Materials Included In Kit

Additional Materials Required

Teacher Tips

• Make copies of the data tables and questions for each student.
• Enough nickels are provided for eight groups of students. Each procedure may be followed by two groups.
• This activity provides an excellent opportunity for graphing exercises. Graphing the data illustrates the nature of equilibrium and how equilibrium is achieved. The approach to equilibrium is rapid at first, when there is a large difference in the “concentrations” of reactants and products, and then slows down as equilibrium is achieved.
• The rates of the forward and reverse reactions at specific time intervals in the approach to equilibrium are equal to the number of nickels moved out of R and P, respectively, in each round. In Part A, the rate of the forward reaction starts out high, then decreases, and eventually levels off. The rate of reverse reaction starts out at zero, then increases, and eventually levels off. At equilibrium, the rate of the forward reaction equals the rate of the reverse reaction.
• Comparing the results of Groups 1 and 2 illustrates the difference between the position of equilibrium and the equilibrium constant. The position of equilibrium—the absolute number of objects in R and P—may be different, but the ratio of objects is always the same. There are an infinite number of possible equilibrium positions but only one value of the equilibrium constant as long as the reacting fractions (the “rate constants”) do not change. At equilibrium, the P/R ratio is equal to the rate constant ratio.
  {12674_Tips_Equation_1}
• The definition of the equilibrium constant should fall out naturally when all group activities have been completed. Illustrate that the P/R ratio is indeed an equilibrium “constant” by having different groups of students start with different numbers of nickels in the Group 1 procedure.
• What happens when the equilibrium condition is disturbed? Comparing the results in Groups 1 and 4 illustrates the action behind Le Chatelier’s Principle. Once both groups have completed the nickel reactions, they may be ready to generalize the results and even formulate the underlying explanation for Le Chatelier’s Principle.
• What happens if the rates of both the forward and reverse reactions are increased by increasing the fractions of R and P that will react in each round? Try the Group 1 procedure again, but change the reacting fractions to one-half of R, one-third of P. The P/R ratio (the equilibrium constant) changes. Increasing the reacting fractions illustrates what happens when the temperature increases. The value of the equilibrium constant depends on temperature.

Sample Data

Group 1. What are the properties of a system at equilibrium?

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

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

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

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

Answers to Questions

1. Based on the results obtained by Group 1, describe the changes observed in the number of nickels in R and P over the course of the “reaction.”

   The number of nickels in R declined from 42 to an unchanging 18; the number in P increased to an unchanging 24 nickels.

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

   Equilibrium is the state at which the concentrations of reactants and products do not change over time.

3. Compare the results obtained by Groups 1 and 2. (a) Does the P/R ratio depend on the initial number of reactants? (b) Predict the number of nickels that would be present in containers R and P at equilibrium if you started with 100 nickels in R, none in P.
   1. No, the ratio of P/R remained 1.3 for Groups 1 and 2.
   2. P + R = 100 and /R = 1.33 1.33 R = 100, R = 100/2.33, R = 43, P = 57.
4. Compare the results obtained by Groups 1 and 3. The P/R ratio may be called the “equilibrium constant” for the nickel reactions. What does this mean?

   The ratio of P/R at equilibrium is the same regardless of where the nickels are initially.

5. Compare the results obtained by Groups 1 and 2 with 4. (a) What happened when the initial equilibrium condition was changed? (b) Predict the number of nickels that would be present in containers R and P at equilibrium if 18 extra nickels had been added to P rather than to R with Group 4.
   1. The reaction adjusted to return the equilibrium ratio to 1.31.
   2. The same numbers as with Group 4, 26 in R and 34 in P.
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.

   The forward and reverse reactions continue, but at the same rate, resulting in no change in the “concentrations” of reactants or products.

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

   The forward and reverse reactions are always occurring.

8. Graph the results obtained by Groups 1 and 3. Plot the final number of nickels in containers R and P versus the transfer round. Use different colors for R and P.

   See graphs in Tips section.

Teacher Handouts

12674_Teacher1.pdf

References

This demonstration has been adapted from Flinn ChemTopic^™ Labs, Volume 15, Equilibrium; Cesa, I., Ed., Flinn Scientific: Batavia, IL, 2003.

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, plastic nickels 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
• Le Chatelier’s principle

Experiment Overview

1. Divide the class into four groups. Choose members of the group for the following defined roles: (a) reactant, (b) product, (c) monitor and (d) recorder.
2. Have each group obtain a counted set of 60 plastic nickels. These will be used to represent reactants and products in a chemical reaction.
3. Have each group obtain two containers to hold the nickels. Label one container R, for reactants, and the other container P, for products.
4. In each group activity, the nickels will be moved in a series of steps between containers R and P.
5. Make a copy of the procedure and data tables for each group. Include a copy of the data table and graph for each student.
6. After each group completes the activity, have the recorder enter their data on the appropriate data table transparency sheet.
7. Graph the results for Groups 1 and 3 on the graph transparency sheet.

Materials

Safety Precautions

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

Procedure

Group 1. What are the properties of a system at equilibrium?

1. Have the first group place 42 nickels in container R, none in container P.
2. In each transfer round, reactant will move one-third of the nickels from container R to P, and product will move one-quarter of the nickels from container P to R. Note: In deciding how many nickels to move, round all calculations down to the nearest whole number.
3. The monitor checks the number of nickels to be moved and gives permission for the actual “reactions” to take place.
4. In the first round, reactant counts out 14 nickels (one-third of 42) to move. Product calculates that one-quarter of zero is zero and does not move any nickels in the first round.
5. Repeat steps 2–5 and carry out a second round of nickel “reactions” in both directions between R and P. Remember that one-third of the R nickels but only one-fourth of the P nickels will react in each round.
6. Continue moving nickels back and forth until no further changes are observed in the number of reactants and products.
7. Calculate the ratio of reactants and products (P/R) and enter the result in the data table.

8. Place 60 nickels in container R, none in container P.
9. Repeat the process followed by Group 1 to move the nickels between R and P until no further changes are observed in the number of reactants and products. Keep the fractions of nickels that react the same as in Part A: ⅓ of R, ¼ of P.
10. Calculate the ratio of reactants and products (P/R) and enter the result in the data table.

11. Place 42 nickels in container P, none in container R.
12. Repeat the process followed by Group 1 to move the nickels between R and P until no further changes are observed in the number of reactants and products. Keep the fractions of nickels that react the same as with Group 1: ⅓ of R, ¼ of P.
13. Calculate the ratio of reactants and products (P/R) and enter the result in the data table.

14. Starting with the equilibrium number of nickels in R and P obtained at the end by Group 1, add 18 extra nickels to container R.
15. Repeat the process followed by Group 1 to move the nickels between R and P until no further changes are observed in the number of reactants and products. Keep the fractions of nickels that react the same as with Group 1: ⅓ of R, ¼ of P.
16. Calculate the ratio of reactants and products (P/R) and enter the result in the data table.

Student Worksheet PDF

12674_Student1.pdf