In the Laboratory
A Simple Calorimetric Experiment That Highlights Aspects of Global Heat Retention and Global Warming
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Joel D. Burley* Department of Chemistry, Saint Mary’s College of California, Moraga, CA 94575-4527; *
[email protected] Harold S. Johnston Department of Chemistry, University of California and Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720;
[email protected] This report outlines a simple calorimetric experiment for the general chemistry laboratory that illustrates a classic principle of chemical thermodynamics and, in a somewhat novel way, gives students an introduction to some broad features associated with global heat retention and global warming. The earth has had a robust greenhouse warming effect for millions of years; otherwise the planet would be much colder than it is now. Scientific measurements show that human activities are increasing the concentrations of atmospheric greenhouse gases, including carbon dioxide, methane, and nitrous oxide. Long-established principles of physics and chemistry indicate that increasing these gases will increase the heat retention by the earth (1). There is an extensive literature concerning global warming and a few representative samples are cited here: materials appropriate for introductory students (1–3), an article that illustrates the great complexity of the subject (4), and detailed studies of Arctic regions (5–7). The pedagogical goal of this experiment is to highlight the distinction between global heat retention and global warming and demonstrate how long-term melting of surface ice consumes heat with no increase in temperature. Experimental Procedure
Equipment and Chemicals This laboratory exercise is intended for the general chemistry laboratory, with students working in pairs. Each pair of students will need three matched 600 mL beakers (i.e., the same style or model from the same manufacturer, with essentially identical heat capacities and optical properties), three stir plates with magnetic stir bars, three computer-interfaced temperature probes, and two small desk lamps (approximately 50 W). To weigh the initial samples of crushed ice and roomtemperature water, a top-loading digital scale with a capacity of 1000 g is also required. (Multiple student groups may share a single scale.) Temperature measurements can be conducted manually (i.e., with thermometers, stopwatches, and manual stirring of the samples) if computer-interfaced temperature probes or magnetic stir plates are not available. Procedure Calorimetric measurements are carried out on three different samples of water and ice: • Sample 1: 500 g of room-temperature water with heat added by a 50 W desk lamp. • Sample 2: 500 g of an ice–water mixture with heat added by conduction from room-temperature surroundings. 1686
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• Sample 3: 500 g of an ice–water mixture with heat added by a 50 W desk lamp and by conduction from the room-temperature surroundings.
Each pair of students labels and weighs the three matched 600 mL beakers. Beaker 1 is loaded with 500 g of room temperature water, and 500 mL of crushed ice are added to beakers 2 and 3. Cold tap water is then added to beakers 2 and 3 until the net ice-plus-water mass is exactly 500 g for each beaker. After the three samples have been prepared, each beaker is positioned on top of its stir plate, and a magnetic stir bar is added to each sample. For samples 1 and 3, a desk lamp is placed behind or to the side of the stir plate, with the head of the lamp positioned so that it is directly facing the beaker from a distance of approximately 5–10 cm. Sample 2 is situated so that it is completely shielded from the lamps that are used to heat samples 1 and 3. The stir plates are turned on and set to an intermediate stir rate. A computer-interfaced temperature probe is inserted into each sample, and data collection (at one-minute intervals) is initiated. After a few minutes of data collection, the desk lamps illuminating samples 1 and 3 are turned on, and the time of lamp activation is recorded. Data collection proceeds until the ice in sample 2 is completely melted and the sample has warmed to a temperature of about 5 ⬚C, requiring approximately 90 minutes. The data (temperature as a function of time for all three samples) are then imported into Microsoft Excel or a similar spreadsheet program for graphical analysis. In some cases the computer software used to control the data acquisition (e.g., LoggerPro from Vernier Scientific) may also be used to perform the data analysis. Hazards There are no significant hazards associated with this experiment. Results and Data Analysis Sample 1 is initially at room temperature, so there is no heat conduction between it and the surroundings. When the lamp is turned on, the initial slope therefore indicates the heating provided solely by the lamp (Figure 1). Samples 2 and 3 remain at a temperature close to 0 ⬚C until all the ice has melted and are then warmed by conduction and by lamp plus conduction, respectively. Since all three samples contain the same mass of H2O, the initial rate at which the temperature increases for sample 3 (i.e., the initial slope of the temperature vs time graph) should be roughly equal to the sum of the initial rates for samples 1 and 2.
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In the Laboratory
surface, earth emits infrared (heat) radiation towards cold outer space. The atmosphere includes greenhouse gases that absorb outgoing infrared radiation and re-emit it in all directions. The downwardly directed component of the re-emitted infrared represents heat retention by the earth. All of these heat effects are included in the mathematical models that experts use to study global warming, but typically “heat retention”, an important simple concept, is usually not explained in so many words to beginning students or the general public. The following outline presents a more precise terminology for thinking about these concepts: 1. Increasing concentrations of greenhouse gases lead to increased global heat retention by the earth.
Figure 1. Heating curve results for samples 1–3 (typical student data). The straight lines, representing the linear regression fits, extend beyond the fitting ranges to show departures from linearity at longer times.
As the temperature for sample 1 increases above room temperature, some energy is lost by heat conduction and the “lamp-only” approximation for this sample becomes less valid. Over the first ten minutes the heat conduction for sample 1 is driven by a temperature difference of between 0 and 3 ⬚C, which is much smaller than the 23 ⬚C temperature difference driving the heat conduction in samples 2 and 3. We estimate that limited heat conduction from sample 1 to the surroundings reduces the measured value of the “lamp-only” slope for sample 1 by approximately 5%.1 Students should begin their quantitative analysis by determining the slopes for the initial linear regions of curves for all three samples, taking care to exclude points from the baseline prior to the onset of the rapid temperature rise, as shown in Figure 1. In most cases the linear regressions will include approximately 5–20 minutes of data, and students should be encouraged to investigate how the slope changes if minor adjustments are made to the fitting range. The heating rate for each liquid water sample can be calculated (and then plotted, if desired) from
∆T heating = ms rate ∆t
(1)
where ∆T∆t is the initial slope of the temperature versus time plot, m is the mass of the water sample, and s is the specific heat capacity for liquid water. If students are careful with the initial setup (matched beakers, all three samples with exactly 500 g of H2O, uniform lamp positions and stir-bar settings, etc.), then the accuracy and reproducibility of the student results are usually very good. Typical errors (i.e., deviations from the slope 3 = slope 1 + slope 2 expectation) under these circumstances are usually on the order of a few percent. Discussion The earth is bombarded by radiant energy from the sun. This incoming solar radiation is partially absorbed by the surface, which warms the surface, and partially reflected out of the atmosphere. At all times of day and night and over its full www.JCE.DivCHED.org
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2. Increased global heat retention contributes to two different observables: (i) Temperature increases for gaseous, liquid, and solid earth. (ii) Long-term melting of ice with no change in temperature.
The recent melting of polar ice is a result of increased global heat retention. Some of the retained heat has warmed cold ice up to its melting point of 0 ⬚C, and additional retained heat has melted ice at 0 ⬚C, with no change in temperature during the melting process. Summary There are two lines of direct evidence that increasing atmospheric greenhouse gas concentrations are presently contributing to increased global heat retention: (i) increasing surface, ocean, and atmospheric temperatures, and (ii) observed long-term melting of surface ice. Experimental evidence from satellites, aircraft, and ground-based sensors indicates that both lines of evidence support the hypothesis of increased heat retention (5–9). While recent increases in the observed global average surface temperature have been small—approximately 0.17 ⬚C per decade (9)—worldwide observations of melting ice have been more conspicuous (5– 7, 10). At the present time, the directly observed melting of surface ice indicates more strongly than the observed temperature increases that the earth has entered into an era of increasing greenhouse effects. Students who have completed the laboratory exercise described here have emerged with a stronger understanding of these key issues and are able to connect them to the simple calorimetric principles that are typically covered in most first-year chemistry courses. Acknowledgments The authors would like to thank the students and support staff of the general chemistry laboratories at Saint Mary’s College (SMC), who enthusiastically debugged the experimental procedure during the spring and summer of 2005. Thanks are also due to the SMC Chemistry Department, which provided financial support for this project. The work at the University of California at Berkeley and the Lawrence Berkeley National Laboratory was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences Division of the U.S. Department of Energy under contract DE-AC02-05CH11231.
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Supplemental Material
Further descriptions of this experiment including quantitative, homework-style problems, the student laboratory handout, and alternative experimental configurations are available in this issue of JCE Online.
2.
Note
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1. A more sophisticated analysis can be used to reduce the error introduced by possible heat conduction in sample 1. In this case, the students plot the observed sample 1 slope for fixed, 4minute time intervals (i.e., 1 to 5 minutes; 3 to 7 minutes; etc.), and extrapolate the results back to t = 0. The extrapolated value obtained for the slope should yield a good estimate of the true, “lamp-only” heating rate desired for sample 1.
1. Climate Change Information Kit; United Nations Environment Programme, 2003 http://unfccc.int/essential_background/
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