In the Classroom
tested demonstrations
A Modified Demonstration of the Activation Energy Concept Submitted by:
Jin Li, Le-Sui Dai, and Li-Shu You Chemistry Department, Hunan Medical University, Changsha, 410078 People’s Republic of China
Checked by:
Arnold George Clarion University, Clarion, PA 16993
Methods for demonstrating the concept of the activation energy have been reported (1, 2). These show the exothermic reaction only, and the equipment used is not available readily in many chemistry laboratories (2). This paper describes a modified method that can be used to present both the exothermic and the endothermic cases. The apparatus is simple and easy to handle. The concept of the activation energy is clearly illustrated.
Materials The materials required for the demonstrations are 150 mL of ethanol colored with NaOH and phenolphthalein, 100 mL of 6 mol/L ammonia colored with phenolphthalein, and a large beaker of hot water. The equipment needed is set up as shown in Figures 1 and 2. A and B are two 100-mL round-bottom flasks. A represents the “reagents” flask and B the “products” flask. C, D, and H are rubber stoppers, two 2-hole and one 1-hole. E, F, and
Figure 1. Apparatus used to demonstrate an exothermic reaction.
Figure 3. Activation energy for an exothermic reaction.
Figure 2. Apparatus used to demonstrate an endothermic reaction.
Figure 4. Activation energy for an endothermic reaction.
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Journal of Chemical Education • Vol. 73 No. 10 October 1996
In the Classroom
G are three pieces of glass tubing of 2 mm internal bore, two straight pieces and one U-shaped piece. Procedure
The Exothermic Case The arrangement of the apparatus shown in Figure 1 mimics the typical energy vs. reaction coordinate plot for an exothermic reaction (see Fig. 3) (3). A comparison of the apparatus with that found in the literature (2) indicates that it can be assembled in any chemistry laboratory without the need for special facilities. The principles and procedures of this demonstration are similar to those reported (2). The Endothermic Case The apparatus used to demonstrate the endothermic
reaction is shown in Figure 2, and Figure 4 (4) shows the usual reaction coordinate plot. Fill the reagents flask (A) with 100 mL of ammonia colored with phenolphthalein and stopper it as shown in the figure. Immerse the reagents flask in hot water to provide energy. As the partial pressure of NH3 increases, the liquid will rise along the tubing (G) until the energy barrier is surmounted, and the liquid will flow into the products flask (B). When heating is stopped, the liquid flow will stop too. Literature Cited 1. Hansen, R. C. J. Chem. Educ. 1984, 61, 804. 2. Rocha-Filho, R. C. J. Chem. Educ. 1988, 65, 157. 3. Masterton, W. L.; Slowinski, E. J.; Stanitski, C. L. Chemical Principles, 6th ed.; Saunders: Philadelphia, 1985; p 494. 4. Chen, B. T.; Shi, J. E.; Mei, R. L., et al. Inorganic Chemistry [Chinese]; People’s Education: Beijing, 1981; p 129.
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