edited bv TOMTIPTON Unlverslty of Nebraska
learning activitie~ A Laboratory Learning Cycle: Hot Stuff Robert G. Silberman State University of New York College Cortland. NY 13045 One of the first lahoratory exercises in many general chemistry courses involves calorimetric measurements. A typical experiment might involve the determinations of AH for an acid-base reaction or the specific heat of an element. When students are carefully questioned after completing such an exneriment. it becomes clear that manv of them have only the vaguest idea of the relationship between temperature changes, heat capacity, and amounts of heat liberated or exchanged. At Cortland we have developed a series of learning cycle laboratory experiments for general chemistry.' One of these seems to he suited ideally for teaching students about calorimetric measurements. In addition, this learning cycle allows students to actively carry out and design an experiment to solve a nroblem, rather than simply . follow directions in a lab m a n ~ a l . ~ - ~ Exploration The class is divided into groups of three or four students that will work together. Each group is given the following equipment: 40°C thermometer, a balance, several styrofoam cups, two sizes of graduated cylinders, beakers, and 25-ml pipettes. (NOTE: A 40°C thermometer can be made by dipping the end of a standard thermometer into latex paint so that all of the scale above 40°C is obscured.) The students are told that styrofoam cups are very good insulators and will maintain the temperature of hot liquid fairly well. A large coffee not. filled with distilled water and maintained a t a ~
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problem: As an enerm conservation measure you have decided to turn your water heater down so that it only heats water to 55T. (NOTE: Usually a hot water heater keeps hot water between 48" and 70°C. Unfortunately, when you decide to measure the temperature of water in your hot water heater, the only thermometer you have is unmarked above 40°C.) In class is a simulated hot water heater (i.e., alarge coffee pot with hot water). Using your 40°C thermometer and any of the materials on the desk (i.e.,
a resource person who answers questions, asks questions, provides encouragement, and provides other equipment if i t is requested. After the initial puzzled and confused reaction of the class, everyone starts experimenting and trying out ideas. It takes a typical class about an hour to solve the problem and develop a general equation for the hot and cold water system. Invention This part of the experiment involves a class discussion of the hypothesis, experimental method, and results of the exploration activity. The discussion centers around the following kinds of questions: 1. What was the relationship between the volume of water you used in your determination and the final temperature you measured with your thermometer? 2. Compare your temperature measurement with that of another lab group. Explam any differences.
Llncoln. NE 68508
3. What quantities, if any, did you have to keep constant to determine the temperature of the hat water? 4. What will the final temperature he if a 25-ml sample of Hz0 at 10°C is mixed with 25.40.50.60 ml of the hat water?
one degree, how do you determine how much heat was involved in raising or lowerug the water temperature in question 4? 7. Does a large volume of water contain more heat than a small volume ofwater? Does hot water containmore heat than cold water7 The result that each group obtains seldom agrees or matches the actual tem~eratureof the hot water. These results serve as the starting point for a student-centered discussion of the exverimental method, technique, and design. iem. ~ g a & there is some variety in the approaches. Actual equations, correct and incorrect, are shown below:
+ final temp. = temp, of hot water. 2. (Vol.cold water) (temp. eold water) (Vol,hot uater) (temp hot water) = (final Vol.) (final temp.). 3. (moss hot water) AT = (mass eold water) AT Application Several applications can be used for the conclusion of this learning cycle. For example: (1) the determination of heat capacity of an element; (2) determination of AH for a reaction; (3) determination of the heat of solution; (4) determination of the coefficient of a reaction by measuring the heat evolved using the method of continuous variation. From the student's point of view, the most popular application has been designing a hot pack. Hot packs and cold packs are available in most large drug stores. They are designed to allow the emergency application of heat or cold to an injury. In class the cold pack, which consist.? of a fragile pouch of water surrounded by ammonium nitrate, is demonstrated. When the pouch of water is broken by hitting it, the water and ammonium nitrate combine and the pack gets cold. The students are asked to design a hot pack, using CaCb and HzO. It is assumed that the packagingmaterials are available, and the problem is to determine the amounts of Hz0 and CaClz to be mixed. Most of our students found this to be an interesting problem. Again, students get experience in designing an experiment and solving a problem. Student response to this learning cycle has been overwhelmingly positive. Typical comments are 1. Final temp. -temp. tap water
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1. This is the first chemistry lah I have ever understood. 2. This lah really made you think. 3. It was fun. 4. Why don't we have more labs like this?,
Acknowledgment This experiment is one of a series of learning cycle lab exercises developed with funds from N.S.F. Loci Grant No. 8000839. .- , at 2nd ' S loerman. R G.. Cognit d o Labs .n Chem s t ~ present80 Annua Conference on Reason'ng P aget, and hlgher Eoucal'on. Metropolitan State College, Denver,l981 James, H. J., J. CHEM. EDUC., 58, 477 (1981). Herron, J. D., J. CHEM. EDUC., 55, 165 (1978). Karplus, R.. "Science Curriculum Improvement Study," University of California. Berkeley, CA, 1974. Volume 59
Number 3
March 1982
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