Probing Student Misconceptions in Thermodynamics with ln-Class Writing Herbert Beall Worcester Polytechnic Institute, Worcester, MA 01609
In-class writing assignments (1)have been used during the teaching of general chemistry a t Worcester Polytechnic Institute to determine student problems and misconceptions. Examples of the use of this technique in the teaching of the thermodynamics section of the course will be presented here. Some of the students' problems that were identified were familiar and expected, but some were rather surprising. The prerequisite topics for this coverage of thermodynamics included the nature of gases, which was useful for studying pressure-volume work, internal energy and entropy, and atomic and molecular particles, which were a part of the discussions of temperature, thermochemistry, and disorder. Because the conceptual problems of college freshmen may be related to the knowledge base with which they enter the general chemistry course, we will first briefly outline a few relevant findings on school-age students. Atoms and molecules are not functional components of the reasoning of many school age students (2),a i d many of them see matter as continuous and static (3);the dynamic picture of molecules in constant motion and separated by empty space is not accessible to many. Many have erroneous views of heat and temperature (4)including: (1) heat is substantive and can be possessed, gained or lost by a n object; and (2)heat and temperature are the same thing, that is, the temperature of an object is the amount of heat it possesses. That a gas has all the regular properties of matter and can exert a force is a mystery to many school students (5). The kinetic theory of gases provides additional problems (6). Many students have trouble with the concepts that the soaces between ~, eas oarticles are entirelv " emotv and that it is thc intrinsic motion ofgas panicles that causes gases to fill their containers comoletelv and rvenlv. Misnmceotions of school students reg&dingthermodyn"amics are prevaconfusion exists in the nature lent (7):for examole.. meat of the relationships among exo- or endothermic processes, spontaneous processes, and reaction rate. The studies of freshmen a t Worcester Polytechnic Institute involved a class of mostly engineering majors with generally solid backgrounds in mathematics and science. Three five-minute, informal writings were spaced during the presentation of the material to examine the student understanding of thermodynamics and the related concents. Each assienment vielded about 150 aaners to be read. Several good papers were selected from each group and.oroiected for the entire class the next dav to show the . " students answers written by their own peers. In the first in-class writing question, the students were asked,
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What is enthalpy and what is it used for? The third question was How do you calculate AG" for a chemical reaction, and what does the value of AG" tell you about the reaction?" In each case the writing assignments were given after the relevant material had been covered in lecture. A number of conclusions were drawn from the student writings, and all of the examples given below were stated expressly on a substantial number of papers. On question one, the expanding gas, 11%of the answers were generally correct. However, many students went directly to their intuition to solve this problem, disregarding the logic that was presented in class. Thus, a CO, fire extinguisher or a refrigerator were used as proof that a n expandcnggas cools even though these systems involve the change of a liquid to a gas. The students often "locked" onto certain notions that were presented in class but not applicable to the problem at hand. For example, because isothermal conditions often are specified for ideal gas problems, a few assumed that all ideal gas processes are isothermal; therefore, the gas expansion was isothermal. A full 17% of the students assumed that a gas expanding against the atmosphere is a free expansion because they recalled the coverage of free expansions in the lecture and textbook. These students went on to predict that work was zero; thus, the temperature change was zero. The students often reverted to prior discussions to solve problems, ignoring more recently presented material. In the opening of the cylinder of gas, 12% attempted to use the ideal gas law to predict the temperature change. Some of these reasoned that the volume would increase and then applied Charles's law to predict that the temperature would increase, that is,
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Predict the effectthat opening a cylinder of compressed gas in the lecture hall would have on the temperature of the gas. The students were instructed to define the system and state any assumptions that were made in order to reach a solution to this problem. The second question was,
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Some reasoned that the temperature would decrease using a similar argument involving pressure. Aaarticular surnrise was the laree number of students ( l l % jwho failed td appreciate tbatibjects in contact, the gas cylinder and the air of the room, would reach a temperature equilibrium. Two factors probably are involved here. The notion of temperature equilibrium is counterintuitive because objects with different heat conductivities "feel" to be a t different temperatures. Thus, a block of metal at room temperature feels cold. Furthermore, prior discussion of the kinetic theory of gases, a topic found to be difficult for school students, produced additional confusion. Many students predicted a differencein the temperatures of the tank and the air because the gas a t higher pressure in the tank would undergo more collisions that would generate a higher temperature. The difficulties in understanding the distinction between heat and temperature that are exhibited by school students persisted in these college freshmen. In the first
question, 15%wrote that change in temperature is synonymous with the flow of heat. In question two, enthalpy, which had been defined in class as a state function, was explained a s being the heat contained in a system, the change of the heat of a system, or the same as heat capacity In both auestions two and three on enthalov ."and free energy change, many of the students lacked an appreciation that the A before H or G means change in enthalpy or free energy. This may he a problem that we teachers propaeate because of imnrecise terminolow. For examole. AH*.. and AH"f often are ieferred to as the &halpy of f;sion the standard enthalpy of formation. When responding to question three on AG" and what it tells you about chemical reactions, the students showed a lack of understanding of the conditions needed for this particular thermodynamic function to be useful. In particular, in using Go to predict if a reaction is spontaneous, the students did not appreciate that this function applies to a reaction where all reactants and products are at standard conditions and pressure and temperature are held constant. On 73%of the papers there was a stated recognition that AG" is somehow a predictor of reaction spontaneity. However, only 1%of the students wrote that standard conditions and constant pressure were necessary for the application of AGO. In some cases, laboratory experiments can limit rather than broaden the student outlook on a concept. In our case, an experiment on predicting precipitation reactions led many of the students to believe that AG" is useful only for precipitation. Most of the students did not appreciate what is normally calculated from what. For &le, because they perform exercises calculatingKfrom AG" as well AG" from K, many indicated that one direction of calculation is as common as the other. They missed the point that tabulated data make the calculatioh of AG" for huge numbers of re-
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actions verv easv and that K can then be calculated from AGO.
The conclusions reached by reading the in-class writings of students in thermodynamics provide evidence of the utility of this pedagogical tool in evaluating the students' level of understanding and the misconceptions they hold during the teaching process. This is a powerful means for identifying student problems and misconceptions so that they can be remedied at the time. Showing good student writings to the students during the next lecture can help clarify troublesome topics. Furthermore, the lecturer can go on to give additional coverage in the identified topics if necessary. Because the writings are informal and ungraded, inclass writing probes student knowledge in an effective, but nonthreatening, manner. Students have responded particularly favorably to swing the writings of other students ( I , . T h ~technique s can be used in large or small clas~esbut has particular utility in the large class where it promotes an interaction between the lecturer and the students and injects an alternative teaching style into the lecture environment (8). Acknowledgment The author is grateful to Edward Shane of Morningside College, Sioux City, IA, for valuable discussions. Literature Cited 1. Beall. H. J. Cham. Edue. 1991. 68, 1 6 1 4 9 . 2. Anderasan, B. Stud in Sci. Educ 1890.18.53-85. 3. Pfundt. H. Chrmico Didortico 1981,7544.
4. E ~ C ~ G.: S Onberghien& ~ , childmn% in-scknep: D ~ ~ vR.; ~T, E.; nberghien, A. Eds: Open University Press:Philsdelphis, 1985: pp 5 2 8 4 . 5. Sere, M. G. Childnn's Idpa. in Science; Driver, R.; Cuesne, E.; nbexghien, A. Eds: Open University Press: Philadelphia, 1985: pp 10&123. 6.Nusabsum, J. Childrenk I& in Science; Dtiver, R.;Guesne, E.; Tiberghien,A Eda; O p n University Press: Philadelphia, 1985; pp 124-144. 7. Johnstone, A H . : MacDonsld,J. J.: Webb, G. PhysicsEduc. 1977, 12, 2 4 M 1 . 8. Birk. J. P.:FosterJ. J. Chrm. Educ. 1993.70. 180-182.
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