CH302%20Zumdahl%20(posting)%20Chapter%2017b%20solutions%20sp08

CH302%20Zumdahl%20(posting)%20Chapter%2017b%20solutions%20sp08

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1 CHAPTER 17b Chapter 16 Topics 2 Chapter 16 We will cover the material in chapter 16 (16.10 -16.11) This material covers vapor pressure and phase diagrams. 3 Vapor Pressure A liquid phase and its vapor phase reach equilibrium with each other. The pressure of the vapor present at equilibrium is called the equilibrium vapor pressure.
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4 Figure 16.44: Behavior of a liquid in a closed container a) Evaporation occurs as the molecules move from the liquid to the vapor phase. b) As the number of vapor molecules increases, the rate of condensation increases until the rate of condensation equals the rate of evaporation. forward reverse evaporation condensation equilibrium vapor pressure is established → ← 5 Vapor Pressure At a fixed temperature, as long as some liquid is present, the vapor exerts a characteristic pressure Regardless of the amount of liquid present. A simple barometer can measure the vapor pressure of a liquid. When the equilibrium vapor pressure is established, the vapor pressure can be determined from the change in the height of the mercury column. atm vapor Hg column vapor atm Hg column P P P P P P = + = 6 Figure 16.46: The vapor pressure of a liquid can be measured easily using a simple barometer of the type shown here. The liquid is injected at the bottom of the tube of the mercury and floats to the surface because of the high density of Hg. The liquid evaporates producing a vapor that pushes some of the Hg out of the tube. Molecules that are the least strongly bound at the surface are able to escape more easily. diethyl ether ethanol water
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7 8.2 Volatility and Intermolecular Forces When the intermolecular forces between molecules of a liquid are weak, the vapor pressures of that liquid are high. Weak intermolecular forces = high vapor pressures Dispersion forces (van der Waals, London) are the weakest. Hydrogen bonding is a strong intermolecular force and is responsible for the lower vapor pressures of some liquids. 8 Volatility and Intermolecular Forces Both of these molecules have the molecular formula, C 2 H 6 O Both have the same number of electrons, and it would be expected that the dispersion forces would be the same. Ethanol, however, has an —OH bond that can form a hydrogen bond to another ethanol molecule. This H-bond lowers the vapor pressure of EtOH, it is a liquid at room temperature. —OH bond •• •• •• •• Gas at room temp. Liquid at room temp. 9 Volatility and Intermolecular Forces If the intermolecular forces between molecules in a liquid are strong: It is expected that the vapor pressure will be low. It is expected that its enthalpy of vaporization is high. H vap is high. The liquid requires more heat energy to break all the intermolecular bonds.
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10 Vapor Pressure At ordinary temperatures, liquids with high vapor pressures are volatile .
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