5/19/2016 Chapter 11: Properties of Solutions Their Concentrations and Colligative Properties Chapter Outline 11.1 Energy Changes when Substances Dissolve 11.2 Vapor Pressure 11.3 Mixtures of Volatile Substances 11.4 Colligative Properties of Solutions 11.5 Osmosis and Osmotic Pressure 11.6 Using Colligative Properties to Determine Molar Mass 1 5/19/2016 Enthalpy of Solution Dissolution of Ionic Solids: • Enthalpy of solution (∆Hsoln) depends on: » Energies holding solute ions in crystal lattice » Attractive force holding solvent molecules together » Interactions between solute ions and solvent molecules • ∆Hsoln = ∆Hion-ion + ∆Hdipole-dipole + ∆H ion-dipole • When solvent is water: » ∆Hsoln = ∆Hion-ion + ∆Hhydration Enthalpy of Solution for NaCl 2 5/19/2016 Calculating Lattice Energies Using the Born-Haber Cycle Lattice energy (U) is the energy required to completely separate one mole of a solid ionic compound into gaseous ions. It is always endothermic. e.g. MgF2(s) Mg2+(g) + 2F-(g) 𝐸𝑒𝑙 = 2.31 𝑥 10−19 𝐽 ∙ 𝑛𝑚 𝑄1 𝑥𝑄2 𝑑 Q1 is the charge on the cation Lattice energy (E) increases as Q increases and/or as r decreases. Compound Lattice Energy (U) MgF2 2957 Q= +2,-1 MgO 3938 Q= +2,-2 LiF LiCl 1036 853 radius F < radius Cl Q2 is the charge on the anion d is the distance between the ions 3 5/19/2016 Born-Haber Cycles – Calculating Ulattice Hess’ Law is used to calculate Ulattice The calculation is broken down into a series of steps (cycles) Steps: 1. sublimation of 1 mole of the metal = ΔHsub 2. if present, breaking bond of a diatomic = ½ ΔHBE 3. ionization of the metal(g) atom = IE1 + IE2 etc 4. electron affinity of the nonmetal atom = EA1 + EA2 etc 5. formation of 1 mole of the salt from ions(g) = ΔHlast = -Ulattice In General for M(s) + ½ X2(g) MX(s) M+(g) + M+(g) + X(g) X(g) EA1 + EA2 etc IE1 + IE2 etc M+(g) + M(g) + X(g) M(g) + ½ X2(g) M(s) + ½ X2(g) X-(g) ½ ΔHBE ΔHlast = -Ulattice ΔHsub ΔHf MX(s) 4 5/19/2016 Calculating Ulattice ΔHf° = ΔHsub + ½ ΔHBE + IE + EA + ΔHlast Rearranging and solving for ΔHlast - ΔHlast = ΔHf − ΔHsub − ½ ΔHBE − IE − EA Ulattice = -ΔHlast Calculating Ulattice for NaCl(s) Na+(g) + Na+(g) + Cl(g) Cl(g) EA1 IE1 Na+(g) + Na(g) + Cl-(g) Cl(g) ½ ΔHBE Na(g) + ½ Cl2(g) Na(s) + ½ Cl2(g) ΔHlast = -Ulattice ΔHsub ΔHf NaCl(s) 5 5/19/2016 Calculating Ulattice for NaCl(s) 6 5/19/2016 ΔHlast = ΔHf − ΔHsub − ½ ΔHBE − IE1 − EA1 Sample Exercise 11.1 Calculating Lattice Energy Calcium fluoride occurs in nature as the mineral fluorite, which is the principle source of the world’s supply of fluorine. Use the following data to calculate the lattice energy of CaF2. ΔHsub,Ca = 168 kJ/mol BEF2 = 155 kJ/mol EAF = -328 kJ/mol IE1,Ca = 590 kJ/mol IE2,Ca = 1145 kJ/mol 7 5/19/2016 Lattice Energies and Solubility 𝐸𝑒𝑙 = 2.31 𝑥 10−19 𝐽 ∙ 𝑛𝑚 𝑄1 𝑥𝑄2 𝑑 Q1 is the charge on the cation Q2 is the charge on the anion d is the distance between the ions 8 5/19/2016 Chapter Outline 11.1 Energy Changes when Substances Dissolve 11.2 Vapor Pressure 11.3 Mixtures of Volatile Substances 11.4 Colligative Properties of Solutions 11.5 Osmosis and Osmotic Pressure 11.6 Using Colligative Properties to Determine Molar Mass Vapor Pressure Pressure exerted by a gas in equilibrium with its liquid Where are we going with this? Colligative Properties – boiling point elevation, freezing point depression, osmosis 9 5/19/2016 Vapor Pressure of Solutions • Vapor Pressure: – Pressure exerted by a gas in equilibrium with liquid – Rates of evaporation/condensation are equal The tendency for a liquid to evaporate increases as 1. the temperature rises 2. the surface area increases 3. the intermolecular forces decrease 10 5/19/2016 Boiling Point “If you have a beaker of water open to the atmosphere, the mass of the atmosphere is pressing down on the surface. As heat is added, more and more water evaporates, pushing the molecules of the atmosphere aside (wsystem < 0). If enough heat is added, a temperature is eventually reached at which the vapor pressure of the liquid equals the atmospheric pressure, and the liquid boils.” Normal boiling point the temperature at which the vapor pressure of a liquid is equal to the external atmospheric pressure of 1 atm. Increasing the external atmospheric pressure increases the boiling point Decreasing the external atmospheric pressure decreases the boiling point 11 5/19/2016 Location San Francisco Salt Lake City Denver Mt. Everest Elevation (ft) b.p H2O (oC) sea level 4400 5280 29,028 100.0 95.6 95.0 76.5 Intermolecular Forces and Vapor Pressure Diethyl ether - dipole-dipole interactions Water - hydrogen bonding (stronger) 12 5/19/2016 Clausius-Clapeyron Equation Vapor Pressure vs Temperature Pressure ln Pvap H vap 1 C R T liquid solid gas Temperature Clausius-Clapeyron Equation Vapor Pressure vs Temperature Plot of ln(P) vs 1/T yields straight line: • Slope = −ΔHvap/R • Intercept = constant 13 5/19/2016 Sample Exercise 11.2: Calculating Vapor Pressure Using Hvap The compound with the common name isooctane has the structure shown below. Its normal boiling point is 99 oC, and Hvap = 35.2 kJ/mol. What is the vapor pressure at 25 oC? Practice Exercise, p. 473 Pentane (C5H12) gas is used to blow the bubbles in molten polystyrene that turn it into Styrofoam, which is used in coffee cups and other products that have good thermal insulation properties. The normal boiling point of pentane is 36 oC; its vapor pressure at 25 oC is 505 torr. What is the enthalpy of vaporization of pentane? 14 5/19/2016 Chapter Outline 11.1 Energy Changes when Substances Dissolve 11.2 Vapor Pressure 11.3 Mixtures of Volatile Substances 11.4 Colligative Properties of Solutions 11.5 Osmosis and Osmotic Pressure 11.6 Using Colligative Properties to Determine Molar Mass Fractional Distillation A method of separating a mixture of compounds based on their different boiling points Vapor phase enriched in the more volatile component 15 5/19/2016 Solutions of Volatile Compounds Raoult’s Law: • Total vapor pressure of an ideal solution depends on how much the partial pressure of each volatile component contributes to total vapor pressure of solution • Ptotal = 1P1 + 2P2 + 3P3 + … where i = mole fraction of component i, and Pi = equilibrium vapor pressure of pure volatile component at a given temperature 16 5/19/2016 Sample Exercise 11.3 (simplified): Calculating the Vapor Pressure of a Solution What is the vapor pressure of a solution prepared by dissolving 13 g of heptane (C7H16) in 87 g of octane (C8H18) at 25 oC? Real vs. Ideal Solutions Deviations from Raoult’s Law: Due to differences in solute–solvent and solvent–solvent interactions (dashed lines = ideal behavior) Negative deviations Positive deviations 17 5/19/2016 Concept Test, p. 478 Which of the following solutions is least likely to follow Raoult’s Law: (a) acetone/ethanol, (b) pentane/hexane, (c) pentanol/water? Chapter Outline 11.1 Energy Changes when Substances Dissolve 11.2 Vapor Pressure 11.3 Mixtures of Volatile Substances 11.4 Colligative Properties of Solutions 11.5 Osmosis and Osmotic Pressure 11.6 Using Colligative Properties to Determine Molar Mass 18 5/19/2016 Colligative Properties of Solutions • Colligative Properties: – Set of properties of a solution relative to the pure solvent – Due to solute–solvent interactions – Depend on concentration of solute particles, not the identity of particles – Include lowering of vapor pressure, boiling point elevation, freezing point depression, osmosis and osmotic pressure Vapor Pressure of Solutions • Raoult’s Law: • Vapor pressure of solution is equal to the vapor pressure of the pure solvent multiplied by the mole fraction of solvent in the solution • Psolution = solvent·P solvent • Vapor pressure lowering: • A colligative property of solutions (Section 11.5) • Ideal Solution: • One that obeys Raoult’s law 19 5/19/2016 Sample Exercise 11.4: Calculating the Vapor Pressure of a Solution of One or More Nonvolatile Substances Most of the world’s supply of maple syrup is produced in quebec, Ontario, where the sap from maple trees is evaporated until the concentration of sugar (mostly sucrose) in the sap reaches at least 66% by weight. What is the vapor pressure in atm of a 66% aqueous solution of sucrose (MW=342) at 100 oC? Assume the solution obeys Raoults’s Law. Colligative Properties of Solutions • Colligative Properties: Vapor pressure lowering Consequences: in b.p. and f.p. of solution relative to pure solvent 20 5/19/2016 Solute Concentration: Molality • Changes in boiling point/freezing point of solutions depends on molality: m nsolute kg of solvent – Preferred concentration unit for properties involving temperature changes because it is independent of temperature – For typical solutions: molality > molarity Calculating Molality Starting with: a) Mass of solute b) Volume of solvent, or c) Volume of solution 21 5/19/2016 Sample Exercise 11.5: Calculating the Molality of a Solution A popular recipe for preparing corned beef calls for preparing a seasoned brine (salt solution) that contains 0.50 lbs of kosher salt (NaCl) dissolved in 3.0 qt of water. Assuming the density of water = 1.00 g/mL, what is the molality of NaCl in this solution for “corning” beef? Chapter Outline 11.1 Energy Changes when Substances Dissolve 11.2 Vapor Pressure 11.3 Mixtures of Volatile Substances 11.4 Colligative Properties of Solutions 11.5 Osmosis and Osmotic Pressure 11.6 Using Colligative Properties to Determine Molar Mass 22 5/19/2016 Colligative Properties of Solutions Colligative Properties: • Solution properties that depend on concentration of solute particles, not the identity of particles. Previous example: vapor pressure lowering. Consequences: change in b.p. and f.p. of solution. © 2012 by W. W. Norton & Company Boiling-Point Elevation and Freezing-Point Depression Boiling Point Elevation (ΔTb): • ΔTb = Kb∙m • Kb = boiling point elevation constant of solvent; m = molality. Freezing Point Depression (ΔTf): • ΔTf = Kf∙m • Kf = freezing-point depression constant; m = molality. © 2012 by W. W. Norton & Company 23 5/19/2016 Sample Exercise 11.6: Calculating the Boiling Point Elevation of an Aqueous Solution What is the boiling point of seawater? Assume that the average concentration of sea salt is equivalent to 0.560 m (molal) NaCl, and that the average density of seawater is 1.025 g/mL. Sample Exercise 11.7: Calculating the Freezing Point Elevation of a Solution What is the freezing point of an automobile radiator fluid prepared by mixing equal volumes of ethylene glycol (MW=62.07) and water at a temperature where the density of ethylene glycol is 1.114 g/mL and the density of water is 1.000 g/mL? The freezing point depression constant of water, kf, is 1.86 oC/m. 24 5/19/2016 The van’t Hoff Factor Solutions of Electrolytes: • Need to correct for number of particles formed when ionic substance dissolves. van’t Hoff Factor (i): • number of ions in formula unit. • e.g., NaCl, i = 2 ΔTb = i∙Kb∙m & ΔTf = i∙Kf∙m Deviations from theoretical value due to ion pair formation. © 2012 by W. W. Norton & Company Values of van’t Hoff Factors © 2012 by W. W. Norton & Company 25 5/19/2016 Sample Exercise 11.9: Using the van’t Hoff Factor Some Thanksgiving dinner chefs “brine” their turkeys prior to roasting them to help the birds retain moisture and to season the meat. Brining involves completely immersing a turkey fro about 6 hours in a brine (salt solution) prepared by dissolving 2.0 pounds of salt (NaCl, MW=58.44) in 2.0 gallons of water. Suppose a turkey soaking in such a solution is left for 6 hours on an unheated porch on a day when the air temperature is 18 oF (-8 oC). Are the brine (and turkey) in danger of freezing? The kf of water is 1.86 oC/m; assume that i=1.85 for this NaCl solution. Chapter Outline 11.1 Energy Changes when Substances Dissolve 11.2 Vapor Pressure 11.3 Mixtures of Volatile Substances 11.4 Colligative Properties of Solutions 11.5 Osmosis and Osmotic Pressure 11.6 Using Colligative Properties to Determine Molar Mass 52 26 5/19/2016 Molar Mass from Colligative Properties ΔTf = Kfm and ΔTb = Kbm Step 1: solve for molality ΔTf or ΔTb m = ΔTf/Kf kg of solvent Step 2: solve for moles Step 3: calc. MW molality moles MW = g sample/moles Sample Exercise 11.12: Using Freezing Point Depression to Determine Molar Mass Eicosene is a hydrocarbon used as an additive in the highpressure fracturing of rock layers to enhance recovery of petroleum and natural gas, a process sometimes called “fracking”. The freezing point of a solution prepared by dissolving 100 mg of eicosene in 1.00 g of benzene is 1.75 oC lower that the freezing point of pure benzene. What is the molar mass of eicosene? (kf for benzene is 4.90 oC/m) 27
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