1s:i'F Granville. Ohio 43023
Demonstration-Tests
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C. David Schmulbaeh Southern Illinois Uniuersity Carbondale, Illinois 62901 I. Y. Ahmed The Uniuersity of Mississippi University, 38677 George H. Gilbert Denison University Granuille. Ohio 43023
Lecture demonstrations provide a n effective oedaeoeical device for stimulating interest in chemical topiis an2 reinforcine students' com~rehensionof principles and concepts. The value of the lectire demonstration is-further enhaneed if it provides an opportunity for students to apply newly acquired principles and concepts to the analysis and interpretation of experimental data and for instructors to evaluate the students' &derstanding of those principles and concepts embodied in the demonstration. The intent of this report is to generate interest in the development of demonstration-tests and encourage their utilization by lecturers. The principles of chemical thermodynamics as they commonly appear in popular textbooks of general chemistry (1) represent a topic of some difficulty for which demonstrations serve to clarify and reinforce concepts. The simple lecture demonstration by Laswick ( 2 )illustrating the thermodynamic principles associated with the elastic property of a rubber band is an effective vehicle for reinforcing these basic ~ r i n ciples. The system is particularly appealing in that it is familiar to the student while a t the same time the thermodynamics of elasticity is yet another application of thermodynamics rarely discussed in textbooks of general chemistry. The thermodyna@c analysis of elastic behavior provides a unique oooortunitv for students to extrapolate principles illustrated primarily with examples taken from gaseous systems and chemical processes. That the laws of thermodynamics apply equally to physical processes as well as chemical changes is reemphasized. The concept of enerm conservation and the transformation of mechahical worG rather than the more familiar PV-work, to other forms of enerm becomes evident. The concept of spontaneity is as vivid in-this demonstration as the well-known and much-used illustration of the irreversible expansion of a gas into an evacuated bulb. Other demonstrations suitable for demonstration tests are available (3).A topical list of demonstrations appearing in this Journal through 1976 may he obtained from the authors on request. Demonstration-Test For convenience the apparatus described earlier ( 2 ) is shown in the figure. The sharpened dowel marker on the ring stand can be adiustpd u, mark the it it ion of the rubber band assembly.
Set-up for demonstration of thermodynamic properties of rubber elasticity. A = d e n dowel. B = rubber band. C = nylon or silk thread. D = 5009 weight. and E = wwden dowel painter. The procedure for administering the demonstration-test is as follows 1) The test (see below) is distributed to the students. The students are instructed to record their observationsas directed in the test and base their response to the questions on these observations. The system is identified as the rubber band and the weight as that part of the surroundings which interacts with the system. The freehanging rubber band is described as being in the relaxed state. 2) The weieht (500 e) is connected to the rubber band throueh the thread l & ' ~ h e i a n d is allowed to stretch slowly until it to a new state of equilihrium-the stretched state. The position uf the pointer is marked. :Ir The stretched hand is heated as uniformly a? possible wilh n henr gun for a period 4 - 1 mi". The position of rhc polnrrr is marked for the new stretehed state and &pared to the original position before heating. In order to get maximum contraction upon heating, the instructor should exoeriment with different rubber bands of varying Iengthr and rrusr-sectiuns.Conrrnerlc,ns of 1-2 em were obtained a d werr readily deterred hy the rtudents. 41 Cut the thread witha pair uf sr~sst,rsr#nsrpsratc thesyrtcm f n m the external force, whereupon the system spontaneously returns to the state of minimum free energy-the relaxed state.
comes
~
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Sample Test All answers are to be based on your observation of the demonstration experiment involving a stretched rubber band. 1) Record your observation of the effect of heat on the rubber
band: Based on your observation record the heat term (A) on the appropriate side of eqn. (I), and state whether the enthalpy of the stretching process, AH-.s, is negative or positive; whether it is endothermic or exothermic. RB (relaxed)+ RB (stretched)
(1)
Volume 53,Number 12, December 1976 / 775
where RB = Rubber Band AH,-, = 2) Record your observation of what occurs when the weight is removed: Based on your observation what is the sign of AG for
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RB (stretched) RB (relaxed) AG,-,
(2)
=
:I) Draw the free e n q ? ddm~rarnfor the rtrcrrhmg proems. 4, \Vhat the sign of AS.-,h,r the process slrrtched alate w i n g tu relaxed srnte" Show thr relatiumh~pYOU used tc, crbta~n).our a n -
l y e experimental results and a mechanism for instructors to evaluate the effectiveness of a demonstration. Literature Cited i l l Spe lor example. Sionku. M. J., and Plane, R. A,. "Chemical Principle and Properties", 2nd Ed., New Y w k , 1974: Mortimer, C. E. "Chemistlv", 3rd Ed.. Van Nostrand Reinhold Co.. New York, 1975. 12) Isrwick,l'. H.. J.CHEM. EDUC..49.169(1972). 811 Alyea, H. N..and Dullon. F. B.,"TestpdDemons~ationainChemlslry",6thEd.. Division cilChemical Education of Amer. Chem. Snc, Easton. Pa. 14) Hill.T.I..,"ThemmlpamialorChemisllsndBiolo~isll",Addiwn-WealoyPublirhing Compan~.Inc,Reading. Mess., 1968, p p 2,LW. 161 Wall, R. T.."Chemical Thermadynamics". 2nd Ed.. Freeman and Company, San Prancisco.Califurnia. 1965.pp. 211-25.
swer. 5) Howdo youaccount for thesign of ASaS,, in question (4) interms of molecular processes?
This test was designed to evaluate the students' ability to assign the proper signs for the AH and AG of the stretching and the contraction processes from experimental observations, t o determine the sign of AS from the relationship AG=AH-TAS and to interpret the meaning of the sien for each of the parameters. ~dditionalqucstio&, to test students'aldity to deal with uuantirative relatiunshivs from thr demonstration, could he introduced. For example, the student may be asked to calculate the external force (in dvnes), the work done on the system (in calories), and the change in the internal energy of the s y s t ~ mfur an adiat~atir stri,tching prncess given that (constantfa, T) where f, is the external (applied) force and AL the change in length of the ruhher hand. In this case, the length change should he recorded and the mass of the weight and acceleration should he given as well as the necessary conversion factors. AE=q+f.AL
Thermodynamic Principles Stretching of a ruhher band is accompanied by appreciable thermal effects, and thermodynamic eiuations describing its propertv are discussed in several texts ( 4 , 5 ) .The "equation bf state" for a piece of rubber of uniform cross-sectibn and chemical composition is of the form f. = f(T,L) where fa is the external force, T is the absolute temperature and L is the length of the hand. Upon stretching, the volume and the potential energy of "ideal" rubber remain constant, and its internal energy E is a function of temperature only, i.e., (dEl a L h = 0. The mechanical work done on the system as the result of an external constant force fa is given by
where Lo is the rest length and L I is the length of the stretched rubber hand. The negative sign occurs because the effect of an external force results in an extension of the rubber band, in contradistinction to the effect of pressure on gases which results in compression. All of the basic thermodynamic equations developed for gases can be written for rubber by simply substituting -fa for P and L for V. The change in the internal energy is given by
+
AE = q f.AL (constantf,,T) The heat absorbed by the system a t constant force is
(2)
q, = TAS = AH (constantf,,T) (3) Conclusion Demonstration-tests add yet another dimension to lecture demonstrations by providing a stimulus for students to ana-
776 / Journsl of ChemicalEducation
The Ketene Generator: Simultaneous ExoEndothermic Reactions Robert D. Whitaker Thurman McGarian University of South Florida Tampa, 33620 Madeline P. Goodstein Central Connecticut College New Britain, Connecticut 06050
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Drill a small hole near the nerimeter of a comer .. coin and thread with a wire. Heat the coin to redness and quickly susvend i t verticallv over the surface of some acetone in a beaker. 'rhe coin will continue to glow redly for some time, and the pungent odor of ketene may he detected issuing from the beaker. The well-known thermal decomposition of acetone to ketene and methane is endothermic.
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CHsCOCHa CH2=C=0 + CHI AHo = 19.3 kcal Clearlv, a classroom explanation for the "ketene generator" demands more than the above equation. We have found that the coin is rapidly extinguished if the beaker is covered so that the convkctional flow of gaseous products and air is interrupted. Furthermore, the infrared spectrum of the effluent gases identifies COa and Hz0 as products along with ketene. We have concluded that the combustion of acetone prohably provides the energy necessary to cause the coin to glow continuously and decompose part of the acetone to ketene and methane.
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CHsCOCH?+ 4 0 2 SCOz+ 3Hz0 AHo = -403.9 kcal We have not made a quantitative determination of the yield of ketene relative to the vidd o i r O > a n dH,O. but the mfrared data suggest that ketene is a mi& product. We have made estimates of the enerw ~roductionnecessarv to maintain the coin at an assumed temperature of 8 W C . The rate of loss of acetone in a typical demonstration apparatus (8 X 10W5 molelsec) is such that if only 50%of the acetone is burned to C 0 2 and HzO, the energy production is more than sufficient to decompose the remaining acetone to ketene and maintain the coin a t a temperature of 800°C. We assumed that thermal losses would occur through convection and radiation and calculated an upper limit of 2 callsec for the former and 12 callsec for the latter. The demonstration might more accurately be termed an acetone burner which also produces some ke&ne. NOTE:The hazardous nature of ketene requires that this demonstration be performed briefly in an adequately ventilated area.
