DONNABOGNER Wichita State University Wichita, KS 67208
Simple Models For Teaching Equilibrium and Le Chatelier's Principle Joan M. Russell Prospect High School 18900 Prospect Road Saratoga, CA 95070
One of the most important challenges to the high schook chemistry teacher is helping students form visible images of invisible processes. Perhaps the best way to accomplish this task is by using appropriate physical models to introduce chemical concepts. This article will present three models that have been effective in teaching chemical equilibrium and Le Chatelier's Principle: (1) the liquid transfer model, (2) the fish model, and (3) the teeter-totter or seesaw model. The Liquid Transfer Mods1
The basic liquid transfer model was explained by Carm0dy.l In the present article this model employs two identical 100-mL graduated cylinders and two 25-cm lengths of glass tubing with different diameters. The first graduated cylinder is filled with 50 mL of colored water representing the reactants. The second graduated cylinder is left empty representing no initial products. The progress of the reaction is demonstrated by dipping tubing # 1to the bottom of cylinder # 1and dipping tubing # 2 to the bottom of cylinder # 2. The demonstrator then places a thumb over the top opening of each tube to trap the liquid in the tube. The entrapped liquid is then transferred to the opposite cylinder (see figure). This process is continued for a few minutes until the volumes in each cylinder no longer change. A state of dynamic equilibrium has now been established between the two cylinders.
happen if more liquid is added to the reactants' cylinder, it can be demonstrated that when 10 mL of liquid is added to the reactants' cylinder, part of the added liquid will transfer to the products' cylinder to reestablish equilibrium. An initial increase or decrease of liquid in either the reactants' or the products' cylinder, followed by the transfer of liquid in opposite directions to establish equilibrium, will effect the directional shift towards reactants or products proposed by Le Chatelier. The quantitative aspects of Le Chatelier's principle in respect to concentration changes can also be demonstrated by the liquid transfer model. For the sample data (see Table 1) the cross-sectional area of tube # 1 is four times as great as the cross-sectional area of tube #2. Fifty milliliters of liquid is added to cylinder #I. The transfer process is begun, and, when equilibrium is reached, 40 mL of liquid will have been transferred to cylinder #2. Students are then asked what will happen if an additional 10 mL of liquid is added to cylinder #I. The results are demonstrated by adding 10 mL of liquid to cylinder # I and then continuing the transfer process until equilibrium is again established. When equilibrium is reestablished, there will be 12 mL left in cylinder # 1and 48 mL in cylinder # 2. Students are now asked what the quantitative results will be if an additional 20 mL of liquid is added to cylinder # 2 and then this addition
The Liquid Transfer Model and the Properties of Equilibrium
While the liquid is being transferred back and forth trying to establish equilibrium, the following questions can be directed to the students: (1) Will all of the liquid from cylinder # 1be transferred to cylinder # 2? (2) How much liquid will be transferred? (3) How will you recognize equilibrium when it is reached in this model? (4) What will be equal at equilibrium? The demonstration clearly shows that the amounts of reactants and products are not equal at equilibrium; rather, the amount of liquid being transferred in opposite directions is what is equal at equilibrium. This equality can be verified by placing the liquid trapped in each tubing at equilibrium into separate 10-mL graduated cylinders and then measuring the volumes. The volumes, which represent the rates of the forward and reverse reactions, should be the same within experimental error. Thus the liquid transfer model demonstrates the dynamic nature of equilibrium, the amounts of reactants and products present at equilibrium, and the amounts of reactants and products being transferred a t equilibrium. The Liquid Transfer Model and Le Chatelier's Principle
The effect of increasing or decreasing the amounts of reactants or products can be easily demonstrated with the liquid transfer model. After asking the students what will -
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Carmody, W. R. J. Chem. Educ. 1960, 37,312.
The liquid transfer model.
Volume 65
Number I0 October 1988
Table I. The Liquid Transfer Model and Le Chatelier's Principle
Process
Total Liquid (mu
Liquid in Cylinder
Liquid in Cylinder
#I at Equilibrium (mL)
#2 at Equilibrium (mL)
Ratio #21#1
50 60 80
10 12 16
40 48 64
40110 = 4.0 48/12= 4.0 64/16= 4.0
Add 50 mL to cylinder #I Add 10 mL to cylinder #I Add 20 mL to cylinder #2
Table 2.
Process
Total Fish
Fish in Tank #I at Equilibrium
Fish in Tank #2 at Equilibrium
Ratio #2/#1
30 45 24
20 30 16
10 15 8
10120 = 0.50 15/30 = 0.50 8/16 = 0.50
Add 30 fish to tank #I Add 15 fish to tank #2 Remove 21 fish from tank #I
Table 3.
The Fish Model and Le Chatelier's Principle
The Teeter-Totter Model and Le Chatelier's Principle
Process
Total Blocks
Blocks on Side #I at Equilibrium
Blocks on Side #2 at Equilibrium
Ratio #21#1
Add 5 blocks to side #I Add 5 blocks to side #I Add 5 blocks to side #2
5 10 15
2 4 6
3 6 9
312 = 1.5 614 = 1.5 916 = 1.5
is demonstrated. This time when equilibrium is reestablished, there will be 16 mL in cylinder # I and 64 mL in cylinder #2. (Data will vary slightly from that in Table 1 due to the error introduced by the different volumes of submerged glass in the two tubes.) In this model the concentration of reactants and products is represented by the volume of liquid in the two cylinders. It thus becomes obvious to the students that when the concentration (volume) of reactants or products is changed, a shift in the reaction occurs such that the ratio of products to reactants remains constant a t equilibrium. The liquid transfer model can also be extended to Le Chatelier7sprinciple governing catalysts. The model can be altered by hypothetically or actually cutting off the tops of two polypropylene cylinders. A new pathway is thus provided for the transfer of liquids. This new path requires less time to establish equilibrium and yet results in the same amount of reactants and products at equilibrium as were present without the use of the catalyst. Fish Model
The fish model for equilibrium is well known from the . ~ this mode1 two glass CHEM Study film, E q u i l i b r i ~ m In fish bowls are connected by a glass pathway. The fish are free to move back and forth between the first fish bowl (reactants) and the second fish bowl (products). The one disadvantage of this model as presented in the film is that the fish bowls are the same size and students often draw the erroneous conclusion that the amount of reactants and products are equal at equilibrium. This misconception can be avoided by making the first tank twice as large as the second one. If this model is used after viewing the film7it is not necessary to use a real fish tank. Since students have a visual image of the model from the film, a chalkboard drawing is sufficient to develop the model. Le Chatelier7sprinciple can be discussed in terms of adding or removing fish in one tank and observing the results. Sample results are shown in Table 2. Since the liquid transfer model showed that the products were greater than the reactants at equilibrium, it is important to reverse this relationship in the fish model so that the students do not draw
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Journal of Chemical Education
the conclusion that the products are always greater than the reactants a t equilibrium. The fish model can also be used to demonstrate Le Chatelier's principle regarding the effect of a catalyst on equilibrium. To accomplish this, the pathway between the two fish tanks can be widened allowing more fish to move back and forth between the two tanks. Equilibrium will be attained sooner?but the amounts of reactants and products at equilibrium will remain the same as they were without the catalyst. The Teeter-Totter or Seesaw Model
Students are familiar with trying to balance themselves and their friends on a teeter-tooter. This model can be easily constructed by using a condenser clamp mounted on a ringstand for the fulcrum and by balancing a meter stick on the fulcrum for the teeter-tooter. Children's blocks make easy objects to balance. Any simple ratio can be established by balancing the blocks a t various distances from the fulcrum. Once the ratio is established7the effect of concentration can again be quantitatively predicted and demonstrated by adding or removing blocks from one side of the teeter-totter and then observing the shift in blocks necessary to reestablish the balance. (See Table 3.) Although this model does not illustrate the dynamic properties of chemical equilibrium in the same way as the other two models do, it does very simply show the quantitative relationship between reactants and products at equilibrium. Conclusion
When the foregoing models are used, equilibrium and Le Chatelier's principle are easily visualized and understood by beginning chemistry students. It is hoped that the students will then be able to transfer the concepts and processes seen in these physical models of equilibrium to the invisible and otherwise difficult concepts and processes encountered in chemical equilibrium.
* Equilibrium Chem. Study Film Series: Berkeley, CA, 1962.
