Introduction

What is equilibrium? What happens to the amount of reactants and products when equilibrium is reached? What if more reactants or products are added to a system already at equilibrium? In this activity, pennies will be used as reactants and products in a reversible reaction to answer these questions and learn more about the fundamental nature of equilibrium.

Concepts

• Reversible reactions • Equilibrium

• Equilibrium constant • LeChatelier's principle

Materials

Small objects, such as pennies, pop-it beads, paper clips, bingo chips, etc., 60 Beakers or other large containers, 2

Safety Precautions

Although this activity is considered nonhazardous, observe all normal laboratory safety guidelines.

Overview of the Activity

1. Each member of the group chooses a defined role: (a) reactant, (b) product, (c) monitor, and (d) recorder.

2. Obtain a counted set of 60 small pennies. These will be used to represent reactants and products in a chemical reaction.

3. Obtain two containers to hold the pennies. Label one container R, for reactants, and the other container P, for products.

4. In Parts A-D, the pennies will be moved in a series of steps between containers R and P.

5. Read the discussion questions at the end of the instructions.

Procedure

Part A. What are the properties of a system at equilibrium?

1. Place 42 pennies in container R, none in container P.

2. In each transfer round, reactant will move one-third of the pennies from container R to P, and product will move one-quarter of the pennies from container P to R. Note: In deciding how many pennies to move, round all calculations down to the nearest whole number.

3. The monitor checks the number of pennies to be moved and gives permission for the actual "reactions" to take place.

4. The recorder constructs a suitable data table and records results. The following information is needed: the initial number of pennies in R and P, the number of pennies that are moved out of each, and the final number of pennies in R and P after the pennies have "reacted."

5. In the first round, reactant counts out 14 pennies (one-third of 42) to move. Product calculates that one-quarter of zero is zero and does not move any pennies in the first round.

6. Repeat Steps 2-5 and carry out a second round of penny "reactions" in both directions between R and P. Remember that one-third of the R pennies but only one-fourth of the P pennies will react in each round.

7. Continue moving pennies back and forth until no further changes are observed in the number of reactants and products.

8. Calculate the ratio of reactants and products (P/R) and enter the result in the data table.

Part B. Does the position of equilibrium depend on the initial number of reactants?

9. Place 60 pennies in container R, none in container P.

10. Repeat the process followed in Part A to move the pennies between R and P until no further changes are observed in the number of reactants and products. Keep the fractions of pennies that react the same as in Part A: 1/3 of R, 1/4 of P.

11. Calculate the ratio of reactants and products (PIR) and enter the result in the data table.

Part C. Does the position of equilibrium depend on the starting point?

12. Place 42 pennies in container P, none in container R.

13. Repeat the process followed in Part A to move the pennies between R and P until no further changes are observed in the number of reactants and products. Keep the fractions of pennies that react the same as in Part A: 1/3 of R, 1/4 of P.

14. Calculate the ratio of reactants and products (P/R) and enter the result in the data table.

Part D. What happens when more reactants are added to a system at equilibrium?

15. Starting with the equilibrium number of pennies in R and P obtained at the end of Part C, (18 R & 24 P) add 18 extra pennies to container R.

16. Repeat the process followed in Part A to move the pennies between R and P until no further changes are observed in the number of reactants and products. Keep the fractions of pennies that react the same as in Part A: 1/3 of R, 1/4 of P.

17. Calculate the ratio of reactants and products (P/ R) and enter the result in the data table.

Data Table

Use the following table to record the results in each part of the activity.

Transfer Round*

Reactant

Product

P/R at Equilibrium

Number of Pennies (initial)

Number of Pennies Moved

Number of Pennies (final)

Number of Pennies (initial)

Number of Pennies Moved

Number of Pennies (final)

0

1

2

3

4

5

6

*A "zero" round (before any reaction begins) is included to use as a starting point when graphing the results, if desired.

Discussion Questions

1. Based on the results obtained in Part A, describe the changes observed in the number of pennies in R and P over the course of the "reaction."

2. Write a definition of equilibrium based on the answer to Question #1.

3. Compare the results obtained in Parts A and B. (a) Does the P/R ratio depend on the initial number of reactants? (b) Predict the number of pennies that would be present in containers R and P at equilibrium if you started with 100 pennies in R, none in P.

4. Compare the results obtained in Parts A and C. The PIR ratio may be called the "equilibrium constant" for the penny reactions. What does this mean?

5. Compare the results obtained in Parts A, B, and D. (a) What happened when the initial equilibrium condition was changed? (b) Predict the number of pennies that would be present in containers R and P at equilibrium if 18 extra pennies had been added to P rather than to R in Part D.

6. In this activity, the reactions between R and P appeared to stop when no further changes were observed. Do chemical reactions actually stop when this happens? Explain.

7. Chemical equilibrium is best described as a dynamic condition. What does this mean?

8. Graph the results obtained in Parts A and C. Plot the final number of pennies in containers R and P versus the transfer round. Use different colors for R and P.