JAMES
edited by 0 . SCHRECK
UNIVERSITY OF NORTHERN COLORADO
GREELEY. CO 80639
Copper/Aluminum Surprise Thomas H. Bindel Pomona Senior High School, 8101 W. Pomona Dr., Arvada, CO 80005 measured temperature. Ten copper pennies (tied together in a copper-wire basket) and two aluminum rods are massed, then heated toeether in 150 mL of boiline water for abaut 1min. Each metal is transferred to a calorimeter. and the hiehest temDerature of the water ir measured. The temperature changes uf t h b metols and of the watrr in the calorimeters are used tu calrulate the specific heats of the metals. After collecting the data, the students calculate the specificheats of the metals and answer the following questions: "Which Styrofoam cup experienced the greatest ch&& in temperature? Isthis what youguessed? If youguessed incorrectly, why do you suppose it came out this way?"
T h e following is a "discovery-based" calorimetry experiment. It is unique in t h a t the student goes from experimentally derived facts (specific heats) to theory (explaining the law of Dulong and Petit) instead of vice versa. In addition, the experiment is much more exciting than the typical "specific-heat" experiment. Armed only with the definition of a calorie. the definition of soecific heat, and a n idea of what calorimetry is, the studentgains a fundamental understanding about "heat movement" in substances and its effect on temperature, beyond just calculating calories.
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Materials A class set for 24 students (12 pairs): T h e student results are presented in the table below. The author held classesin Laboratory 1 and a fellow reacher held classes in 1.aburatory 2:I'he numher of student pairs (Pairs) representinr each lahoratorv averare is also listed. The literar"ret valu& for the specif& heats& copper and aluminum are 0.092 cal,g-deg C and 0.215 cal/g-deg C, respectively. As the can be seen,-the-class data comp&e favorably. important thing is t h a t Aland C u a r e different, with A1 being higher. This is very good considering "alcohol" thermometers were used for safety reasons.
120 pennies (have each pair bring in 10 pennies (abaut 28 g)) copper wire 24 4-cm-long, 12.7-mm-diameter aluminum rods ( a ring-stand rod can he cut into these leneths; . the mass is about 28 K for two rods (about 1mol)) 12 250-mL beakers 24 Styrofoam cups 36 alcohol thermometers (graduated to the nearest degree) 12 forceps
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BU~T
Experlment The following senario is posed to the students: "If equal masses of copper and aluminum at a temperature equal to the boiling point of water are placed separately into Styrofoam cups containing equal masses of hater at about the same temperature, which cup do you think will have the highest temperature rise?" After each student has answered the question, the students experimentally determine the specific heats of copper and aluminum. The two calorimeters each contain 70 mL of cool water at a
Laboratory
Pairs
T.CB
Metal
Sp.H.O
'T.c.-average temperature change of the water. a Sp.H.-average specific heat.
Most students (75%)intuitively guess copper will give the highest temperature rise. They know from experience t h a t when copper and aluminum are heated, copper becomes
' Weast, R. C.. Ed. Handbook ofChemistryandPhysics; The Chem-
ical Rubber Co.: Cleveland. OH. 1987.
About the Column Editor James 0. Schreek received his BA degree from the University of St. Thomas (Houston) and his MS and PhD (oreanie ehemistrv) . .. . . from Texas A&M Universitv. Durine 196P66 he held portdoctoral pornam at Georgia Institute olTwhnology ruith Jack Hino and at Louisiana State l'nivcrsity \with William Pryor). In 1966 he juineu the rllk~railyof Northern Culurado and is Prc,ferior 01 Chemistry. He wacher undergraduate and graduate courses in organic chemistry and an undergraduate course and laboratory in general education chemistry (Consumer Chemistry). During the summers of 1980-86 he served as coordinator of the Science Program in UNC's Summer Enrichment Program for Talented and Gifted Kids. He developed and taught chemistry courses for high-ability students in grades 5-7 and another for those in grades 8-10. During Summer 1988 he directed the "Chemistrv Can Be Fun Dav C a m ~ "for 6-8th eraders in the Institute for Chemical Education worksho&. With financial suoook . . from the ~oiorsdaSection uf the American Chemical Society, he prepared and edited a newsletter tChem,C,, .Yews1 disrributed toall Colorndo hiahaehoalrhem~myteachers I n n 1981-8- He iscurrently servine as directnr for a Title II-funded, two-year project entitled "Science and Applied Mathematics Enhancement for Rural Elementary Teachers". He is a member of the ACS and has served the Colorado Section in several positions (treasurer, Section Awards chairman, and High School Awards chairman). In 1985he received the Colorado Section's Service Award for his efforts on ChemiCo News. His administrative experience consists of six years as Chair of the Chemistry Department and, more recently, as Chair of the Undergraduate Council, which is charged with maintaining the overall quality of the University curriculum. ~~~~~~~~~~~
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Volume 67
Number 2
February 1990
165
4
C/
hotter faster. In applying this to the experiment, the students think the copper in the boiling water is "hotter" (higher heat content) than the aluminum. But of course this is incorrect! What thev fail to ask themselves is whv aluminum does not experience as fast a temperature rise as copper. Aluminum has the hieher specific heat. I t should be pointed o i t that it makes no difference whether "coooer" pennies or the "new" cooper-olated zinc pennies are used in the experiment. ~ 0 t h - i h o u i dgive the same specific heats within experimental error. (In fact. I do not make mention of the fact ihat penniesare differentuntil the exam, and then I ask the students why we did not worry about it.) I then discuss theoretically why aluminum hasahouttwice the specific heat as copper. The students are guided into thinking about the numbers of atoms. The students eventuallv realize there are about twice as manv aluminum atoms as copper atoms in the experiment or pe; gram of metal. Of course, acalculation of the literature molar heat caoacities of copper and aluminum are found to be 5.84calfmol-heg C and ~
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166
Journal of Chemical Education
5.80 cal/mol-deg C, respectively. Ah ha! The differences are revealed as being due to the relative numbers of atoms. Next, molar specific heats of some selected monatomic and diatomic gases are presented. A discussion of the differences ensues that eventually unravels why polyatomic gases have higher molar specific heats than monatomic gases. The discussion leads into modes of motion, the dependence of temperature on the types of modes of motion, and equipartition of energy. These, of course, can be complicated topics and are dealt with in advanced courses at the college level. Nonetheless, they can be successfully introduced a t an elementary level in a beginning course. In conclusion, I have found this experiment to he tremendous in terms of both student success and understanding, far surpassing any of the "traditional" experiments. Acknowledgment The author is indebted to Charles Mueldener for his support and data acquisition.
