JAMES 0.S
filtrates e residues
C
H
University 01 Northern Colorado Greeley, CO 80639
Determination of Heats of Fusion Using Differential Scanning Calorimetry for the AP Chemistry Course Susan M. Temme Cheshire High School, Cheshire, CT 06410
In 1978 Earnest ( I ) discussed in this Journal the slow rate of introduction of thermoanalytical techniques into the undergraduate teaching laboratory. He maintained that this did not reflect the increased prominence of these techniques in a wide spectrum of scientific applications. During the subsequent 15 years, progress in incorporating the particular technique of Differential Scanning Calorimetry (DSC)into the undergraduate chemistry cumculum has been reported. A survey of articles published in this Journal revealed six on this topic during that time period. In 1979 Brown (2) discussed a "dry-lab" exercise for determining purity of a substance by DSC. In 1985 Mjojo (3) presented a classroom experiment on phase equilibria involving orientational disordering in crystals. An undergraduate experiment observing the decomposition of urea by both DSC and thermogravimetry (TG)was described by Wynne ( 4 )in 1987. Choi and Larrabee (5)discussed the use of DSC within a multitechnique laboratory experiment using the tetrachlorocuprate (11) anion as part of a n advanced integrated laboratory course. The authors also used DSC and Atomic Absorption Spectrometry in an experiment to determine the per cent lead in brass (6). In 1990, Hardgrove et al. described an integrated laboratory experiment for the physical chemistry course utilizing DSC in the characterization of polymers (7). While DSC (and other thermoanalytical techniques) are being taught in some undergraduate laboratories, the limitations imposed by lack of DSC experience among instructors and limited equipment budgets remain problematic in the Advanced Placement Chemistry Course. Experiments are traditionally limited to calorimetry, and students are not exposed to thermal analysis despite the rigorous attention oaid to such thermodvnamic entities as heat of fusion. entropy, and Gibbs Free ~ n e r ~ ~ within t h e Advanced PlacementIGenera1 Chemistry curriculum. One approach to overAH..^.. coming these limitations is for Substance the AP Chemistryteacher to utilCut & weigh ize the resources of either a uni(mcal Img) versity or industrial research center. Such a setting can pro- Indium standard vide both accessibility to instru13.1 mentation not available in the Tin secondary school laboratory as Zinc 21.7 well as the assistance needed by 5.45 secondary educators to learn a Lead particular technique. Data gath- KN03 22.0 ered by a n instructor can be stored on a computer disc and BenzoicAcid 34.1 subsequently used by students 17.0 during their course of study. This NH4N03 916
Journal of Chemical Education
paper describes an exercise designed to be used in an AP chemistry course to accompany the study of themodynamics. The data described in this oaner was obtained a t the Olin Research Center in ~heshcre;~onnecticut. Aseries of substances were analvzed bv the author usina Differential Scanning calorimetry. students were taught Low to access the resulting thermograms and each student was asked to determine heats of fusion using the two techniques of integration and a "cut and weigh" graphical technique. The heats of fusion were subsequently used in problem solving to obtain corresponding values for changes in entropy and Gibbs Free Energy during the isothermal phase changes as suggested by Earnest (1). Experimental
Descriptions of the technique (and theory) of DSC can be found in several textbooks on thermoanalytical techniques (8).For this project, the melting of a series of pure substances was carried out under nitrogen on a Dupont 990 Calorimeter. The heating rate was 10 "Clmin. Sample size was in the range of 0.75-8 mg. All samples were sealed in aluminum pans, and an empty pan and lid were used for a reference. The substances chosen for this study included common metals used by students in a preceding experiment on specific heat, as well as organic and inorganic compounds that were both available and familiar to students from the existing AP laboratory program. The instrument was calibrated using indium, tin, and zinc in order to allow for the broad temperature range being considered. Data was stored using software produced by PL Thermal Sciences, 160 Old Farm Road, Amherst, MA 01002. Data files and software used for integration are available from the author upon request. Heats of Fusion
Melting Point
AHusion
Integration (mcal Imp)
Literature (mcallmg)
pc)
Experimental Literature PC)
6.81
156.4
156.6
14.5'
230.6
231.9
~
Results and Discussion Heats of fusion were determined using two different techniques, and the results obtained compared to literature values. With both techniques, the heat absorbed during the phase change of each substance was determined by comparing the peak area with the area obtained from a known mass of the standard. The first method involved integration of the area under the peak and was achieved by use of a computer momam written to be used with the DSC.The seiond rneth;l'd w;ii :ld;tpted from one described by 1)ani~is1.9,. It involved o r t n t ~ n ec n ~ i e sof each tht!rmogram that had been normaiized s o t h a t the ranges of both temperature change (x-axis) and heat in mcal/s O.-axis) were identical. The peak area of each thermogram was cut with sharp scissors and weighed on a n analytical balance. The corresponding weight per milligram 01each sample was calculated. Each of these calculated values was compared to the weight per milligram for indium determined by the same method. Because the weight of each peak is proportional to the energy change, the heat of fusion of each substance could be determined by comparison with the accepted value for the heat of fusion of indium of 6.81 mcaWmg (9).A sample calculation for determining the heat of fusion of tin is shown below. mass of In peak = 0.0428 g; mass of In sample = 4.440 mg mass of Sn peak = 0.1553 g; mass of Sn sample = 8.400 mg mass of peak per mg of In = 0.0428 g14.440 mg = 0.00964glmg mass of peak per mg of Sn = 0.1553 g18.400 mg = 0.01849 glmg AH fusion Sn = 15.81~ (0.0184910.00964)= 13.1mcallmg Results obtained are recorded in the table. While the accuracv of the values for heats of fusion compared to literature values (8,I O , 4 , 11)was limited by thebroad range of the calibration, the precision between the cut and weigh technique and electronic integration was within 5%. Conclusion This exercise was found to be of value in the teaching of thermochemistry and thermodynamics in a n AP chemistry course for several reasons. The particular type of data generated by thermal analysis on the DSC can be contrasted with t h a t collected by the students during other calorimetry experiments. This activity demonstrates to stu-
dents one method by which the values found in their textbooks are determined. I t provides them with "real data2'for problem solving, found to be much more motivating than the use of numbers from the problems a t the end of the chapter. The two methods used to determine heat of fusion and the comparison to literature values provided a n opportunity to discuss and contrast precision and accuracy of measurements. Because most of the students in this AP chemistry class were concurrently taking AP calculus, they were interested in both the application and graphic representation of the integration process. Finally, students were intrigued by and attentive to the author's experiences working in a thermal laboratory. As a result, this opportunity provided a basis for sharing information on possible car% choices in industri;tl chrm&rry Furth6.r work dune by the author using the 1)SC' to recognize glass transitionstates of polymersand the thermogravimetric analyzer (TGA) to study decomposition reactions may prove to be useful in further broadening student understanding of the applications of thermoanalytical techniques Acknowledgment This project was developed during a two-week mentorship designed as part of a sixth-year program of the Institute for Science Instruction and Study (ISIS) under the auspices of Southern Connecticut State University. The author would like to thank the staff a t the Olin Chemical Research Center in Cheshire, CT. Thanks are due particularly to Richard Wedlich of the Analytical Department for his advice, instruction, and enthusiastic suggestions. Without his support, this project would not have been possible. Literature Cited Earnest.C. M. J. Chem. Educ 1978.55.A331-A336. Blown. M. E . J. C1z.m. Educ. 1979.56.310313. Mjojo. C. C. J. Chem. Educ. 1985,62,834& W m n c , A . M. J. Chem. Educ 1987.64,lRO-182. Choi, S.:Larrabee.J.A. J Chem Educ. 1989,66,776776. Choi, S.;Larrahee. J.A. J. Chem. Educ 1989,66,86&865. Hardyrove. 0.L.:Tam, D.A.:Miessler G. L. J. Chem Edne 1980.67,979981. Wendlandt. W . W. Thermal Anniysis. 3rd ed.:. Wilev: . New York. 1986. Daniel%F Malh~moliealPm~oroiionfor Physical Chsmisfw:McGraw-Hill: New York. 192s. p 242. ID. Gray. A. P Tllermol Analpis Appiiiitiii Study No. I ; Perkin-Elmer Corp., 1972. 11. Watson, E. 3.: O'Neill, M. J.; Justin, J.; Brenner, N. A n d Chsm. 1964.36, 1233123s. 1. 2. 3. 4. 5. 6. 7. 8. 9.
Volume 72 Number 10 October 1995
917
