Chemistry Everyday for Everyone
Equilibrium Principles: A Game for Students
W
Lionel J. Edmonson Jr.* and Don L. Lewis Division of Science, Coastal Bend College, 3800 Charco Road, Beeville TX 78102
A few basic ideas about chemical equilibrium can be illustrated with a laboratory exercise which we call Egame. In the exercise, dice are tossed to simulate forward and reverse reaction rates. Egame exhibits the dynamic character of equilibrium and reinforces the notion that the equilibrium state is defined for a closed system. Kinetics games illustrating reaction rates and rate constants have been previously described (1–3). Egame was developed from Castillo’s kinetics game (1). After playing Egame, students calculate an equilibrium constant and equilibrium populations by equating the forward and reverse reaction rates.1 Calculated equilibrium populations are then compared with experimental Egame results.
25
Number of cubes
Pt Products
15
10
5
0
Playing Egame
0
One member of each pair of participants in Egame is furnished with 20 sugar cubes. Each cube has a red X on one face and a blue Y on another face. The student initially given the 20 cubes is designated custodian of the reactant collection, R. The other student who initially has 0 cubes is custodian of the P collection. The cubes in the R collection are tossed, and those cubes that display either a red or a blue marked face are transferred to the P collection. After the first toss, both the R and P collections are tossed. Cubes in R that show either a red or blue marked face are set aside for transfer to the P collection. Cubes in the P collection that display a red marked face are set aside for transfer to the R collection. The transfer process is completed, and the throw number as well as the number of number of cubes in the R collection and in the P collection are recorded.2 The process continues for eight throws and the entire game is repeated for ten trials. Discussion Egame rules are summarized by eq 1. A graph of student data is shown in Figure 1. R
red or blue
P
(1)
blue only
Computer simulations for a kinetics game and for Egame were constructed. The computer simulations had much less instructional potential than that offered by a “hands-on” experience. W A report sheet for Egame is available on JCE Online at http://JChemEd.chem.wisc.edu/Journal/issues/1999/Apr/ abs502.html.
*Email:
[email protected].
502
Rt Reactants
20
2
4
6
8
10
Throw number, t
Figure 1. Outcome of Egame averaged over 10 repetitions of the game.
Calculations Equation 1 is treated as if it were an elementary reversible chemical reaction.1 At equilibrium the forward and reverse rates are assumed to be kf Req and krPeq , respectively. Equating the forward and reverse rate generates eq 2, from which an equilibrium constant and equilibrium populations can be predicted. K eq = k f /kr = (20 – Req)/R eq = Peq/(20 – Peq )
(2)
Elementary probability considerations suggest that k f = 2/6 and k r = 1/6. Thus, Keq = 2. Calculated values for the equilibrium populations are R eq= 20/3 and Peq= 40/3. Notes 1. The relationship between kinetics and equilibrium may be more complex than this for real chemical reactions. See King (4 ) for details. 2. Recording the number of product cubes serves to remind students that equilibrium is a condition of a closed system (P + R = 20).
Literature Cited 1. 2. 3. 4.
Castillo, R. Phys. Teach. 1968, 6, 467. Harsch, G. J. Chem. Educ. 1984, 61, 1039. Schultz, E. J. Chem. Educ. 1997, 74, 505. King, E. L. J. Chem. Educ. 1984, 63, 21.
Journal of Chemical Education • Vol. 76 No. 4 April 1999 • JChemEd.chem.wisc.edu
