Heat of Vaporization Experiment

 

 

 

Purpose of the experiment:

 

Here is a simple experiment that can be performed in almost any laboratory.  No expensive or special equipment is required in order to obtain relatively accurate results.

 

This procedure has been performed in our freshmen chemistry laboratories for many years with excellent results.  The experiment is useful in enumerating several important basic chemistry concepts.  These concepts include the following:

 

1.      The effect of external pressure upon boiling point is easily illustrated1.  One can show that as the pressure inside of the boiling flask is reduced, the boiling point is lowered.  Starting with the lowest pressure that can be obtained by using a good aspirator, a dynamic demonstration is presented. One can hold their hand for an indefinite period of time on the surface of the boiling flask while the water is boiling.

 

2.      This experiment provides an excellent opportunity to introduce the concept of logarithms to the freshmen students.  Many students have scientific calculators that have logarithm functions; yet, they are totally unaware of the purpose of these keys.

 

3.      This experiment can be used to help students to develop new techniques in the preparation of graphs.  In connection with this, the concept of reading slopes and the meaning of the slope intercept form of an equation for a straight line, should become apparent following successful completion of the experiment.

 

4.      If computers are available, this experiment provides an excellent opportunity to use a spreadsheet program to perform a regressional analysis.  Ultimately, the task can be performed using an appropriate scientific calculator.  The slope of the best-fit line is provided either by the spreadsheet calculations or the calculator.

 

5.      This experiment provides an avenue to discuss evaporation and vaporization. Factors including temperature and intermolecular forces that influence equilibrium vapor pressure can be explained.  The definition of boiling can be explained relative to the vapor pressure of a liquid and the external pressure applied to that liquid.  Finally, students are exposed to experimental technique wherein a value, in this case the heat or vaporization of water, is obtained indirectly by measuring other quantities.

6.      If an aspirator is used to develop the required vacuum, its operation can be illustrated and explained.  Contrary to popular belief, most aspirators do not operate upon Bernoulli’s principle where streamline flow is expected2.  Please refer to figure III.

 

Experimental apparatus:                        Figure I

 

The experiment is composed of a round bottom boiling flask, a distillation condenser or multiple condensers, a heat source (a burner or a heating mantle), a vacuum gauge (an open mercury manometer however, a Bourdon type gauge will work and will eliminate the mercury hazard), and an aspirator or trapped vacuum pump.  Also, a pressure-regulating device (a needle valve that is part of a Bunsen burner base will work), and a thermometer are necessary.  Figure one shows the experimental set-up that has been employed in our laboratories.  Some modifications can be introduced without adversely affecting the validity of the experiment.

 

Procedure:

 

It has been found that it is most convenient to start the experiment at the lowest possible pressure.  This condition is accomplished by closing the regulator (bleed) valve.  The water flow to the aspirator is set to maximum to provide highest vacuum.  Also, of course, flow of water through the condensers is initiated.

 

The temperature of the sample in the boiling flask (distilled water) is increased until boiling commences.  At the boiling point, the equilibrium vapor pressure of the liquid is equal to the external pressure.  This is the pressure within the apparatus.  (It is highly recommended that a few boiling stones be placed in the round bottom-boiling flask in order to minimize bumping that occurs even more severely under reduced external pressure.  This provides a good opportunity to discuss the phenomenon of bumping.)

 

At this point it is necessary to try to obtain a stable temperature and pressure.  If a large thermometer in a glass well is being used, anticipate a temperature lag.  Therefore, it may be necessary to wait a minute or two for the temperature to stabilize. 

 

In our labs, this experiment is always performed as a demonstration, with only one set-up.  Therefore, everyone obtains the same raw data.

 

Having obtained the first data set, that is, the boiling temperature and the pressure or manometer reading, it is now time to increase the pressure in the vessel and determine the new boiling temperature at the new pressure.  This is achieved by opening the bleed (the needle valve on the burner base) slightly until a new pressure is established.  Under the increased external pressure, boiling will cease immediately.  The temperature must then be increased to bring the liquid to a boil again.  Boiling can typically be achieved at temperatures of fifty degrees Celsius or lower, at the lower external pressures.  This is a good time to remove the heat source and demonstrate to the class that it is possible to hold one's hands on the bottom of the flask while the water continues to boil.  It should be remembered that glass, being a poor conductor of heat, could have a high surface temperature immediately after removing the heat source.  To burn a finger or jump and scream at this time would destroy the dynamics of the presentation.

 

Treatment of data:

 

A plot of equilibrium vapor pressure versus temperature yields a smooth curve.  If instead, according to the integrated form of the Clausius-Clapeyron equation, the logarithm of the vapor pressure is plotted versus the reciprocal of the kelvin temperature, a straight line is obtained.  The slope of the line, obtained either graphically or through regressional analysis, is used to calculate the molar heat of vaporization as follows3.

 

Equation I         DH = (-2.303)(R)(m)    If the universal gas law constant "R" is inserted as 1.987 cal/K mole and "m" is the slope of the line, the value for the molar heat of vaporization "DH" is obtained with units of calories per mole.

 

The pressure P may be measured in any absolute units.  Conversion factors for pressure units will only affect the intercept.

Table I

Sample Experiment  (Atmospheric pressure-768 mm Hg)

Temp. C

h mm Hg

Temp. K

(1/TK)X103

P mm Hg

(P=768-h)

Log P

41.5

710

314.5

3.18

58.0

1.77

62.5

610

335.2

2.98

158

2.20

75.0

500

348.0

2.87

268

2.43

86.5

300

359.5

2.78

468

2.67

92.2

220

365.2

2.74

548

2.74

101.0

0

374.0

2.67

768

2.89

 

The values to be analyzed are as follows:

 

  Table II

 

Log P

1/T X E3

1.76

3.18

2.20

2.98

2.43

2.87

2.67

2.78

2.74

2.74

2.89

2.67

Text Box: Note: It is not necessary to show both sets of lines.  Typically, one would elect to show the connecting segments of the line fit plot only.
Figure II

Rather than performing a regressional analysis, one may just choose “Add Trendline” with selection “Linear” and “Options – Display Equation on Chart”.  The equation is displayed in the form y = mX + b, where “m” is the slope of the line.

 

Table III

 

 

 

 

 

 

 

 

 

 

 

Coefficients

Standard Error

t Stat

P-value

Lower 95%

Upper 95%

Lower 95.0%

Upper 95.0%

Intercept

8.859436

0.098492

89.95081

9.16E-08

8.585978

9.132894

8.585978

9.132894

1/T X E3

-2.23383

0.034258

-65.2066

3.31E-07

-2.32895

-2.13872

-2.32895

-2.13872

 

 

 

 

 

 

 

 

 

 

The coefficient from the regressional analysis presented in table III has a value of  -2.23 X 103.  This yields a value of 10.2 kcal/mole.

 

The accepted value for water is 9.72 kcal/mole4.  The deviation from the accepted value is then 4.7%.  The results obtained from this experiment seldom exceed 5% deviation from the accepted value. 

Figure III

The construction of most aspirators (Figure III, right) is such that a water jet entrains air.  The water supply must have significant pressure such that when fed to an orifice, there will be a strong diverging jet breaking up into droplets.  When such a jet is led into a matching cylindrical tube of suitable size, the water drops will entrain air, with potentially a much greater flow rate than that of the water.  This device relies on kinetic effects (similar to an oil diffusion pump).  The water magnitude of the water pressure is related to droplet speed, and these droplets are like “fan vanes” that bring the air molecules with them.  In a well-designed aspirator, the ultimate vacuum is limited only by the vapor pressure of the water at the specific temperature.  This same design is used in wastewater aeration.  In this application these devices are known as jet aerators or ejectors.  The high discharge velocity and small bubble size make these devices effective in dispersing atmospheric air or air from a blower5.

 

1. Bassam Z. Shakhashiri, Chemical Demonstrations (Madison: The University of Wisconsin Press 1985), 85.

2. Victor L. Streeter and E. Benjamin Wylie, Fluid Dynamics, 7th ed. (New York: McGraw Hill 1989), 101.

3. Raymond Chang, Chemistry, 7th ed. (New York: McGraw Hill 2002), 447.

4. J. A. Dean, Ed., Lange’s Handbook of Chemistry, 12th ed. (New York: McGraw Hill 1979).

5. W. W. Eckenfelder, Jr. and D. J. O’Connor, Biological Waste Treatment (New York: Pergamon Press 1961), 98.