nm to 1000 nm in size Examples Opal (water in solid SiO2) Aerosols (liquids in Gas) Smoke (solids in Air) Milk (fat droplets & solids in water) Mayonnaise (water droplets in oil) Paint (solid pigments in liquid) Biological fluids (proteins & fats in water) Characteristics Larger particle sizes: translucent, cloudy, milky Smaller particle sizes: can be clear 05/12/09 Zumdahl Chapter 17 27 Light Scattering by Colloids: The Tyndall Effect 05/12/09 Zumdahl Chapter 17 28 05/12/09 Zumdahl Chapter 17 29 Chapter 17 Properties of Solutions 17.1 Solution Composition 17.2 The Thermodynamics of Solution Formation (skip) 17.3 Factors Affecting Solubility 17.4 The Vapor Pressures of Solutions 17.5 Boiling-Point Elevation and Freezing-Point Depression 17.6 Osmotic Pressure 17.7 Colligative Properties of Electrolyte Solutions 17.8 Colloids 05/12/09 Zumdahl Chapter 17 30 ...
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