Thomas P. Chirpich Memphis State University Memphis, Tennessee 38152
Ideal and Non-Ideal Gases An experiment with surprise value
A t times, chemistry requires considerable abstract thinking on t h e p a r t of students. Chemical theories demand conceptualization of the unseen a n d a r e verified primarily through quantitative experiments. Such quantitation is fundamental to chemistry and n o student can really understand chemistry without a n appreciation of its essentially quantitative nature. For this reason, most laboratory classes contain a n abundance of auantitative experiments. However, too many such experiments reinforce the common rvncept among nunscience students that chemistrv is dull and that ~ c i e n t i s t s are too concerned with arcane fo&ulas that are not worth the students' trouble. Conseauently, i t is good t o include some experiments t h a t are more dramatic a& t h a t arouse student curiosity. T h e experiment reported here is one such attempt. I n t h e first p a r t of t h e experiment, Charles' law is qualitatively illustrated b y c o o l i ~ gt h e air in a n Erlenmeye;flask from approximately 100°C t o room temperature, with the decrease in gas volume manifested as a volume of water drawn into t h e flask. I n t h e second part of the experiment, t h e non-ideal gas, water vaoor., is similarlv cooled. Initiallv. t h e cooling is slow, d u e t o t h e low specific h e a t of t h e surrounding air, b u t t h e n increases ereatlv once the first water is drawn into t h e flask. T h e suddgnness of this transition, t h e rapidity with which water rushes into t h e flask (sometimes complete with sound effects), and the amount of water drawn into the flask contrast with t h e first p a r t of t h e experiment a n d arouse student euthusiasm a n d provoke questions.
.
.~~
This eroeriment orimarilv asks the student to understand the four posrulntrs of rhe kmetic mdecular theory of gaser and the consequences whrn one of those pvstulnter rs nut satisfird. It qualitativrlg dlustrates horh (!hnrlw' law and the flow of wnrer m rerpome to pressure differences. The marked change in the rate of cooling can be used to illustrate the difference in the specific heats of air and water, with a fuller explanation bringing in the densities of air and water and the thermal conductivity of the flask. One practical effect of cooling a non-ideal gas can also be presented. One assumes an almost, but not quite, empty storage tank for a volatile liquid and a painter who paints the tank on a warm day with the lid open and who then closes the lid when he is finished. A rough calculation can be made of the force acting on the tank when it coals during the night, and it is not hard for the students to visualize the appearance of the tank the next morning. This experiment can easily be performed in 1hr and so combined with other experiments in the same laboratory period, including a number of quantitative experiments an the properties of ideal gases.' A cautionary note is in order. Although we have had no flasks collapse, a partial vacuum is created in the second part of the experiment. To reduce this vacuum as rapidly as possible, the inlets to the flasks are made from %hole No. 507 5% stoppers containing short L-shaped pieces of glass tubing (4 mm id., 1mm wall thickness) with smooth bends. Fiftv centimeters of moderatelv thick-walled rubber tnhine is then nttaihed to each short oieee of: elass tubine. .. In nre. liminary ~xperimentu.mly a I-hole stopper and thin-walled tuhmp. that collapsed was used. E w n in this case, nc, accidenr occurred. Nevertheless, it still is advisable to test the apparatus before student use since tubing, flasks, etc. can vary with the source of supply. ~
~
~
~~~
~~
~~
~~~
~~
~~
~
378 1 Journal of Chemical Education
-
Student Procedure Part 1 Add 200-250 ml of water and 3-4 hoiling chips to a 400-ml beaker and begin heating the water. Take a dry 125-ml Erlenmeyer flask and insert a 2-hole stopper fitted with two outlet tubes (these are provided on the side shelf). Fasten a utility clamp around the neck of the flask and immerse the ends of the outlet tubing in a 250-ml beaker filled with water. Set the Erlenmeyer aside, hut not near the flame heating the water in the 400-ml beaker. Once the water in the beaker is boiling, adjust the flame until the water boils gently; then immerse the Erlenmeyer as fully as possible. Hold the Erlenmeyer at a slight angle to reduce splashing. Be sure the ends of the outlet tuhing are still immersed in the water in the 250-ml heaker. When air has stopped bubbling from the outlet tubing, remove the Erlenmeyer from the hailing water but keep the ends of the outlet tubing under water. Turn off the burner and carefully remove the beaker containing the hat water with a pair of tongs. Attach the Erlenmeyer to the ring stand about 20 cm up (Caution: The ring stand is still hat.) and let it cool while you consider the following. What molecules were in the Erlenmeyer flask when you began the experiment? Are they in motion? Do they strike the side of the flask with any force? Do they strike the insides of the outlettubing and the surfaceof the water in the outlet tubing with the same force per unit area (pressure)as they do the inside of the flask? Do air molecules in the room strike the surface of the water outside the outlet tubing? What would happen if the pressure exerted on the water outside the tubing were not equal to the pressure exerted on the surface of the water inside the tubing? When the air in the flask is heated by the boiling water, what happens to the motion of the molecules in the flask? What happens to the pressure differential between the inside of the flask and the air outside the flask? What is the result of this pressure differential? What is the pressure differential when the bubbling stops? When the flask cools down, what happens to the motion of the molecules in the flask and to the pressure they exert? How does this change the pressure differential between the inside af the flask and the air outside the flask and what is the result of this? Because of the low specific heat of air, the Erlenmeyer flask cools slgen molecules, hnve fairly qtronr intermolrrular forrw that cnuse the rnolrnlles mcundense as thr temperaturr lalls below 100°C so that lhrrr nrr few rnol~culesletr in the vapor phase and the internal preswre falls. Charles' Law docs not apply in such a case. In generalIthe gas laws apply only t o ideal g s s e c t h a t is. when the attractive forces between molecules can be neeleeted and u hrn the other I hrre assumptions of the kinetic rnolcculnr theor).of gasrs hold true. Hral gases brhave like ideal gases only under renain cwditiunc. Water wpt>rm i l l bchnve like an idpalgas ifthe temperature is raised enough so that the molecular motion is vigorous enough t o overcome the attractive forces. Oxygen and nitrogen will hehave as non-ideal gases near the temperatures a t whieh they condense to liquids (-183'C and -196%)
Volume 54. Number 6. June 1977 1 379
