edlted by
J. DUDLEY HERRON Purdue Un#vers#ty West Lafayene, lndmna 47907
The Experimental Determination of the Heat of Vaporization of Volatile Liquids F r a n c e s Chames Nina F a r v e r Catherine Grieve Archie Lynch Wayne Memorial High School 3001 Fourth Street Wayne, MI 48184
Michelle M a c Renee Rickel J e r r y Sears'
WATER
In this Journal, Laidler2 illustrated several applications of the Arrhenius law using examples of research in biological fields. The chriping of tree crickets, the creeping of ants, the flashine of fireflies. the rhvthm of human alpha brain-waves, were demonstrated to obey and otKer biologic& the Arrhenius law within specified temperatures. Brennan, Shapiro, and Watton, in this Journal3 extended the list of examples to include a physical phenomenon. They proposed an experiment whereby the heat of vaporization of a volatile liquid could he determined from an Arrhenius plot. That exnerimental orocedure has been modified by the authors of this article to enable the experiment to be su&essfully performed in any high school laboratory.
I Figure 1.
I
COOLER
Apparatus far determining heat of vaporization.
Background
In 1887, Svante Arrhenius proposed an equation to explain the variation of the rate constant (k) of a chemical reaction with temperature. loglok = A - -
B
T
T is the absolute temperature and A a n d B are constants that can he determined from a plot of loglo k versus 1/T for specific reaction. The constant B is directly related to the activation energy (E,) of the reaction. B = E.12.3 R , where R is the gas law constant (1.99 cal mol-I deg-1). When loglo k is plotted against 1/T, a straight line is obtained which has the slope equal to -Ea/2.3R. In working with the rate of evaporation of volatile liquids Brennan. S h a ~ i r o .and Watton2 observed that activation energies dctvrl;iinid from an Arrhenius plot had n numerical corrrsrxmdence to the latent hears of varmrization (H,). Their work kcludes an analogy between the evaporation process and a chemical reaction as it relates to the Arrhenius equation. Equipment
A cooler made of expanded polystyrene with a volume of approximately 10 1is used as a constant temperature bath. A piece of Styrofoam approximately 2 cm thick is cut to size so that it will float on the surface of the water when the cooler is nearly filled with water. The Styrofoam float contains a hole in the center into which a clear evaporating dish fits tightly
EDUC., 51,276 (1974) 3 Aylward, G. H., and Findley,T. J. V., "SI Chemical Data," John Wiley and Sons, Inc., New York, 1971.
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362 / Journal of Chemical Education
1.8
2.9
3.0
I
3.2
3.3
3.4
1000/ TEMPEIATUIE, KELVIN Figure 2.
Arrhenius plot far carbon tebachlaride.
(see Fig. 1). The bottom of the evaporating dish, on the exterior surface is painted black. This greatly increases the visibility of a drop of liquid when it is placed in the dish. Experimental Procedure 1) Put approximately 8 1 of water into the expanded polystyrene
container. The desired water temperature can be acquired by mixing boiling water with tap water. 2) Set the Styrofoam float containing the evaporating dish an the surface of the water. Wait 5 min. for temperature equilibrium to be established. Record the temperature of the water. 3) Using a lambda pipet, dispense 0.10 ml of the volatile liquid into the evaporating dish. 4) Using a stopwatch, record the number of seconds it takes for the liquid to disappear. Start the watch the moment the liquid first touches the evaooratine dish. more than two trials. 6) Repeat the procedure at several different temperatures, ranging
Heats of Vaporlzatlon Literature
Llquid Acelone 2-Butanone Carbon Tetrachloride Chloroform Cyciahexane Cyclahexsne Ethanol Hexane Methanol
('C)
Experimental Hv (kcallmale)
Value far HW3' (kcallmole at 25'C)
56.1 79.6
18-54 20-72
7.4 8.1
7.4 8.4
76.5 61.7 80.7 83.0 78.3 68.7 64.5
28-73 21-57 25-75 24-74 21-70 24-68 18-60
7.8 7.1 7.0 8.0 10.1 8.3 9.3
7.6 7.4 7.9 8.1 10.3 7.6 9.1
Boiling Point
Temp.
(OC)
Range
From reference in footnote 4.
from near the boiling point of the liquid to room temperature. 7) Graph the logarithm of llfirne (seconds)versus 1000/temperature (Kelvin).
8) Determine the slope of the line. 9) Using the relatiouship,slope = HU/2.3R,determine the heat of vaporization. An Arrhenius plot for carbon tetrachloride is shown in Figure 2.
The nrocedure described has several advantaees. E x c e ~ t for a stopwatch, no special equipment is needed. The styrofoam container is larae enouah and sufficiently insulated so that the water tempe;ature $11 remain c o n s t k t over several minutes. The painting of the clear glass evaporating dish makes the liquid being tested very visible. There is no doubt when the last trace of liquid has vaporized and the times required for the vaporization are reproducible. Only a small amount of liquid is required for the experiment; however, due to the potential health hazard associated with the vapors of volatile compounds, the experiment should be performed in a hood. The table shows the heats of vaporization that were exnerimentallv determined for several volatile liauids as compared to the-literature value.4 The e~~erimentafvalues are in reasonable agreement with the literature value. S
Volume 57, Number 5. May 1980 1 363
