GEORGE
edited by L. GILBERT
Denison University Granville, Ohio 43023
Entropy as a Driving Force
Demonstrating Tetrahedral Bonding Using Soap Films S U S M ~ E DSV
SUBMITTEDBY
John C. Whitmer
John C. Salzsleder
Western WarhingIon University Eellingham. WA 98225
Phillips University Enid, OK 73701
CHECKEDBY
CHECKEDBY
George . F. Wollaston Clarion State College Clarion. PA 16214
H. Slotnlck Central Connecticut State College New Britain. CT 06050
A simple demonstration which illustrates the driving force of entropy uses the familiar effects of the negative thermal expansion coefficient of rubber. Related demonstrations and discussions have often appeared. (See, for example, (1)-(3) and references therein.) This particular demonstration has several nice features.
The tetrahedral bonding in carbon and other similar atoms can be demonstrated effectively with an open tetrahedral frame and a soap solution. The frame is dipped into the solution and Slowly withdrawn. With care and perhaps a few dips one can form a figure of six soap film planes which connect with the minimum surface area the six edges of the tetrahedron (see figure). The planes intersect within the frame
1) It is cheap, requires virtually no setup time, and always
works. 2) It is done by the students themselves in the classroom. J ) I t can be repeatrd many timrs in a few seconds. 41 I t allows the teacher mdo~omethrngseemingly w r y silly with the class uften a welcome break in a discwsion uf thermudv-
namics. For the demonstration, balloons are passed out to the class. A hallmn is slipped onto the index finger up to abuut the second ioint. l'he neck of the balloon is held with the thumb of the same hand to prevent the balloon from coming off. Takine the bulk of the balloon in the other hand. the balloon is rapiky stretched. After pausing for a moment; the balloon is allowed to rapidly relax. Students will notice the intended effects without being coached. When the balloon is stretched, the finger is warmed, and when the halloon relaxes, the finger is cooled. Repeating the cycle a few times help to contrast the two effects. Heavier, more expensive balloons give . a more pronounced warming and cooling. The analysis of the demonstration can be done in two ways. The details are discussed by Dole1 and by Nash3 and willnot be repeated here. The first analysis considers the spontaneous contraction with the final state being the shorter, cooler balloon. This process is essentially adiabatic because of its speed. The second analysis of the contraction is to consider the final state as the balloon after it has absorbed heat from the finger. The state functions are then compared to the state functions of an isothermal process, and the internal energy change is' approximatelg In this case you have an engine which eificiently converts heat to work. Dole, Malcolm, J. CHEM.EDUC., 54,754 (1977). Mandelkern, Leo, J. CHEM. EDUC., 55,177 (1978). Nrqh, Leonard K., J. CHEM.EDUC., 56,363 (1979).
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forming four lines re~resentinethe tetrahedral hondine. These lines meet at a central point. with a small diameter &ss tube a bubble can be blown at the central ~ o i n to t remesent the carhon atom. Lnlikc ball and stick rnudil, ahirh ihvw wily the urienttltion d t h e honds (thetetrshedron is irnpl~edj.this demonstration shows clearly the tetrahedron as well as the bond orientation. The frame shown here was made from 10-cm steel rods soldered together. Frames can be made also by gluing dowels or wooden cotton-tip ao~licatorsticks. or hv hendine No. 12 -or 14 hnred househoid wke into a tetrahedraishape. l'he soap solution is made hv addine 20 ml liauid soao and 150 ml rlvcerine to 1l distilled water and mixing welf The addition bf glycerine improves the lasting aualities of the soan films. visibility may be a problem in a large lecture hall hecause of the relatively small size of the frames. The demonstration could become a laboratory exercise particularly in a high school class if wire is used and each student constructs hisher own frame.
a tormat convenient for classroom use. Readers inter~ s l e din either submining or checking demonstrations should contact the column aditor. An outline of format requirements was given an p. 166 of lhe March 1976 iswe of THIS JWRNAL. W g a Gllberl's interest in chemisby demansOatimr traces from Hanaver High School (Mass.)through un-
280
Journal of Chemical Education
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.... . ..-. ....-. ..=-. . .,.. tional areas of interest in ehe ~- ----. ~self-pacing: TIPS, and project-style laboratories. He mnsulk wilh science mueums and is adive in fostwing scienceawareness in public arenas. Recreational in@rests include ca-vipping, backpacking and outdca activities. He suffers tmm frequent attacks of paronomasia which is best treated by studied forbearance. ~
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