General References Baal" Conosion Course, Ed. NACE. Haustoto (1976). Boekria, J . O'M; Fkddy, A. K. N., Eds. Modem Electmchmistry; Plenum:New York, 197Q V d 11. Evam, Uliek R. Tho Corrosion ond Oxidation ofMefola Scisntilic Plincipler and Roctieel Applications: Arnold: Landan, 1971.
A Simple Demonstration of the Activation Enerav Concept SUBMmED BY
Romeu C. Rocha-Fllho Unlveroldade Federal de Sio Carlos Calm Postal 676 560 Carlos-SP, 13560 Brarll CHECKED BY
Mel Mosher
M l ~ o o u r Soulhem l College
Joplln, MO 64801
Activation energy is a concept that has different definitions in the various theoretical approaches to chemical kinetics, as has been clearly shown by Dacey (I). qhese different concepts of "activation energy" are related to each other hut are numerically distinct (I). Nevertheless, activation energy may always he visualized simply as an "energy" barrier that has to he surmounted for the reaction to proceed to ~roducts. Although activation energy is presented to students quite earlv. it often seems mvsterious and difficult to visualize. This; the purpose of this note is to present a simple visual demonstration that has moved useful to h e l students ~ overcome this difficulty. This demonstration is based on the high coefficient of thermal expansion of organic liquid substances. Ethanol-a = 11.2 X 10 -4 K-1 @-was chosen to he used due to its commonness and low toxicity.
Equipment and Materials A glass apparatus such as the one shown in the figure provides an analogy for the typical "energy" vs. reaction coordinate plot for a spontaneous reaction ( 3 , 4 ) . The "reagents" and "products" flasks are interconnected by acapillary tube. A capillary tube is also placed above each flask so that pressure balance is always present. Ethanol (200mL),colored with a dye for visual enhancement, and a 250-mL heaker of hot water are all that is needed for the demonstration to be carried out.
Demonstration Procedure First fill the "reagents" flask with the wlored ethanol up to a lpoint where only a small bubble of air is left below the capillarytube above it. Before continuing, the students' attention should he called to the simdarity that now exists between the demonstration setup and a c.ar's gasoline tank connected by rubber tubing to a vessel on the 'floor. Although the vessel is at a lower level of potential energy than the car's gasoline tank, the gasoline does not flow to the vessel. Why? Because there is a potential-energy barrier (the path of the rubber tubing) that has to he surmounted. Now, contmue by immersing the "reagents" flask in hot water: provision of energy. Immediately the ethanol will start to expand slowlv as its tem~eraturerises: this wav. .. the ethanol will rise in the "energy" barrier capillary (as well as in the caprllary ahuve the "reagent*" flatkj. Once the ethanol reaches the t*,p of the bwrier it will be siphoned to the "products" flask: enough energy (amvation energy) has been provided for the harrier to he surmounted. This demonstration characterizes a better analogy for activation energy than one recently reported by Hansen (5), since energy is actually provided to the "reagents". Note: If no facilities for manipulation of glass are available, the apparatus far this demonstration may be assembled using Pyrex tubing for the "reagents" and "products" flasks, small connections of capillary tubing at the top and the bottom of the flasks, and Tygon tubing for making the connection. Nevertheless, the Tygon tubing used should be of small diameter; otherwise the volume available for expansion of the ethanol may become so large as to cause the demonstration not to work.
Literature Cited 1. D s w , P. D. J Chzm. Educ. 1981.58.612. 2. Castellan, G.W. Phyrieol Chem&t~y,2nd ed.: Addlaon-Waley: Reading MA, 1971; p m 3. Ref2, p 181. 4. Atkin., P. W . Phwicol Chemktry, 2nd ed.;Freeman: San Frantiseo, 1982;~ 9 7 9 . 5. Hsnsan,R.C.J.ChemEduc. 1984.61.804, 0".
A Boiling Demonstration at Room Tampiratwe' S U B M ~ ev D
Axel Hablch Kant~mschdeRdrnlbuhl Rdrnlslrarse 54 CH-BOO1 Zurlch, Swltrerland CHECKED BY
George L. Gilbert Denlson Unlverrlty Granville, OH 43023
The apparatus used in the reaction-analagldemonstration.
The dependence of the boiling point of a liquid on the attractive forces between the particles of which it is composed can be strikingly demonstrated simply by mixing two liquids able to form an azeotropicmixture with a sufficiently low minimum boiling point. In such mixtures, the attractive forces hetween the particles of different kinds are weaker than those hetween particles of the same species. Therefore, the particles can respond more easily to the urge of entmpy and, taking up some potential energy, arrange themselves at a greater distance from each other. This is manifested macroscopically hy a significant cooling during mixing, a more than additive increase in volume. and an increased volatilitv or a lowered boiling point of the' mixture. Evaporation has, so to speak, already occurred to some degree within the Volume 65
Number 2
February 1986
157
