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Measuring the Heat of Vaporization Using the Clausius-Clapeyron Equation Submitted by: Checked by:
J e r r y A. Driscoll University of Utah Salt Lake City, U T 84112 Len Grotz University of Wisconsin- Waukeshq Waukesha, W153I86
One of the majm ol~jectivesin presentinga demonstration is to quantify the demonstration to give "more mileage", challenge the student and make a more meaningful experience. Also, when a measurement is made as part of the demonstration, it raises the student's consciousne&that chemistry is experimentally hased. In the following demonstration we would like to measure the heat of vaporation using the Clausius-Clapeyron equation. .
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By measuring the vapor pressure at two temperatures one ean calculate AHwP.in the form:
One should he aware that is the equation assumes AH,. constant over the range of temperatures. This is not completely trce; however, the changes are generally small for most substances near room temperatures. But as the temperature of the substance approaches its critical point, the AH,,. falls off rapidly to zero
(1). To perform the demonstration one needs to assemble the apparatus (see figure). A steam jacket is placed around a barometer tube. The jacket has a steam inlet a t the top ruhber stopper, and the steam and condensate escapes through the bott,om stopper via a tube to a collection vessel (2). A thermometer should he placed inside the steam jacket to observe the temperature. The assernhled apparatus is placed in a container of mercury. It is strongly recommended t h a t t h e container of mercury be placed i n a second vessel to prevent soills. A convenient option on the barometer tuhe is a stopcock or
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a clamp on a ruhber tubing through which a vacuum can he pulled (3).If you use a vacuum pump, do not pull a vacuum on the harometer after the liquid has been introduced. This may damage the vacuum pump and will most certainly lower its capability to pull a vacuum. The success of the demonstration is deoendent on a liauid which has a vapor prassure about 40mm at room temperature and does not exceed 1 atm vapor pressure at the boiling point of water. The chemical used should also be pure, readily accessible and relatively non-toxic. A perusal of the "Handbook of Physics and Chemistry" reveals only a few such liquids (4): toluene, methylcyclohexane, and piperidine. The first step in performing the demonstration is to pull a vacuum on the harometer and observe the atmospheric pressure. Then, the liquid is intnnjuced into the hiirnmeter tuhe with a honk tipped eve dropper. Care should be used in this steo to avoid the introduction of air into the harometer which would give erroneous results. One can also introduce the liauid throueh the sto~cocka t the to^. but this method has n i t proven G o succeskul. The temperature and the level of the mercurv are observed and recorded. Then. the steam is introduced into the steam jacket and in a minute or two the amaratus is a t the temperature of the boiling water. Ohservations of pressure and temperature are recorded. The apparatus should he clamped securely since the steam causes vibrations in the steam jacket and the mercury vibrates up and down vigorously until equilibrium is reached. An example of data we have collected and used to calculate iW,,.for toluene is as follows: PI = 26mm T I= 295.3'K
P2= 48Omm
T2= 368.QeK
AH,, = 8.6 X LO3 cal A variation of the experiment utilizes a pressure gauge on an immersible flask (5). Literature Cited
corridor demonstration Experiments for Display Submitted by:
T. M. Letcher a n d S. W. Orchard University of Witwatersrand Johannesburg, South Africa Checked by: Richard F. Jones Sinclair Community College 44 West 3rd Street Dayton, OH ~i~~~~~ for measure. ment of heat of vaporization.
Recently we were asked t o present a chemistry display which would be both eye catching and challenging. The disVolume 57, Number 9, September 1980 1 667
