CHNotes9 - Chapter Nine Physical Equilibrium CHEM 1211K...

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Chapter Nine Physical Equilibrium CHEM 1211K Fall 2010 1
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Objectives Define vapor pressure and understand its relationship to intermolecular forces Examine the variation in vapor pressure with temperature Examine the driving forces for boiling, freezing and melting Learn to interpret phase diagrams Examine solubility, including effects of temperature, the thermodynamics of dissolution, and Henry’s law Examine and understand colligative properties and how the work Make calculations using the theories of colligative properties 2
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Key Ideas Dynamic equilibrium between two phases is reached when the rate of conversion between them is the same in each direction. The rates are equal when the molar Gibbs free energy of the substance is the same in each phase and therefore there is no tendency to change in either direction. The same concepts apply to the dissolving of a solute. The presence of the solute alters the entropy of a solvent and affects its thermodynamic properties. 3
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Why is it important? This chapter introduces the concept of equilibrium between different phases of a substance, a concept that will prove to be of greatest importance for chemical and biological transformations. This chapter also examines how the presence of solutes is used by the body to control the flow of nutrients into and out of living cells and how the properties of solutions are used to separate the components of petroleum. 4
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Phases and Phase Transitions The unifying concept of this chapter is equilibrium , specifically during phase transitions . At a given pressure, substances undergo phase transitions at a certain temperature. When one phase changes into the other, the two phases are in equilibrium, at which point G = 0. 5
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Vapor Pressure (9.1) Vapor pressure is the pressure exerted by a gas on the surface of the liquid with which it is in dynamic equilibrium . 6 Figure 9.2 H 2 O (l) H 2 O (g)
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Liquids with high vapor pressures at ordinary temperatures are said to be volatile . Solids also have vapor pressures, but they are much lower than those of liquids. 7 Vapor Pressure (9.1) http://tinyurl.com/2ga4bmj http://tinyurl.com/24huaab
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Section summary: The vapor pressure of a substance is the pressure exerted by its vapor when the vapor is in dynamic equilibrium with the condensed phases. A system is in dynamic equilibrium when the forward reaction rate equals the reverse, so any change at the atomic or microscopic levels cannot be detected at the macroscopic level. 8 Vapor Pressure (9.1)
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Intermolecular Forces (9.2) The “easier” it is for gas to escape the liquid, the more evaporation occurs, and the higher the vapor pressure. The stronger the intermolecular force, the lower
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This note was uploaded on 01/16/2012 for the course CHEM 1211 taught by Professor Ford during the Fall '09 term at Georgia Institute of Technology.

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CHNotes9 - Chapter Nine Physical Equilibrium CHEM 1211K...

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