Visualizing the Transition State

Introduction

Learning is child’s play—when you’re teaching with toys! Demonstrate the properties of the transition state with a colorful plastic ball that suddenly switches inside-and-out when tossed into the air with just the right amount of spin.

Concepts

• Chemical kinetics
• Activation energy
• Transition state

Background

The collision model offers a simple explanation for how fast a chemical reaction will take place—in order for a reaction to occur, reactant molecules must first collide. Not all collisions, however, will lead to products. In order for colliding molecules to form products, the collision energy must exceed a certain minimum energy, or energy threshold, called the activation energy (E_a). The role of activation energy in limiting the rate of a chemical reaction is illustrated using a reaction energy diagram (see Figure 1). In this diagram for an exothermic reaction, the activation energy corresponds to the potential energy “barrier” that the reactants must overcome before they can be transformed into products. The species at the top of the activation energy barrier is called the transition state or activated complex. This species is considered highly unstable—it lies on “top” of the energy “hill” rather than in an energy “valley” and thus can never be isolated. Some of the bonds in the reactant molecules are partially broken in the transition state, while other new bonds are being formed to make products. Use the Switch Pitch ball to model the transition state of a chemical reaction.

Materials

Safety Precautions

Although the materials used in this demonstration are considered nonhazardous, please observe all normal classroom safety guidelines. Do not throw the ball at or near anyone in the room.

Procedure

1. Bring out the Switch Pitch ball. Note the color of the ball. Lightly toss the ball in the air (just a little bit and as straight as possible). Try not to spin the ball! Note: You do NOT want the ball to change color immediately. The students should see that the ball is not rigid—its parts move.
2. Toss the ball up a little harder so that it inverts and therefore changes color. Note: This may take a couple of tosses. Say “Wow!” or “Cool!” or “Did you see that?”
3. Using a guided-inquiry format, challenge students to imagine or explain how the Switch Pitch ball can be used as a model for the reactant, product, transition state and activation energy in a reaction.

1. The initial color of the ball represents a reactant molecule (R).
2. The final color of the ball represents a product molecule (P).
3. The activation energy for the reaction is related to how high the ball must be thrown (and the minimum spin that is required) in order to change color and thus convert from R → P.
4. The transition state corresponds to when the Switch Pitch ball is in the air and fully extended or opened up—both colors are visible.

4. Pull the Switch Pitch ball apart to show the geometry of the transition state. To show that the transition state is unstable, pull the Switch Pitch ball apart until it is fully extended, and place it on the desk or tabletop. The ball will instantly collapse back to one of its colored forms (either R or P).
5. (Optional) Visit the toy manufacturer website at http://hoberman.com/fold/Switchpitch/switchpitch.htm (accessed July 2018) and “toggle” on the action of the Switch Pitch toy—the axis being controlled in this way is a model for the “reaction coordinate” or the progress of the reaction.

Teacher Tips

• The Switch Pitch ball simulates a rearrangement reaction or isomerization of a molecule. Isomers are molecules that have the same molecular formulas but different bonding arrangements of atoms. An example of an isomerization reaction is the conversion of butane (CH₃CH₂CH₂CH₃) to isobutane [(CH₃)₂CHCH₃]. Although the mechanism for a specific rearrangement reaction may not involve the direct collision of two molecules in the transition state, the collision model is still valid. Energy is transferred between molecules through collisions. Reactant molecules must possess enough kinetic energy from collisions to stretch, bend or break chemical bonds and rearrange into product molecules.
• Two Switch Pitch balls may be collided together to simulate a bimolecular reaction mechanism leading to products.

Discussion

This demonstration takes abstract concepts such as the transition state and activation energy and makes them more concrete and accessible to students. A child’s toy (The Hoberman Switch Pitch) is used as a model for what happens to a reactant as it passes through a transition state before being transformed into a product. The demonstration device illustrates that the transition state is inherently unstable. The height that the ball must be thrown is a good analogy for the potential energy barrier, that is, the activation energy for a reaction.

References

Thanks to Jeffrey E. Fieberg, Ph.D., Centre College, Danville, KY, for sharing this demonstration with Flinn Scientific.

Atkins, P. and de Paula, J., Physical Chemistry, 8th Ed., W. H. Freeman and Company: New York, 2006; pp. 887–890.

Birladeanu, L., J. Chem. Educ., 1998, 75, 603–606.