edited by GEORGEL. GILBERT Denison University Granville, Ohio 43023
Kinetic Hydrogen isotope Etfect SUBM~D BY
Davld A. Blnder and Robert Ellason Soulhwast Stale Unlverrny Marchall, MN 58258 CHECKEO BY
Darrell D. Axtell
The Armand Hammer Unlted World Coll%e o( fhe Amlcan P.O. Box 248
We.(
Kinetic hydrogen isotope effects are often used t o support mechanistic theories presented in beginning organic chemistry courses. In addition, the effect of isotopes on reaction kinetics may be introduced in general chemistry courses. I n neither instance is there a simple, convenient laboratory exercise to provide students with some practical experience in the reactivity differences of isotopes. The only demonstrationl, that we are aware of. to illustrate kinetic isotone effects invol\,es the use of an isotopically labeled compouid, which must he synthesized. Described herein are three simple demonstratibns or laboratory exercises that illustrate kinetic hydrogen isotope effects. I n each case, the hydrogen isotope reacts faster than the deuterium isotope. Except for deuterium oxide (heavy water), each demonstration requires only common reagents likely to be found in any chemical stockroom. Procedure
All demonstrations described below should he conducted in a hood or well-ventilated area, and the usual precautions for handlina acidic and basic solutions should he taken. Disposal o f t h e resulting solutions is the same as for any acidic or basic aqueous solutions. 1) In separate 18- X 150-mm test tubes are placed 5 ml of Hz0
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Alyea. H. N.; Dunon. F. 6. 'Tested Demonstrations in Chemistry". 6lh ed.; Journal of Chemical Education: Easton, PA. p 130.
536
Journal of Chemical Education
and 5 ml of D20. Two drops of phenolphthalein indicator are added to each test tuhe. Two equal pea-sized Lumps of sodium are selected, being careful to limit the size of the sodium lump, and washed with ether to remove any oil. The sodium lumps are simultaneouslydropped into the test tubes. Caution: The reaction of sodium with AzO is violent. The hydrogen gas released bursts into flames and a mild explosion usually occurs. Care must be taken to watch out for splattering of strong basic solution. The reaction in D90. . . while vieorous. " . is not as vident as the reaction in H20.The rodrum lump scoots about on the D10 surface without flame or explusion. The color in each tuhe turns red due to therhemical8imilarityof the reaulting alkaline solutions. 2) In separate 12- X 100-mm test tubes are placed 1g of magnesium eranules. Two milliliters of HqO are added to one test tube, and 2 ml of D20are added to thk other. To each test tube is added three drops of bromophenol blue indicator. Two drops of concentrated HzS04are added simultaneouslyto each test tube, which are each immediately mired. The solutions instantly turn yellow, and reaction of magnesium with the acid commences. The reaction intensity appears nearly equal in both tubes. Reaction in the H20 is perhaps more vigorous. Within a short time. the color in the H.0-containine" tube begins to turn blue as the acid is ronrumed. At this time, the tuhe with the D20 is still obviously yellow. Hydrogen end deuterium gas is released so care should he taken to vent or dilute the gas. 3) The results of the following experiment are somewhat less predictable than the ones heretofore mentioned; thus, the experiment requires a little more practice than the others. Equal amounts (two lumps consisting of approximately 0.25 g) of calcium carbide are placed in two 18- X 150-mm test tubes. Next, 4 ml of HzO and 4 ml of Dz0 are measured out and are added simultaneously to the test tubes containing calcium carbide. The tube with the Hz0 froths more rigorously than the tube with D20. At this point, both tubes are ignited. In the tube containing D20, soot descends and adheres to the inner surfaceof the tube. Such is not observed in the tube with HzO. The acetylene is liberated faster from the tuhe with H20 than the tube with D20. This is indicated by the larger flame protruding from the H20 tuhe. As the reaction progresses, the flame from the DzO tube becomes larger relative to the flame from the Hz0 tube. This is due to a decrease in acetylene production in the Hz0 tube because of the consumption of calcium carbide. ~
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