STAN STATE CHEM 1112 - Vapor Pressure Modified (9 pages)

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Vapor Pressure Modified



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Vapor Pressure Modified

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Pages:
9
School:
California State University Stanislaus
Course:
Chem 1112 - General Chemistry II Laboratory
General Chemistry II Laboratory Documents

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Vapor Pressure Enthalpy of Vaporization and Intermolecular Forces Dr Mike Perona California State University Stanislaus Procedure modified by Dr Scott Russell 8 30 2007 Purpose Measure the equilibrium vapor pressure of several liquids at room temperature and explain the results in terms of intermolecular forces By measuring the equilibrium vapor pressure of a liquid at two temperatures it will be possible to calculate the molar enthalpy of vaporization for that liquid Introduction You have probably observed that if water is left in an open container at room temperature it will eventually evaporate completely even though its temperature is never raised to the boiling point The explanation for this behavior is that some of the molecules in the liquid have enough kinetic energy to overcome the intermolecular forces which hold the molecules of the liquid together and are able to escape into the vapor phase The molecules in the vapor phase then diffuse away and a net loss of liquid occurs The evaporative process results in a momentary loss of the most energetic molecules from the liquid sample Heat energy is continuously absorbed from the surroundings and so the supply of energetic molecules is maintained This continues until all of the liquid has evaporated If the liquid water is placed in a closed container at constant temperature however the amount of liquid water will at first decrease and then remain constant It will appear as if evaporation has stopped In fact evaporation does not stop What happens is that since the vapor phase molecules cannot escape their number initially increases This results in an increase in the frequency of their collisions with each other and with the liquid surface Collisions with the liquid surface lead to condensation Ultimately the rate of condensation becomes equal to the rate of evaporation and the relative amounts of water in the two phases remains constant This corresponds to a condition of dynamic equilibrium The word dynamic



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