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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. pressure. 17.3 Discuss Raoult's Law in a quantitative and qualitative method Raoult' for describing vapor pressure. 17.4 Outline the role of colligative properties and provide mathematical and qualitative descriptions for colligative properties. 9/30/2008 Zumdahl Chapter 17 1 Solution Composition The solute and solvent can be any combination of solid (s), liquid (l), and (s (l gaseous (g) phases. (g Dissolution: Two (or more) substances spread out, or disperse, into each
other at the level of individual atoms, molecules, or ions. Solution:
9/30/2008 Zumdahl Chapter 17 2 1 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
9/30/2008 Zumdahl Chapter 17 3 Molality msolute =
Units: moles per kg or mol kg-1 Molarity csolute =
Units: moles per liter or mol L-1
9/30/2008 Zumdahl Chapter 17 4 2 Section 17.2 The Thermodynamics of Solution Formation (skip) skip will cover Thermodynamics later in course, Chapters 9 & 10. 9/30/2008 Zumdahl Chapter 17 5 Factors Affecting Solubility 1. Structure Effects Review chapter 4 "Like Dissolves Like" concept Like" Polar molecules, water soluble, hydrophilic (water loving) molecules, E.g., Vitamins B and C; water-soluble Non-polar molecules, soluble in non-polar molecules, Nonmolecules, nonhydrophobic (water fearing) E.g., Vitamins A, D, K and E; fat-soluble
Vitamin C Vitamin A 9/30/2008 Zumdahl Chapter 17 6 3 Factors Affecting Solubility (con't) 1. Structure Effects 2. Pressure Effects
Henry's Law (for dilute solutions) Henry'
The vapor pressure of a volatile solute in a sufficiently dilute solution is proportional to the mole fraction of the solute in the solution. solution. kH is the kH depends on temperature and on strength of solute-solvent interaction solute- 9/30/2008 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 Henry' in water at this temperature. . temperature Given PN cN = 9.20 atm
2 2 -3 = [N 2 ] = 5.76x10 mol/l Henry' s Law PN = k N X N
2 2 2 2 XN =
2 nN + nH 2 O nN nH 2 O
8 2 9/30/2008 Zumdahl Chapter 17 4 Given PN cN PN X = 9.20 atm
2 2 -3 XN2 nN2 / nH2O
mol/l = [N 2 ] = 5.76x10 = kN X
2 Henry' s Law
2 N 2 2 = 2 O 5.76 x 10 mol/L _________________________ (1000 g/L) / (18.0 g/mol) -3 N =
2 nN nN
2 nN nH + nH 2 O 2 Therefore, = 9.20 atm / 1.04 x 10-4 kN2 = 8.86 x 10-4 atm
9/30/2008 Zumdahl Chapter 17 9 Factors Affecting Solubility (con't)
1. 2. 3. 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 temperature, increasing temperature 9/30/2008 Zumdahl Chapter 17 10 5 Factors Affecting Solubility (con't)
1. 2. 3. Structure Effects Pressure Effects Temperature Effects for Aqueous Solutions (b) The aqueous solubilities of most gases decrease with increasing temperature. temperature. In solvents other than water, gas solubilities often increase with increasing temperature.
9/30/2008 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.
9/30/2008 Zumdahl Chapter 17 12 6 Raoult's Law Consider a non-volatile solute (component 2) in a solvent non(component 1) that has a measurable vapor pressure X1 = mole fraction of solvent Raoult's Law Raoult' Solvent always obeys Raoult's Raoult' Law in the limit where X1 1 Positive (Negative) Deviation from Ideality solute-solvent attractions < (>) solute- solvent-solvent attractions solvent-
9/30/2008 Solute(s) always obey Henry's Solute(s) Henry' Law in the limit of Zumdahl Chapter 17 infinite dilution 13 Ideal Solution P1 = X1 P1 P2 = X2 P2 9/30/2008 Zumdahl Chapter 17 14 7 Vapor Pressure of a Solution of Two Volatile Liquids 9/30/2008 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. Properties. Colligative Properties include V B F O 9/30/2008 Zumdahl Chapter 17 16 8 The vapor pressure of a solvent above a dilute solution is always always less than the vapor pressure above the pure solvent. The boiling point of a solution of a non-volatile solute in a volatile nonsolvent always exceeds the boiling point of a pure solvent
Boiling: Liquid in equilibrium with vapor Boiling Point: Temp. at which Psolvent = Pext
lim a ti GAS
on n tio sa en on nd at i Co or ap Ev Normal boiling point: Psolvent = 1 atm Su b De po sit SOLID ion Melting Freezing LIQUID 9/30/2008 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 Phase diagrams for pure water (red lines) (red lines) and for an aqueous solution containing a nonvolatile solution (blue lines) (blue lines)
9/30/2008 Zumdahl Chapter 17 18 9 9/30/2008 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 = Molarity or moles/L = n / V 9/30/2008 Zumdahl Chapter 17 20 10 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 9/30/2008 Zumdahl Chapter 17 21 Dialysis A cellophane (polymeric) tube acts as the semisemipermeable membrane Purifies blood by washing impurities (solutes) into the dialyzing solution. Representation of the functioning of an artificial kidney
9/30/2008 Zumdahl Chapter 17 22 11 Sample Problem A dilute aqueous solution of a non-dissociating compound contains non1.19 g of the compound per liter of solution and has an osmotic pressure of 0.0288 atm at a temperature of 37C. Compute the 37 molar mass of the compound. Strategy 1.) use = cRT to find the c in mol/L g Molar mass g Given g l = = 3 .) Rearrange M = c mole mole l 2.) Recall that mole =
9/30/2008 Zumdahl Chapter 17 23 A dilute aqueous solution of a non-dissociating compound contains 1.19 g of the noncompound per liter of solution and has an osmotic pressure of 0.0288 atm at a 0.0288 temperature of 37C. Compute the molar mass of the compound 37 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
9/30/2008 Zumdahl Chapter 17 24 12 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 1901 awarded first Noble Prize in Chemistry
9/30/2008 Zumdahl Chapter 17 T = - i m K 25 Colligative Properties of Electrolyte Solutions
Elevation of Boiling Point Tb = m Kb
(m = molality) molality) The Effect of Dissociation (i = the number of particles released into the solution per formula unit of solute) e.g., NaCl dissociates into i = 2 (Na+ + Cl-) e.g., Na2SO4 dissociates into i = 3 (2 Na+ + 1 SO42-) e.g., acetic acid (a weak acid and weak electrolyte) dissociates only slightly so i 1 Analogously, Depression of Freezing Pt. 9/30/2008 Zumdahl Chapter 17 26 13 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
9/30/2008 Zumdahl Chapter 17 27 Light Scattering by Colloids: The Tyndall Effect 9/30/2008 Zumdahl Chapter 17 28 14 9/30/2008 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 BoilingFreezing17.6 Osmotic Pressure 17.7 Colligative Properties of Electrolyte Solutions 17.8 Colloids 9/30/2008 Zumdahl Chapter 17 30 15 ...
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