In the Classroom
Experiencing and Visualizing the First Law of Thermodynamics: An In-Class Workshop Pamela A. Mills,* William V. Sweeney, and Waldemar Cieniewicz Department of Chemistry, Hunter College, New York, NY 10021-5085; *
[email protected] Syringes have been used often to illustrate the concepts of the gas laws (1–3). The fire syringe can be used to demonstrate the temperature rise that accompanies an adiabatic compression (4). Fire syringes, however, do not allow students to observe directly the temperature rise accompanying the adiabatic path, nor can students use them to observe an isothermal path. We have a designed a workshop appropriate for general chemistry students that uses a handmade syringe device to illuminate the concepts of heat, work, energy transfer, and thermodynamic path. The Workshop The workshop is done in two parts. In the first, students focus on the macroscopic aspect of the first law of thermodynamics, and in the second they develop a microscopic explanation for their macroscopic observations. Central to the workshop is a device designed at Hunter College to give students a feel for heat and work during gas compression. This device consists of a plastic syringe with a temperature probe sealed into the needle end. The probe is connected to an integrated circuit with a fast response time, which displays temperature. Figure 1 is a diagram of the device.
Figure 1. A standard 3-cm3 plastic syringe is fitted with a thermocouple installed in the needle end. When turned on, the device samples the ambient temperature and zeroes to the current temperature. The display shows the maximum positive temperature change (denoted Hi°C of 6.8), the maximum negative temperature change (denoted Lo°C of 0.0) and the real-time temperature sampled every 0.1 seconds (denoted ∆T°C of 3.5).
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First Workshop Session In the first workshop students are asked to rapidly compress gas contained in the syringe. This mimics an adiabatic compression, and thus they observe a temperature increase in the gas of 5 to 10 °C. Students are also asked to do an isothermal compression. They quickly determine that this can be achieved by depressing the piston slowly. Although students are able to do isothermal and adiabatic gas compressions, they appear perplexed by the clear demonstration of work causing a rise in temperature of the gas in the adiabatic compression, and they are unable to provide either a microscopic or macroscopic explanation for the temperature rise. To help organize their thinking, students work in groups of five or six to discuss the following questions and formulate responses, which are then presented in class. Commentary and sample responses are given after the questions. Case I: Adiabatic Compression Q. Did the temperature change? A. The temperature increased 6.8 degrees (see Fig. 1). Q. Was any work done on the gas? How do you know? A. Work was clearly done on the gas because students depressed the syringe, thereby applying a force over a distance. Q. Did any heat enter the system? How do you know? A. This is a difficult question. The temperature rise may be due, in part, to heat, but the effect is likely to be small. To get a feel for this, students can be directed to cover the syringe with their hand, thereby heating the gas. They will notice a small and slow temperature rise in contrast to the instantaneous temperature rise accompanying the fast compression. Thus students can conclude that the temperature increase observed with the fast compression is not due to heat. Q. Did the energy of the gas change? How do you know? A. The internal energy of the gas increased because the temperature of the gas increased. Q. What caused the energy of the gas to change? A. The temperature rise comes from the work done on the gas. Case II: Isothermal Compression Q. What did you have to do to compress the gas isothermally? A. The gas has to be compressed slowly in order to allow the temperature of the gas to remain in equilibrium with the temperature reservoir (the room temperature). Q. Did the energy of the gas change? A. The energy of the gas did not change because the temperature of the gas did not change. Q. Was any work done on the gas? How do you know? A. Clearly work was done on the gas because a force was applied over the distance the plunger was depressed.
Journal of Chemical Education • Vol. 78 No. 10 October 2001 • JChemEd.chem.wisc.edu
In the Classroom
Q. Where did the work energy go, since the temperature of the gas did not increase? A. The energy due to the work was transferred out of the syringe into the ambient atmosphere as heat.
Second Workshop Session By the end of the first session, students have experienced the macroscopic relationships among work, heat, temperature, and energy for gas compressions. However, they are still unable to answer the question: what is the microscopic explanation for the observed temperature rise of the gas when the gas is compressed adiabatically? To address this, we have constructed a 90-minute workshop that develops a microscopic model for the adiabatic and isothermal processes. Students are asked to model a gas particle as a tennis ball, and to compute the change in speed and energy that occurs as a result of collision between the ball and a huge plunger (effectively a moving wall), neglecting gravity and friction.1 For this exercise, the plunger is assigned a mass of 1000 kg and a velocity of 2 m/s and the ball is assigned a mass of 0.10 kg and a velocity of ᎑5 m/s. Students perform the calculation for this elastic collision readily, and find an increase in the speed of the ball of 4 m/s. To encourage them to think more deeply about energy transfer, the groups are asked to address the following questions. Q. Compare the energy of the ball before and after the collision. A. Since the speed of the ball increased, the kinetic energy of the ball increased. Before the collision, the kinetic energy of the ball was 1.25 J. After the collision, the kinetic energy of the ball was 4.05 J. Q. Recall that for elastic collisions, energy is conserved. If the kinetic energy of the ball changed, where did the energy come from? A. The energy must have come from the plunger. Q. What was the change in the kinetic energy of the plunger as a result of the collision? Does your answer make sense to you? Why or why not? A. The kinetic energy of the plunger after collision is calculated to decrease by 2.8 J. This is reasonable because the sum of the kinetic energies of the plunger and ball before and after the collision is the same. Q. If you wanted to keep the speed of the plunger exactly the same before and after the collisions, would you need to do work on the plunger? A. Yes, because you would have to add energy to the plunger to bring its kinetic energy back to its initial value. A similar process is repeated for the isothermal case. Often, the individual groups do not construct coherent answers to the above questions. Rather, reasonable microscopic pictures of the adiabatic and isothermal compressions occur after a few groups present the results of their discussion to the class. It takes about three to four group presentations before the class has arrived at a consensus understanding of the micro-
scopic model. At the conclusion of this class session, students articulate a macroscopic and microscopic understanding of the first law of thermodynamics as it is applied to gas expansions and compressions. General Applicability of the Workshop The development of the microscopic model relies on the students’ prior knowledge of mechanics. These workshops are presented in our integrated chemistry–physics introductory science course. The students in the interdisciplinary course have studied the topics of momentum and collisions earlier in the semester. In a general chemistry class in which students are not required to have completed an introductory physics course, it will be difficult to have students perform the computations. In addition, students who have no prior knowledge of physics may find the concepts of momentum and collisions opaque. However, it may be possible to modify the second workshop to enable students to develop a physical feel for the microscopic properties of the gas compression without a detailed understanding of mechanics, momentum, and collisions. The first workshop does not require any indepth understanding of physics and can easily be applied to a general chemistry class. The Syringe Temperature Probe We have designed and built the device pictured in Figure 1. It uses a standard plastic disposable syringe with a thermocouple installed in the needle end. In early versions of this device we discovered that the temperature changes rapidly decay. While increases of 8 °C can be generated easily, the temperature returns to the starting temperature in as little as one or two seconds. For this reason we designed a device that samples the temperature every 0.1 s and has a real-time display in addition to the maximum positive and negative temperature changes. The displays of the maxima are rezeroed after 20 s. The device is powered by a 9-V battery. Construction requires the ability to make printed circuit boards. The materials for each device cost approximately $60. On request we will be happy to provide details of the construction. Note 1. Students in a General Chemistry course that does not require physics will not be able to do this calculation—see section entitled General Applicability of the Workshop.
Literature Cited 1. 2. 3. 4.
Lewis, D. L. J. Chem. Educ. 1997, 74, 209–210. Lieu, V. T. J. Chem. Educ. 1996, 73, 837. de Berg, K. C. J. Res. Sci. Teach. 1995, 32, 871–884. Hayn, C. H.; Baird, S.C. Phys. Teach. 1985, 23, 101–102.
JChemEd.chem.wisc.edu • Vol. 78 No. 10 October 2001 • Journal of Chemical Education
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