In the Laboratory edited by
Cost-Effective Teacher
Harold H. Harris University of Missouri—St. Louis St. Louis, MO 63121
An Updated Equilibrium Machine Emeric Schultz Department of Chemistry, Bloomsburg University, Bloomsburg, PA 17815; *
[email protected] Numerous models and analogies (1–4), computer simulations (5), demonstrations (6–9), and even games (10–12) have been described that attempt to facilitate an understanding of equilibrium, kinetics, and related energy concepts. In fact, three “home-built” apparatuses intended to demonstrate these concepts have been described in the pages of this Journal (13–15). Alden and Schmuckler (13) describe a mechanical device in which small glass beads, stirred by motor driven paddles, can be exchanged between adjoining chambers. Sawyer and Martens (14) describe a wooden box with a Plexiglas front in which benchtop air pressure is used to facilitate the exchange of Styrofoam balls between two chambers. Most recently Ellis and Ellis (15) have described an apparatus that improves significantly on the glass bead exchange device; this device also gives reproducible quantitative results that could not be obtained with the earlier device (15). In this article an apparatus that improves on the Styrofoam exchange device and also gives reproducible quantitative values is described. The apparatus is a modification of the device previously described in an earlier article on modeling hydrogen bonding (16) and is shown in Figure 1. An electric commercial leaf blower is mounted on a wooden box and secured to the box by screws through wooden brackets at the front and back of the blower. Only one type of blower was tested, but in principle other models could be used. The box has a hole in the top for “exhaust” air from the blower that vents through the open back end of the box (not shown). The business end of the blower is duct taped to 90° PVC connector (2.5 in. i.d.) that, in turn, is connected to a PVC vent with a metal face plate (11 cm, or 4 in., diameter) attached to a plastic screw plate. Tightening the screw plate allows for a secure fit of the plate to the bottom of the plastic container. The plastic container (20 cm × 30 cm × 28 cm) has ridges on the bottom surface that fit into grooves cut in two strips of wood that are screwed into a support frame for the container. The frame in turn is attached by screws to the wooden box and a wood support such that the device is level. The container requires a top that snaps on so as to provide a tight seal during experiments. The floor of the plastic container requires a network of drilled holes in order to vent air (Figure 1, bottom). A selection of wooden barriers, held in place by screws, can be placed across the middle of the vent to separate the container into two equal compartments.
side. The time until equilibrium is reached and the demonstration of kinetic–energetic concepts can be shown by varying the height of the barrier or the amount of air pressure. The effect of a catalyst can be demonstrated by comparing the time to equilib-
Experiments The blower has a broad range of air pressures that can be reproduced by turning the control valve to a certain setting. The height of the fence can be varied between 5 and 22 cm. Equilibrium can be approached from Styrofoam balls exclusively on one side or the other or from any combination of balls on either
Figure 1. Equilibrium machine: (top) side view, (center) front view, and (bottom) top view.
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 8 August 2008 • Journal of Chemical Education
1131
In the Laboratory
rium with the 22 cm high fence versus a 22 cm high fence with a significant hole. Balls can easily be labeled with markers such as to demonstrate the dynamic nature of equilibrium. Adding and removing balls can be used to demonstrate the Le Châtelier principle. Figure 2 shows one such experiment in progress. All these types of experiments have been possible with the previous equilibrium machines and demonstrations (6–9, 13–15). What is new with this device is the ease with which an “energy differential” can be added to the demonstration. Figure 3 shows the exhaust port before and after electrical tape has been used to cover a number of the openings on one side of the container. With a total of 26 one inch Styrofoam balls (25 or more balls give consistently reproducible results in all experiments), starting from any configuration prior to the addition of tape, a final state in which each side has 13 ± 1 balls (with rare statistical variations) is reached after 30 seconds (moderate blowing). Under the same conditions with the amount of air lessened in one chamber by the tape covering shown in Figure 3, the final equilibrium state reached has 16 ± 1 balls on the side with the tape and 10 ± 1 balls on the side without the tape (the result of 10 experiments). In the extreme case of all the holes covered on one side and the balls being placed on that side, none of the balls leaves that side of the chamber even when the blower is quite strong (given a moderately high fence). Likewise, if the balls are initially placed in any other configuration, the final equilibrium state (under the same conditions described above) will have all the balls on the side with the vent completely taped over. Values for “equilibrium constants” for various intermediate states of vent closure can be obtained. Conclusion The equilibrium machine described can be easily made and is highly portable such that it can be taken to smaller classrooms. All that is required is an electrical outlet. The machine can be used to demonstrate any number of kinetic, equilibrium, and thermodynamic concepts. One simply changes a limited number of parameters: the number, labels, and size of the balls; the nature and size of the energy barrier; the amount of air flow from the blower (total flow as well as differential flow between the compartments). All these changes can be done in minutes, if not seconds, and can be in response to questions asked by students. Results are easily recordable and can be processed quickly. The aspect of this device that is most appealing is that at the end of each experiment matter (balls) is distributed with respect to its energy content, and that the lowest energy level available is the one that is most populated. The author would like to point out that the initial reviews for this article came back just before the appearance of the Ellis and Ellis article (15). This author believes that that article does a far better job of quantifying both enthalpy and entropy effects using a device that is “tuned” much finer than the blower device described in this article.
Figure 2. Styrofoam balls being exchanged during experiment.
Figure 3. (left) Manifold without alteration and (right) manifold with taped over openings in left side.
4. 5. 6. 7. 8. 9.
10. 11. 12. 13. 14. 15. 16.
Schultz, E. J. Chem. Educ. 2005, 82, 401. Cullen, J. F., Jr. J. Chem. Educ. 1989, 66, 1023. Birk, J. P.; Gunter, K. K. J. Chem. Educ. 1977, 54, 557. Edmonson, L. J.; Lewis, D. L. J. Chem. Educ. 1999, 76, 502. Hanson, R. M. J. Chem. Educ. 2003, 80, 1271. Huddle, P. A.; White, M. W.; Rogers, F. J. Chem. Educ. 2000, 77, 920. Hambly, G. F. J. Chem. Educ. 1975, 52, 519. Harsch, G. J. Chem. Educ. 1984, 61, 1039. Wilson, A. H. J. Chem. Educ. 1998, 75, 1176. Alden, T. A.; Schmuckler, J. S. J. Chem. Educ. 1972, 49, 509. Sawyer, D. J.; Martens, T. E. J. Chem. Educ. 1992, 69, 551. Ellis, B. E.; Ellis, D. C. J. Chem. Educ. 2008, 85, 78. Schultz, E. J. Chem. Educ. 1997, 74, 505.
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2008/Aug/abs1131.html
Literature Cited
Abstract and keywords
1. Bartholow, M. J. Chem. Educ. 2006, 83, 48A. 2. Garritz, A. J. Chem. Educ. 1997, 74, 544. 3. Meyer, E. F.; Glass, E. J. Chem. Educ. 1970, 47, 646.
Full text (PDF) Links to cited JCE articles Color figures
1132
Journal of Chemical Education • Vol. 85 No. 8 August 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
