Vapor Pressure of a Pure Liquid
Jonathan Smith
In this investigation we will examine the relationship between vapor pressure and temperature by considering the properties of benzene, water, and n-heptane at various temperatures and within a vacuum apparatus.� Some background is given by Shoemaker, Garland, and Nibler.2
We are considering the equilibrium between the vapor and liquid phases.� The Clapeyron equation (1) applies to this situation.� (See Atkins1 section 6.6)
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������������������������������������������������������������� (1)
Where DS is the change in entropy and DV is the change in volume for the phase transition.� Using the pressure and temperature points at which this vapor-liquid equilibrium holds we can apply the Clapeyron equation in a modified form to get DHm,vap..
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������������������������������������������������������ (2)
By varying the pressure above the liquids under examination and measuring the temperature above the liquid at the boiling point (under a particular pressure), we can map out several pressure-temperature points for the three liquids under examination.� Z is the compressibility factor.
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������������������������������������������������������������� (3)
Z can be assumed to be 1.0 (ideal gas) as a first approximation.� For refinement the van der Waals equation can be used or for our purposes we can use a graph of Z vs. Tr which is available for water, n-heptane, and benzene (Toluene will behave much like benzene).� Tr is the reduced temperature and Tc is the critical temperature which is available in any standard reference (NIST Webbase, CRC handbook of Chemistry and Physics).
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��������������������������������������������������������������� (4)
Method:
����������� We will develop a procedure to determine DHvap for water, benzene, and n-heptane. The apparatus available consists of a vacuum pump, a capacitance manometer, a boiling flask, a thermometer, and glassware and tubing to connect these items.� Before coming to carry out the investigation diagram an apparatus and detail the steps you will take to determine DHvap including any data analysis.
����������� Consider the differences in DHvap between the compounds and why some compounds are higher then others.
1. �� Atkins, P. Physical Chemistry, 6 ed.; W. H. Freeman and Company: New York, 1998.
2. �� Shoemaker, D. P.; Garland, C. W.; Nibler, J. W. Experiments in Physical Chemistry, 6 ed.; McGraw-Hill: New York, 1996.
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Kinetics and Thermodynamics Page
Created by Jonathan M. Smith
Updated October 24, 2005
Gustavus Adolphus College