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)

                                                                                                                               (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 DH_m,vap..

                                                                                                                 (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.

                                                                                                                                (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. T_r which is available for water, n-heptane, and benzene (Toluene will behave much like benzene).  T_r is the reduced temperature and T_c is the critical temperature which is available in any standard reference (NIST Webbase, CRC handbook of Chemistry and Physics).

                                                                                                                                   (4)

Method:

            We will develop a procedure to determine DH_vap 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 DH_vap including any data analysis.

            Consider the differences in DH_vap between the compounds and why some compounds are higher then others.

 

References

 

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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Created by Jonathan M. Smith

Updated October 24, 2005

Gustavus Adolphus College