In the Laboratory edited by
Cost-Effective Teacher
Harold H. Harris University of Missouri—St. Louis St. Louis, MO 63121
An Inexpensive Solution Calorimeter Emma Kavanagh, Sam Mindel, Giles Robertson, and D. E. Peter Hughes* Westminster School, 17 Dean’s Yard, London SW1P 3PB, United Kingdom; *
[email protected] almost exactly balanced by the heat produced by the stirrer. The temperature rise due to the reaction was found using a computer program that evaluated the best straight line fit for the initial and final periods of the run and identified the take off point, This temperature rise can also be found by using a computer printout of the reaction run; best straight lines were drawn through the points in the pre- and post-reaction periods and the vertical height measured at the take off point. Measurement of the temperature change at the take off point is the method recommended for a fast reaction (6). Hazards Concentrated sodium hydroxide is a strong base and highly caustic. Dilute hydrochloric acid and dilute ethanoic acid are corrosive and may cause burns. Results and Conclusion The apparatus is easy to use and the results of a run can be obtained in ten minutes; this made it possible to carry out several determinations in a lab session. For example we have used the apparatus to determine the enthalpy of neutralization of ethanoic acid. Three calibration runs for the neutralization of HCl with NaOH were done and three runs for the neutralization of CH3CO2H with HCl. The standard molal enthalpy change of neutralization for 0.100 mol dm‒3 solutions of HCl and NaOH can be found from published tables of data (7); it is ‒56.5 kJ mol‒1. This value and the measured temperature changes enabled us to calibrate the apparatus and hence determine the enthalpy of neutralization of CH3CO2H. Our value of ‒56.3 kJ mol‒1 was close to that obtained from the tables of data (‒56.4 kJ mol‒1; ref 7 ). For more accurate work additional calibration runs can be done. Our best value for the temperature rise with HCl and NaOH was 1.096 ± 0.007 K.
Temperature Increase / K
The principal defect when using an open Styrofoam cup as a solution calorimeter (1, 2) is the substantial heat loss due to evaporation. It is difficult to estimate the value of this heat loss over the 10 minutes that is needed to carry out the experiment. In our apparatus, heat loss was minimized by using a sealed calorimeter, fitted with a magnetic stirrer and a miniature bead thermistor that had a rapid response time of 6 s. There is another source of error when using a Styrofoam cup; the solutions may not be at the same temperature when they are mixed with each other. In our apparatus, this error was minimized by having both reactants in thermal contact with each other prior to mixing. In a research instrument, thermal contact is achieved by breaking a glass ampoule containing one reactant or by modifications of the method (3, 4). We put one reactant in a plastic tube sealed with a rubber lamina. The plastic tube was placed in the solution of the other reactant to ensure thermal equilibrium and the rubber lamina pushed out with a glass rod to initiate the reaction. The thermistor was made from one arm of a Wheatstone bridge circuit. The output was fed into an analog-to-digital (AD) converter and then to a computer. Analysis of the output showed a background noise of 5 mV, a level that was not substantially reduced by screening or by grounding the apparatus. The only satisfactory way of reducing the noise was to rapidly sample the output of the thermistor and to average several thousand samples. This averaging was achieved using a computer program. The program also contained instructions to ensure that the output voltage from the thermistor was linearly proportional to the temperature change. The resistors used in the Wheatstone bridge had a 1% tolerance. The output of the AD converter had an uncertainty of 1% and that of the thermistor 3%. It was, therefore, necessary to calibrate the instrument before use. It is usual to calibrate a solution calorimeter using TRIS, tris(hydroxymethyl)aminomethane, which produces 491.5 J when 1.000 g is dissolved in 200.0 cm3 of 0.1000 mol dm‒3 HCl (5); this gives a temperature rise of about 0.2 K. It was decided that it would be better to use the neutralization of 0.1000 mol dm‒3 HCl with NaOH as a standard as this gives a temperature rise of ~1 K. Another advantage of the HCl/NaOH method was that the solutions need to be weighed with a precision of only 0.01 g, while the use of TRIS would need weighing to a precision better than 0.001 g. With a rise in temperature of 1 K, measurement to 0.01 K corresponded to 1% accuracy, which was the value aimed for in designing the apparatus. By keeping the thermistor, Wheatstone bridge, and AD converter together as a set, uncertainties were kept constant and included in the calibration. When the same set is used experimentally, these uncertainties cancel out. A typical trace for the neutralization of HCl with NaOH is shown in Figure 1. Initially, there was a steady temperature rise of 0.002 K s‒1 caused by the heat produced by the stirrer: after the reaction had taken place this increase was reduced to 0.0003 K s‒1, showing that the heat loss from the apparatus was
1.0
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Figure 1. A typical trace for neutralization of HCl with NaOH. The temperature rise was found from the vertical distance at the take off point (22.4 s).
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 8 August 2008 • Journal of Chemical Education
1129
In the Laboratory
The apparatus gave superior results to the use of Styrofoam cup in two main respects; first, it detected smaller temperature changes and second, uncertainties caused by heat losses were largely eliminated. The cost was approximately $60 apart from the cost of the magnetic stirrer, AD converter, and computer. Its accuracy is intermediate between that of the Styrofoam cup calorimeter and that of a Parr 6755 calorimeter; the latter can measure temperature changes with an accuracy of ± 0.002 K (5). Literature Cited 1. Wong, S.-S.; Popovich, N. D.; Coldiron, S. J. J. Chem. Educ. 2001, 78, 798. 2. Randall, J. Advanced chemistry with Vernier; Vernier Software and Technology: Beaverton, OR, 2004; Experiment 26. 3. Bailey, R. A.; Zubrick, J. W. J. Chem. Educ. 1981, 58, 732–733. 4. Diogo, H. P.; Minas da Piedade, M. E.; Moura Ramos, J. J.;
Simoni, J. A.; Martinho Simoes, J. A. J. Chem. Educ. 1992, 69, 940–942. 5. 6755 Solution Calorimeter Operating Instruction Manual; Parr Instrument Company: Moline, IL, 2007; Section 6-1. 6. Canagaratna, S. G.; Witt, J. J. Chem. Educ. 1988, 65, 126–129. 7. Selected Values of Chemical Thermodynamic Properties; Wagman, D. D., Evans W. H., Parker V. B., Schumm R. H., Halow I., Bailey S. M., Churney K. L., Nuttall R .l., Eds.; National Bureau of Standards: Washington DC, 1981.
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2008/Aug/abs1129.html Abstract and keywords Full text (PDF) with links to cited JCE articles Supplement Construction of the thermistor and calorimeter and calibration
JCE Concept Connections: Calorimetery JCE Print
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Web edition of the Journal of Chemical Education
Some additional articles describing laboratory exercises using calorimetry measurements are listed below. The solution calorimeter described by Hughes and co-workers can be used in these exercises. The Stoichiometry of the Neutralization of Citric Acid: An Introductory Laboratory. J. Chem. Educ. 1995, 72, 1029. Heat of Solution: Hot Packs. J. Chem. Educ. 1994, 71, 791. Simple Heat Flow Measurements: A Closer Look at Polystyrene Cup Calorimeters. J. Chem. Educ. 1994, 71, 793. Copper/Aluminum Surprise. J. Chem. Educ. 1990, 67, 165. How Good Is Your Bleach? J. Chem. Educ. 1989, 66, 973.
In a recent article and related JCE Classroom Activity, students perform quantitative calorimetric measurements on samples of ice/water heated by incandescent light bulbs and by room-temperature surroundings. They make connections between the measurements and global warming. A Simple Calorimetric Experiment That Highlights Aspects of Global Heat Retention and Global Warming. 2007, 84, 1686. Hold the Heat: Global Warming and Calorimetry. J. Chem. Educ. 2008, 85, 224A.
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The Web-ready titles in the JCE Software collection are now available on the Web, including ChemPages Laboratory, by Joe L. March, John W. Moore, and Jerrold Jacobsen. ChemPages Laboratory covers more than 30 laboratory techniques and items of equipment as a set of web pages that include text, images, video, and self check questions. The topics discussed are commonly encountered in the first-year chemistry laboratory, including:
Calorimetry, Coffee Cup Description; Assembling the Calorimeter; Measuring the Temperature; Adding Solutions to the Calorimeter; Self Check Exercises
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