Born-Haber Cycle: ΔH hydration

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1 Born-Haber Cycle: ΔH hydration ΔH solution,nacl = ΔH hydration,nacl(aq) U NaCl ΔH hydration,nacl(aq) = ΔH hydration,na + (g) + ΔH hydration,cl (g) Enthalpies of Hydration 1

Vapor Pressure Pressure exerted by a gas in equilibrium with its liquid Where are we going with

2 Sample Exercise 11.3 Use the appropriate enthalpy of hydration values in Table 11.3 and ΔH solution = 0.91 kj/mol for NaF to calculate the lattice energy of NaF. 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 rate of evaporation = rate condensation 2

the intermolecular forces decrease Vapor Pressure In general, vapor

3 The tendency for a liquid to evaporate increases as - 1. the temperature rises 2. the surface area increases 3. the intermolecular forces decrease Vapor Pressure In general, vapor pressure of a solution is lower than vapor pressure of pure solvent. Rate of evaporation < rate of condensation 3

4 Vapor Pressure of Solutions Raoult s Law: vapor pressure of solution is proportional to mole fraction of solvent (non-volatile solute). P solution = X solvent P solvent Vapor pressure lowering: one of the colligative properties of solutions Ideal Solutions: solutions that obey Raoult s Law. Sample Exercise 11.4 The liquid used in automobile cooling systems is prepared by dissolving ethylene glycol (HOCH 2 CH 2 OH, molar mass g/mol) in water. What is the vapor pressure of a solution prepared by mixing L of ethylene glycol (density g/ml) with L of water (density g/ml) at C? Assume that the mixture obeys Raoult s law. 4

5 Solutions of Volatile Components For mixtures containing more than one volatile component: Partial pressure of each volatile component contributes to total vapor pressure of solution. P total = X 1 P 1 + X 2 P 2 + X 3 P 3 Where X i = mole fraction of component i, and P i = equilibrium vapor pressure of pure volatile component at a given temperature. Real vs Ideal Solutions Deviations from Raoult s Law occur due to differences in solute-solvent and solvent-solvent interactions (dashed lines = ideal behavior). Negative deviations: Solute-solvent > solvent-solvent. Positive deviations: Solute-solvent < solvent-solvent. 5

6 Sample Exercise 11.7 Calculate the vapor pressure of a solution prepared by dissolving 13 g of n-heptane (C 7 H 16 ) in 87 g of n-octane (C 8 H 18 ) at 25 C. By what factor does the concentration of the more volatile component in the vapor exceed the concentration of this component in the liquid? The vapor pressures of n-octane and n-heptane at 25 C are 11 torr and 31 torr, respectively. Vapor Pressure vs Temperature (Increasing attractive forces.) Normal boiling point: the temperature at which the vapor pressure of a liquid equals 1 atm (760 mmhg). 6

Diethyl ether - dipole-dipole interactions Water -

beaker of water open to the atmosphere, the mass of the

As heat is added, more and more water evaporates, pushing

If enough heat is added, a temperature is eventually

7 Diethyl ether - dipole-dipole interactions Water - hydrogen bonding (stronger) 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. 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. 7

Normal boiling point - the temperature at which the

increases the boiling point Decreasing the external

Location Elevation (ft) Boiling Point H 2 O ( o C)

8 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 Location Elevation (ft) Boiling Point H 2 O ( o C) San Francisco sea level Salt Lake City Denver Mt. Everest 29,

9 Pressure 5/21/2012 Clausius-Clapeyron Equation Vapor Pressure vs Temperature ln H vap 1 P C vap R T solid liquid gas Temperature Plot of ln(p) vs 1/T yields straight line: Slope = ΔH vap /R Intercept = constant 9

10 Practice Exercise, p. 513 n-pentane, C 5 H 12, boils at 36 o C. What is its molar heat of vaporization in kj/mol if its vapor pressure at 25 o C is 505 torr? 10

Chapter 11: Properties of Solutions - Their Concentrations and Colligative Properties. Chapter Outline

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