edited by: RON DELORENZO Middle Georgia College Cochran. Georgia 31014
Enthalpy and "Hot Whealsw-An Analogy Marcla C. Bonneau Coltland Jr.-Sr. High School Cortland, NY 13045
When studving chemical kinetics, the relations hi^ hetween acti\xtnnenergy and heat of reaction for a rhekical rrnction is usually ill~tstratedhy n graph with the potential energy of the &em shown on the Gertical axis-and the reaction coordinate on the horizontal axis. These concepts can be visualized better by using a "Hot Wheels" track and car to simulate the potential energy diagram. Although the car can he driven up the incline by hand, a "Rod Runner", a rubber-hand-driven catapult used t o shoot the car up the incline, contributes to the excitement of the demonstration. Large wooden hlocks and ring stands can he used to oron uo the track as shown in the accom~anvinz - . photogriphiFig. 1). At an initial ~ o t e n t i aenergy l (Ed, the car (the reactants) isvatapulted up the incline inan effort togoover the topand down thr other side. A discussionof rheamount ofactivation energy needed to form the activated complex is easily started if the car doesn't have enough speed to reach the top of the incline. The height of the incline can be changed to show different activation energies. Once the car attains the proper activation energy and drops to a lower potential energy (Ed,
Figure 1. Demonstration using a Hot Wheels Rod Runner.
Figure 2. Analogy for exothermic (left) and endothermic (right) reactions
486
Journal of Chemical Education
the difference in the initial and finals levelsof the car can be related to the enthaluv or heat of reartion ( A H ) of an exothermic change (Fig: 2, left). The car can be rolled in the reverse direction to illustrate an endothermic change (Fig. 2, right). I would like to acknowledge the help of Marcia Frye, David Newton, and Tom Gath for their assistance in this project.
Another Auto Analogy: Rate-Determining Steps David W. Ball Rice University P.O. B a x 1892 Houslon, TX 77251
A recently published article in this feature' described several analogies involving automobiles that were useful in the conception of certain chemical ideas. The article prompted me to descrihe an analozv that I have used successfullvin the past, an analogy to desciihe the physical meaning of"a ratedetermining step. In a reaction mechanism, the rate-determining step is usually considered to he the slowest step. Since the overall reaction can go only as fast as the slowest step, the slowest step determines the overall rate. A common line of thought for a mechanistic process is that all reaction steps that come before the rate-determining step are kept from proceeding by the slow rate-determiningstep. One can say that the steps before the rate-determining step are "backed up". It is also common to represent a reaction by using an energy-reaction coordinate plot, as shown in Figure 1. The hump in the curve represents the energy barrier for the reaction, known as the activation energy or transition state energy. The overall energy of reaction is the difference between the initial energy and the final energy, indicated on Figure 1. For the sake of this analoev, the rate-determinine st& can be thought of as occurring ai the peak of the hum; This energyrmrtion coordinate olot can he thought of as a hill, and t h e reaction steps in t h e mechanism"can he thought of as cars going up the hill. Cars going up the hill will slow down due to the slope of the hill unless one presses down on the accelerator (which is not permitted in this analogy). On a crowded road, cars will he backed up going up the hill, just like reaction steps in a mechanism are "backed up" by the rate-determining step (see Fig. 2). When the cars go over the top of the hill and start going down the hill, the speed increases and the "hacking up" disappears. This can he equated to molecules reacting in steps after the ratedetermining step, which all occur a t a faster individual rate. A chemical mechanism with a rate-determining step can
' Potts. Richard A. J. Chem. Educ. 1985, 62,579
