A Reaction-Solution Calorimeter for the Undergraduate Laboratory H. P. Diogo, M. E. Minas da Piedade, J. J. Moura Ramos, J. A. ~irnoni,' and J. A. Martinho Simdes
Departamento de Engenharia Quirnica, lnstituto Superior Tecnico, 1096 Lisboa Codex, Pottugal The energetics of molecules and chemical processes is one of the most basic topics in chemistry and also is a theme of intense current research (1).Although undergraduate students frequently deal with concepts such as standard enthalpies of formation, bond dissociation enthalpies, lattice energies, and solvation enthalpies, most of the ex~erimentaltechniques that have been used to determine those quantities ;nd the assumptions that affect their values are usuallv irmored. While maw of the modern thermochemical tods&e too sophisticated and too expensive to be available in undergraduate laboratories, very profitable experiments can be made with a reactionsolution calorimeter, which, being the most basic of the thermochemical techniques, is, nevertheless, irreplaceable in modem thermochemistry ( I ) . Despite its simplicity, a fairly accurate commercially-producedcalorimeter still is considered too expensive for most student laboratories. As a result, many solution calorimeter designs have been presented in this Journal, some for demonstration purposes, and others capable of obtaining respectable results in the undergraduate laboratory (2-6).It is believed that the readion-solution calorimeter described here is a good compromise between cost and accuracy, and is particularly suited to a physical chemistry laboratory course. It can be upgraded easily, if desired, and it was designed in order to avoid some of the features that may hinder the introduction of good quality calorimetry in an undergraduate laboratory: the need for thin glass ampoules and the handling of compounds that are moderately sensitive to oxygen or moisture. Calorimeter Description
The design of the calorimeter described below was based upon models developed for research (71,and consists of four parts: 'the calorimetric vessel, where the physicochemical process takes place, the measuring system, t h e calibrating unit, and the thermostatic bath. The calorimetric vessel (Fig. 1) consists of a 180-mL transparent glass Dewar, 1, glued with an epoxy resin, such as Araldite, to two acrylic flanges, 2, and a brass lid, 3. The vessel is tightly closed by using two o-rings, 4 and 5, and six brass screws, 6, two of which are used to hold the vessel in the thermostatic bath. Attached to the brass lid are the ampoule holder, 7, two thin glass tubes (one for the thermistor probe, 8, and the other for the calibrating resistance, 91,the stimngsystem, 10, and aninletloutlet Teflon tube, 11. Thin glass ampoules, containing the sample to be mixed with the solution in the Dewar vessel, are normally used in reaction-solution calorimetry, and experiments are initiated by breaking the ampoule. While the procedure presents no inconvenience in a research laboratory, a different 'Permanent address: lnstituto de Quimica,Universidade Estadual de Campinas, 13081 Campinas (SP), Brazil. 940
Journal of Chemical Education
Figure 1. Calorimetric vessel. system, more suitable in an undergraduate laboratory, was therefore designed and built. It consists (Fig. 2) of several Teflon parts. One of these parts, 1, is fixed to the brass lid and holds the ampoule, formed by a detachable cylinder, 2, and a plug, 3. In order to fill the ampoule with sample, 2 + 3 is detached and 3 is partially inserted into the lower part of the cylinder 2. Part 3 is then fully inserted into 2 and the whole set (2 + 3 + sample) is weighed and fixed to the calorimeter lid. The top of cylinder 2 is screwed into 1, and 3 is simultaneously screwed into shaft 4. This tightly fitted shaft will be used to push 3 in order to start the experiment. The fmal position, which is shown in Figure 2A, can he easily adjusted by using spacing rings, 5. As stated above, part 3 is fully inserted into cylinder 2 when the ampoule is loaded with sample, i.e., before the mixing of the reactants. The detachable glass stirrer 12 (Fig. 1)has two pairs of small hut different-sized propellers and is driven by an external 280 rpm motor. The upper blades are at approxi-
Figure 2. Details of the ampoule system.
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mately the same level as part 3 of Figure 2, aRer opening the ampoule. This helps the rapid mixing of the reactants. The movement is transmitted by a Teflon shaR that has two retainers, 13, useful if the process being examined requires an inert atmosphere inside the vessel. The shaR is connected to the motor by a flexible rubber tube in order to reduce vibration of the calorimeter. The inlevoutlet Teflon tube enables purging the calorimeter or the solution contained therein with an inert gas, or introducing a previously degassed calorimetric solvent. This feature is crucial when a reactant or product is sensitive to oxygen or moisture. The tube is also useful when the analysis of the calorimetric products is required. In this case, the rubber lid 14 is replaced by a septum, so that a sample can be taken by using a syringe. The measuring system consists of a Yellow Spring precision thermistor, with 5 kn nominal resistance at 25 T , im-
Figure 3. Electric diagram of the temperature measuring system.
mersed i n silicone oil to improve thermal conductivity (tube 8, Fig. 11, a homemade Wheatstone bridge, (Fig. 3) and a high input impedance Hitachi 561-1004 recorder. The thermistor & is one of t h e four 3 cm arms of the bridge. R1 and R2 are 5 kR resistors, and R3 is a &10 kn variable resistor. The power source is a 1.35 V mercury battery. The Joule effect is 1 used to calibrate the calorimeter (Fig. 4). A 33.266 R resistance immersed in silicone oil (tube 9, Fig. 1) is heated by using an inexpensive A G DC transformer with variable output. The current (0is calculated from the voltage drop across a -12-0 resistance, which is measured with a KD-550C Kingdon LCD digital multimeter (m,). Calibration time ( t ) is measured to 0.1 s with a digital chronometer and the heat output is calculated as
where R, is the heater resistance. The stability of the power source is ensured by a dummy resistance (Rw 33 R). Alternative designs for the measuring system and the calibration circuit have been reported in this Journal (5,8, 9). The calorimeter is immersed in a 65-L water bath, whose temperature is maintained a t 25 'C (f0.05) by a SU6 Grant thermostat. In a typical experiment, the ampoule was loaded with 0.2-1.0 g of sample weighed to flo4 g with a Mettler AE 260 balance. The mass was not wrrected for vacuum, but students should become aware that for high-precision work this correction is necessary. The vessel was filled with 125mL of the calorimetric solution and the whole setup immersed in the water bath. By using the electric heater from the calibration system, the temperature of the calorimeter was then raised to a point slightly below the bath temperature. The system was allowed to "equilibrate", which was indicated by the linearity of the temperature-time curve (this "equi1ibrium"meansthat a constant rate of heat transfer fmm the bath to the calorimeter is achieved). Approximately five minutes later, the so-called "fore period" of the curve (Fig. 5) was recorded. This took -3 min, aRer which the electrical calibration started, yielding the steeply-slopingpart of the curve in Figure 5. Meanwhile, the voltage across the heater and across the 1 2 . 3 4 reference resistance (to calculate the current) were recorded. In high-precision work, the voltage and the time period for the calibration should be chosen in order to match the temperature difference observed in the experiment. Also, the calibration should be performed either before or after the reaction, depending on the sign of the measured heat effect. In the present case, however, the errors caused by neglecting these rules (10)are insignificant. Therefore, calibrations were always made before the reaction, with time periods of -50. s and V 2.7 V. Each calibra-
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Volume 69 Number 11 November 1992
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1 Eraiiond 1
P%zd
After
Period
Time Figure 5. Atypical thermogram.
Figure 4. Electric diagram of the calibration circuit. tion was followed by the "after period" portion of the curve (Fig. 5) where a linear change of temperature with time was again observed. The after period of the calibration often can be used as the fore period of the reaction, particularly when the reaction under study is endothermic. In any case, the thermogram for the experiment was obtained much in the same way as described for the calibration. The only difference is, of course, that the mixing of the reactants replaced the electrical heating. There is, however, a detail that deserves to be mentioned. When a rapid process occurring in the vessel is either very endothermic or exothermic, an oscillation of the recorder (overshoot) may be observed immediately after the opening of the ampoule. In order to avoid this problem, the thermistor and the ampoule must be placed opposite to each other, and the ampoule must be opened fairly slowly. I t is finally noted that true temperature-time records are not obtained with the set-up described above. The change in the thermistor resistance with temperature is detected by a potentiometer and the signal fed to a recorder. This is, however, unimportant, since both the calibration and reaction were made by using the same recorder sensitivity. The enthalpy changes, AH, were obtained from eq 1, where Q is Comparison of Measured Enthalpies of Solutlon of Alkali Metal Halides (MX) with Values Tabulated in NBS Tables (T= 298 K; data in kJImol)
AHw (thiswork)' LiCl
38.39*0.16
~ ( H ~ o ) I ~ ( M x ) ~A&I~(NBS)~
2751
-38.80
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s of MX. 'Obtained by interpoiation of tabulated data (15).
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Journal of Chemical Education
(1)
(14).
At present, two experiments involving the reaction-solution calorimeter are available in our undergraduate laboratory. One aims to discuss the Bom-Haber cycle for several alkali metal halides and the other is related to the energetics of solute-solvent interactions in organic solutions. These experiments will be described in a future publication. Here, a data sample of the measured enthalpies of is given in the solution of several alkali metal halides (MX) table and compared with values quoted h m NBS tables (15).The agreement is good, particularly if it is considered that the samples (pro analysis grade) were not subject to any purfiation. Acknowledgment J. A. S. thanks Funda~Hode Amparo A Pesquisa do Es-
tad0 de S. Paulo (FAPESP, Brazil) for a postdoctoral grant. Literature Cited 1. Ma&, T J., Ed.. Boding Ener@iea in O~gonomidlieCompounds: ACS Symp. Ser No. 428, 1990. 2. Neidig, H.A.;Schneider,H.:lkates, T G.J. Cham. Educ. I=, 42.26and references "red therein. 3. Banle. K D.; Osbmn, P-~ M.J Cham Educ. 1913,50,687. 4. fied;ban, N. J. cham Ed ue is??, 54,248. 5. Bailey, R.A.;Z"hiek, J. \1 J~Chem Edue. lWl.58.732. - - -6. M l l l e ~D. P J Cham. Edue. Illb), 62,I I L . 7. Teixeirs,C. Ph. D. Thesis, Instihlto Superior W m , 1986. 8. Srivastaua, S. B.;Meloan, C. E. J Cham Edue. 1984,61,1027. 9. Fuehs, R. J C h . Edue. 1981,58,594. 10. W a d s , I. Sci. l b d s 1966, 13.33. 11. Csnageratna, S. G;With J . J . Chem. Edue. 1966,65,126. 12. Vandenee,C. E. J. Cham Thermadyn. lW1,13,1139. 13. Dunn, S. R. J. Cham. Thermalyn. l971,3,19. 14. Vandenpe.C. E.; Waugh. D.H.;Hsaa, N C . J C h . Themodyn. 1981,13,1. 15. Wagmem,D.D.:Evans,W.8;Psrker.Y B.;SEh-,RH.;IW-,l;BeilexS Chumey, K L.; NutWl. R.L.J.Phys Cham. Bef Doh 1982,II, suppl. no. 2.
'.
number of d
ATCT,,l m
the heat output during calibration, M the molecular weight, m the mass ofthe sample, and ATcdand AT are the adiabatic temperature differences in the calibration and in the process under study, respectively. As stated above, in our experiments the measured ATs are proportional to the true temperature differences.The methods used to determine these quantities from the thermograms are described in detail elsewhere (Ig13). The calorimeter was tested by measuring the enthalpy of the reaction of hydrolysis of THAM (tris(hydroximethy1)aminomethane) in 0.1 M HCl aqueous solution. The average result six run&-29.91 f 0.33 kJImol, is in good agreement with the recommended value, -29.782 f 0.014 kJ1mol
~~~~~
'Averages of five runs. b~verage values of the ratios between the number of moles of water and the
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