LABORATORY EXPERIMENT pubs.acs.org/jchemeduc
The Heat Is On: An Inquiry-Based Investigation for Specific Heat Deborah G. Herrington* Department of Chemistry, Grand Valley State University, Allendale, Michigan 49401, United States
bS Supporting Information ABSTRACT: A substantial number of upper-level science students and practicing physical science teachers demonstrate confusion about thermal equilibrium, heat transfer, heat capacity, and specific heat capacity. The traditional method of instruction, which involves learning the related definitions and equations, using equations to solve heat transfer problems, and using equations to determine the heat capacity of a metal in a laboratory setting, contributes to this confusion. A simple modification of a traditional high school or introductory college chemistry heat capacity laboratory is described. Based on the learning cycle, rather than a verification of the concept of heat capacity, this experiment is designed to introduce students to the concept of heat capacity and to encourage them to make connections to their personal experiences before being provided with definitions and mathematical equations. Rationale for this modification along with a description of the experiment including pre- and postlab student questions, student instructions, sample student data, and teacher facilitation suggestions are provided. KEYWORDS: First-Year Undergraduate/General, High School/Introductory Chemistry, Laboratory Instruction, Physical Chemistry, Hands-On Learning/Manipulatives, Inquiry-Based/Discovery Learning, Heat Capacity, Metals, Student-Centered Learning
C
Often students are exposed to specific heat capacity, c, by learning the definition and then using c in the relationship
oncepts related to heat and temperature are taught in science courses from elementary school through college. However, a study published in this Journal1 indicates that students with four or more college science courses and practicing middle and high school science teachers still demonstrate confusion about the fundamental topics of (i) thermal equilibrium, (ii) physical basis for heat transfer and temperature, and (iii) the relationship between specific heat, heat capacity, and temperature change. This confusion is not unexpected from a constructivist perspective where people construct knowledge from their experiences. There are a number of everyday experiences that may lead students to naïve or incorrect ideas about these topics. Most students have heard someone say, “Close the door, you’re letting the cold in” or “Put your warm sweater on, it’s cold outside.” The naïve or incorrect ideas that students construct from their personal experiences are tenacious, and accepted scientific ideas will only replace these if students have experiences that require them to confront and revise their initial ideas.2 In teaching for conceptual understanding or conceptual change, students must (i) make predictions to foster awareness of their current thinking about a concept and (ii) make connections to their own experiences, not just think it is some isolated thing they see in science class.2 Though students have some experience with heat capacity and specific heat, for example, they know that when they pull something out of the oven they can pull off the aluminum foil relatively quickly but they need to wait awhile before they grab the aluminum pan with their bare hands, rarely are they required to connect what they learn in science class to these experiences. Copyright r 2011 American Chemical Society and Division of Chemical Education, Inc.
q ¼ mcΔT
ð1Þ
to calculate the quantity of heat energy transferred, q, in a given situation or the quantity of energy required to heat a certain object with a mass m from temperature A to temperature B, ΔT. A typical high school or introductory college laboratory requires students to determine the identity of an unknown metal by heating the metal, placing the heated metal in room temperature water, and then determining how much heat energy is transferred to the water as the metal cools down.36 Some experiments use more modern equipment than the standard coffee cup calorimeter,5,6 and some experiments expand on the concept and also demonstrate Dulong and Petit’s law.5,7 However, these experiments do not help students understand the concept of heat capacity. For example, if you heat metals that have the same mass but different specific heat capacities to the same temperature and drop each of those metals into equal volumes of water, which metal will cause the water to heat up the most? Why? Many students can do the calculations with eq 1 but cannot answer this question, which is central to the concept of specific heat capacity. It is important for the students to understand that the metal with the higher specific heat will release more energy as it cools and thus will raise the temperature of the water more. This phenomenon also explains why cities or towns near large bodies of water Published: August 02, 2011 1558
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Table 1. Student Data and Specific Heat Capacities for the Metals Metala Al
Steel
Zn
Cu
Sn
a
Trialb
Ti,water/°C
Ti,metal/°C
Tf/°C
ΔTwater/°C
|ΔTmetal|/°C
1
20.6
100.0
32.6
12.0
67.4
2
21.0
99.6
33.3
12.3
66.3
Avg
12.2
66.9
1
20.3
100.0
28.4
8.1
71.6
2
20.9
100.0
28.1
7.2
71.9
Avg
7.7
71.8
1
20.9
100.0
28.0
7.1
72.0
2 Avg
21.0
97.8
26.9
5.9 6.5
70.9 71.5
1
20.3
100.0
25.5
5.2
74.5
2
21.6
100.0
27.7
6.1
72.3
Avg
5.7
73.4
1
21.1
99.5
23.7
2.6
75.8
2
20.8
100.0
24.6
3.8
75.4
Avg
3.2
75.6
Lit c/(J g1 °C1) 0.900
0.460
0.388
0.385
0.228
Masses of metal samples and water were 58.0 g. b These data were collected by two teams of students who used the same procedure.
stay warmer in the winter (water releases more energy as it cools than land because water has a much higher specific heat capacity) and conversely cooler in the summer. The experiment described here focuses on conceptual understanding of this topic by introducing students to the concept of heat capacity by having them examine how heated metals of equal mass affect the temperature of water before they are introduced to terms heat capacity and specific heat.
’ EXPERIMENTAL DESIGN A recent study by Meyers and co-workers8 showed that although students liked the assurance in having the “correct answer” and knowing the correct procedure in a direct-instruction laboratory, they liked the thinking involved in a inquirybased laboratory and felt that they learned more with inquirybased laboratory. As such, a typical direct-instruction laboratory was modified based on the learning cycle.9 In the prelab, students are asked to answer questions designed to solicit their prior knowledge about the concepts of heat, heat transfer, and heat capacity. These questions (see Supporting Information) prod students to awareness of their prior knowledge as well as inform the instructor of areas to discuss after the laboratory. These questions also start students thinking about how hot metals can be cooled (put them in water), which helps them design a procedure. Students design a procedure to investigate the question, “Which substance (aluminum, zinc, copper, tin, steel, or water) gains or loses the most heat when it changes temperature?” This is not a trivial procedure for students to design as they have to consider several factors. For ease of comparison, each trial requires the following: • Identical mass for the water and the metals • Same initial temperature of water • Same initial temperature of the metals Several guiding questions are provided to help students design a procedure. Students are also provided with a list of available materials. In addition to standard materials found in most high school and college chemistry labs and those used in coffee cup calorimeter activities, students are also provided with a set of 5
metal cylinders of equal mass and diameter.10 The general experimental setup that students use to collect their data is a standard coffee cup calorimeter, which consists of a coffee cup that has a lid with a hole in it for the thermometer to fit through. The general procedure that the students decided on was to place a predetermined volume of water in the cup and record its initial temperature. A piece of metal was heated in boiling water for 10 min to establish thermal equilibrium and the temperature of the boiling water was recorded to establish the initial temperature of the metal. The piece of heated metal was added to the room temperature water and the temperature of the water was recorded until it stopped rising. Students used digital, metal thermometers that were also used to stir the water. The data collection portion of this experiment goes faster if students work in teams. With five metals, it is possible to have five teams of students, with each team collecting data for one metal and sharing the data. Students also work in teams to design their procedures. Students can design an initial procedure as homework and then work in the teams to refine the procedure. Facilitation of the procedure-design process is critical for this experiment and the team approach allows the instructor to check with each group. Some of the common student-design issues and questions to help the teams solve these issues are provided in the Supporting Information. After students collect and share their data, they answer the guiding question for the experiment by putting the five metals and water in order of the magnitude of heat loss. They are required to support their order with data. Students participate in a group discussion of the results and conclusions, referring to several of the prelab questions to help explain their results as well as reflect on how their thinking has changed as a result of this investigation. Following this discussion, it is important for students to connect these ideas to other contexts so that this experiment does not become something they learned in science that does not apply to the rest of their lives. A set of follow-up questions can be used to assess whether students understand the concepts of heat transfer and heat capacity that they explored in the experiment and whether they can apply this to real-world situations. 1559
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’ HAZARDS There are no significant hazards in this investigation but students should be reminded that hot metals and glass do not look different from cool metals and glass. ’ RESULTS AND DISCUSSION Sample student data and literature values for specific heat capacities are summarized in Table 1. Even though the specific heat capacities for tin, zinc, steel, and copper are close in value and there is some heat lost to the surroundings using coffee cup calorimeters, the results follow the pattern of heat lost predicted by the specific heat capacities. After students have tried to answer the guiding question of the experiment, a postlab discussion involving the entire class can help address any common problems students have drawing conclusions from their data. For example, one common problem was that some students thought that the metal that had the greatest change in temperature, lost the most heat energy. Ideally, during the class discussion students who realized that the metal that increased the temperature of water the most and had the smallest change in temperature would produce the most heat energy would offer this alternate analysis. This would then lead to a productive class debate. However, at this point, the instructor may need to direct the class toward the temperature change for water in each trial by asking them what was constant in each trial. As water is the constant for comparison, students should note that the metal that has the biggest temperature change causes the water to increase in temperature the least. Therefore, it must release the least energy when it changes temperature. The students also had some difficulty evaluating the energy loss of the water. Although it is possible for students to heat a sample of water up to 100 °C and add it to room temperature water similar to the procedure used with the metals, the students did not consider doing this.11 Therefore, they had to compare the changes in temperature of the water to the changes in temperature of the metals. Initially students appeared to believe that because the metals had much larger temperature changes, they had to lose more energy. To help students correct their thinking, they were asked where the energy from the metal was going. The students should realize that the energy is being transferred to the water. As the temperature change of the water is much less than the temperature change for any of the metals, they should be able to reason that the water must require more energy (or loses more energy) to change temperature than the metals do. As this experiment is designed as an introduction to specific heat capacity, further experiences with heat transfer, specific heat, and thermal equilibrium are required for students to develop the deep, conceptual understanding of these topics. However, class discussions and student answers to pre- and postlab questions indicate that students have gained a better understanding of how heat capacity affects changes in temperature of substances. For example, in the prelab questions, 77% (10/13) students indicated that if 100 g of hot iron is added to 100 g of cool water, the same temperature changes are expected for each substance because you have the same quantity of each substance. However, in the postlab discussion, students recognized that the temperature change for water is much less than that for any of the metals tested. Furthermore, all of the students indicated that if you added a piece of hot metal that has twice the mass as the one they used in their experiment to the room temperature water, the
LABORATORY EXPERIMENT
temperature change of the water would double because the energy transferred would be doubled. All of the students were also able to connect these learnings to their observations at the beach indicating that water requires more energy to raise its temperature than sand as “there is the same quantity of energy from the sun on both the sun and the sand, but the water is cooler” and that the water releases more energy when it cools thus resulting in winter temperatures being slightly higher in areas next to large bodies of water.
’ CONCLUSION This relatively simple revision of a common high school and college chemistry laboratory including the postlab discussion can easily be completed in one, 2-h or two, 50-min sessions if students complete the prelab and postlab questions for homework.12 Given that a large number of research studies indicate that learning cycle approach can result in greater student achievement, better retention of concepts, and improved reasoning ability,9 and that students are constructing a conceptual understanding of heat transfer and heat capacity during the experiment and not just verifying what they learned in lecture, this experiment could provide much larger student learning gains. Moreover, after some initial frustration, most students expressed a sense of accomplishment in designing the procedure and constructing an answer to the guiding question based on their data. A colleague did this experiment with her class on a day when she had a sore throat and had trouble talking. She said that as a result, “the students agreed to discuss and negotiate answers, and develop procedures all by themselves. They did an excellent job and surprised themselves by enjoying it in the process.” ’ ASSOCIATED CONTENT
bS
Supporting Information Pre and postlab questions, student instructions, and notes for the instructor. This material is available via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by Grand Valley State University. The author is also grateful to her CHM 201 students who pilot tested this experiment. ’ REFERENCES (1) Jasien, P. G.; Oberem, G. E. J. Chem. Educ. 2002, 79, 889–895. (2) Watson, B.; Kopnicek, R. Phi Delta Kappan 1990, 680–684. (3) Ngeh, L. N.; Orbell, J. D.; Bigger, S. W. J. Chem. Educ. 1994, 71, 793–795. (4) Kokes, R. J.; Dorfman, R. K.; Mathia, T. J. Chem. Educ. 1962, 39, 90–91. (5) D’Amelia, R.; Stracuzzi, V.; Nirode, W. F. J. Chem. Educ. 2008, 85, 109–111. (6) PASCO (2006). Specific heat of an unknown metal. http:// www.pasco.com/file_downloads/experiments/pdf-files/glx/physics/ 31-Specific-heat-SV.pdf (accessed Jul 2011). (7) Bindel, T. H.; Fochi, J. C. J. Chem. Educ. 1997, 74, 955–957. 1560
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(8) Meyers, P.; Hong, H. H.; Fynewever, H. Chem. Educator 2008, 13 (2), 120–125. (9) Lawson, A. E.; Abraham, M. R.; Renner, J. W. A Theory of Instruction: Using the Learning Cycle to Teach Science Concepts and Thinking Skills [Monograph, Number One]; National Association for Research in Science Teaching: Kansas State University, Manhattan, KS; 1989. (10) The metal cylinders can be obtained from a number of different sources including Flinn Scientific and Sargent-Welch. (11) The students did not choose to heat a 58 g sample of water to 100 °C and add that to the room temperature water in the same way they did the metals. Instead they used the ΔT water data from each metal trial to help them determine how water compared to the metals with respect to heat loss or gain. However, treating water like one of the metals may make this comparison easier for students. (12) An experiment “Guided Discovery: Law of Specific Heats” published in the Journal,7 has some similarities to this experiment; however, students are briefly introduced to heat capacity prior to the experiment, it requires 10, 50-min periods, and it introduces students to the law of Delong and Petit.
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