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
-
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
Submitted by:
Checked by:
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.
