By Mark W. Meszaros, PhD
Vice President, Carolina Biological Supply Company

Introduction

Transfer water between 2 aquariums and study what happens to the amount transferred and the amount in each aquarium. Will a steady state or equilibrium be reached? If so, what changes and what stays the same? This guided-inquiry teacher demonstration provides a simple introduction to the concepts of reversible reactions and equilibrium.

National Science Education Standards

Equilibrium is one of the Unifying Concepts and Processes in the National Science Education Standards for all grades.

Safety

No special safety precautions or safety equipment is needed to perform the activity.

Materials

• 2 Plastic Aquariums or Terrariums (e.g., Carolina item #670388 or #674337, or similar flat-bottomed containers)
• 2 100-mL Beakers
• 1 250- or 400-mL Beaker
• 2 100-mL Graduated Cylinders
• Water
• Food Coloring
• Paper Towels
• “Reactants” Sign
• “Products” Sign

Procedure 1—equal beakers

1. Fill 1 of the containers at least half-full of water and add some food coloring to increase its visibility. Leave the other empty.
2. Place the 2 containers alongside each other on a table or desk in the front of the classroom. Place the container with the water on the left, as viewed by the class, and place the “Reactants” sign beside it.
3. Pick 2 student volunteers and assign each to a container. Give each volunteer 1 of the 100-mL beakers.
4. Inform the 2 that each of them is to use the beaker to transfer the contents of his or her assigned container to the other’s container, according to these rules:
   1. Fill the beaker as full as possible for each transfer, but without tipping the large container.
   2. This is not a race; complete each transfer methodically and simultaneously.
5. Ask each volunteer to scoop a beaker of water from their container, show it to the class, and announce the approximate volume of water. Note: One should be the full volume and the other, zero volume.
6. Have each of them pour their beaker of water into the other’s container.
7. Ask the class to predict
   1. What will happen to the amount of water in each of the containers as the volunteers continue to make transfers between the 2?
   2. Whether the amount of water transferred between the 2 containers will ever be equal and, if yes, at what volume?
8. Have the volunteers continue to make simultaneous transfers.
9. After a few transfers, have them announce the approximate volume of water in each of their beakers. If the volume being transferred appears similar, they can use the graduated cylinders to measure the amounts precisely.
10. Ask the class if they would like to change their predictions, and if so, why?
11. Have the volunteers continue with the transfers, stopping every few transfers to measure the amount of water in their beakers.
12. Ask the class how the amount of water being transferred is changing. Eventually, each volunteer’s beaker will contain the same amount. Have them use the graduated cylinders to verify the equal volumes.
13. When equilibrium is reached, show the class that the reaction continues even though the same amount of material is being transferred back and forth.
14. Stop the demonstration and pour all the water into 1 of the containers.

Variation—starting with products only

1. This time, place the container with water on the product side and the empty container on the reactant side.
2. Select 2 new volunteers, ask the class to make predictions, and repeat the process. Eventually, each volunteer’s transfer beaker will contain the same amount of water. Although more water may remain on the “product” side, equilibrium will again be reached when the same amount of material is being transferred in each direction.
3. Stop the demonstration and pour all the water into 1 of the containers.

Procedure 2—unequal beakers

1. Select 2 new volunteers, but this time give 1 of them a larger beaker (250 or 400 mL).
2. Ask the class to make predictions again before the volunteers begin the transfer process, following the same guidelines as before.
3. Eventually, each volunteer’s transfer beaker will contain the same amount of water.
4. When this happens, a state of equilibrium has been reached. Again, show that the reaction does not stop, even though the same amount of material is being transferred in each direction.

Student misconceptions

1. Students frequently believe that equilibrium is reached when the quantity of reactants and quantity of products become equal. This is false—equilibrium is a dynamic process characterized by equal rates of transfer such that the concentration or quantity of reactants and products no longer changes.
2. Students also may think that reversible reactions occur only in 1 direction and then the other direction. In fact, both the forward and the reverse reactions are occurring simultaneously as long as reactants and products are available.
3. Students may consider “equilibrium” and “reversible” to have the same meaning. “Reversible” describes a reaction that proceeds in the forward and reverse direction. “Equilibrium” is the dynamic state in which there is no net change of reactants or products in a reversible reaction.
4. Like other models, this simulation is not perfect and may generate some new misconceptions. Reinforce the following points to avoid creating misconceptions:
   • In this simulation, the “products” and “reactants” are the same substance, but just in different containers. Have your students imagine that the water in the “Reactants” container is different from the water in the “Products” container.
   • In a real reversible reaction, the reactants and products are not separated but occur in the same vessel (and are, of course, not segregated to the right or left side).
   • The forward and reverse reactions do not “take turns.” They occur simultaneously and rapidly as soon as reactants are available for either.

Conclusion

1. Equilibrium is reached when the rate of the forward reactions equals the rate of the reverse reaction. In this analogy, the rate is symbolized by the amount of water being transferred.
2. Equilibrium is a dynamic, not a static condition. Both reactions continue to occur even though the net amount of reactants and products does not change.
3. In a reversible reaction, 1 of the 2 reactions may be the limiting reaction. When unequal-sized beakers are used, the smaller beaker is the limiting reaction. When the 2 reactions occur at the same rate and equilibrium is reached, the smaller beaker will determine the rate.
4. In a reversible reaction, equilibrium can be approached from either direction. One can start with only reactants or only products or with some of both. The equilibrium or steady state will always be the same.
5. To drive a reversible reaction to completion, the product must be removed from the closed system. To demonstrate this concept, add a third person to the demonstration. This person will remove water from the “products” container during each transfer. This will reduce the amount of products that can be converted back to reactants.