the microscale laboratory 19F-NMR(trifluarotaluene internal standard 63.13 ppm) 6 55.01(s. 3F.-CF,. 52%forSS).54.82 (s.3F. -CFs48%f0rS.R): e.e. = 4490: i~ 1C%Cloeast) 3430.2948. i747.166"16.1496.1452: GC-MS retention tim; 1259, 1169, 1122, 10"18,762,69'8 14.02 min; MS(E1) 338 (M*,4. The Mosher's ester of the optically active alcohol (8.2 mg, 0.024 mmol) is prepared in 25% yield. 'H-NMH (CDCI?)6 35fi Is. 9'71, 3.47 Is, 9141, ex. = 8234'; 1YF-lWRS55.021s,Y31 1,54.821s,7Lbl,e.e.=86?4'1;TLC,IR, GC-MS are identical to thore of the rneemic pmduct. Literature Cited 1. Lee,M.L.J. Chem. Educ l993,70,A155-A158. 2. la1 Nakamura, K,Ushlo,K:Oka,S.; 0hno.A.TktrohdmnIatt. 1984,25,39795982. lbl Bucdarelli, M.; Forni, A ; Moretti. I.; lbm, G.Synthesis 1983, 897-899. 1 4 Elie1,E.L. J hChem. . Sac.I949,71,397W3972. 3. Pouch&, C.J. TheAldrichLibroryofNMR Spaclm, 2nd ed.;Ald?~chChemicalCompany,he.: Milwaukee, 1988;Vol.1, pp 921C. 922A. 4. Yamaschi, S. In Asymmtric Syntheses, Morrison, J.D., Ed.; Aesdemie Resa, he.: New York. 1983: Vol. 1. DO 125-152.
8. Downer, E.; Kenyan,J. J. Chom. Soc. 1939, 1156.
Temperature Control for a Small-Scale Kinetics Experiment Patrick lash' Kent State University-Ashtabula Campus Ashtabula, OH 44004 This note describes a microscale version of the classic iodine clock kinetics experiment with iodide and peroxydisulfate under temperature-controlled conditions that also permits calculating the activation energy. The iodine clock experiments available in current microscale general chemistry lab books ( I , 2) do not offer a way to control temperature using microwell plates. Thompson (3)uses a styrene coffee cup to provide temperature control for a methylene blue decolorization study. Construction of the Bath One-inch, blue polystyrene insulation foam is cut into pieces 6.5 x 8 in. (for bases) and 1 in. x 1 in. strips (for edges). Sauare edges are essential. The l-in.-sauare strips are cut tdthe length and fastened to the base and each other with bathtub caulk, which seals all the seams to provide a shallow water bath that fits a 24-well plate perfectly. Four to SIX 3 8-in. holes are dnlled in the base of the 24-well olate between the wells and each edee of the d a t e is notchLd with a paper punch to allow wat& circulacon. Use of the Bath A 60-drop volume of stock ~eroxvdisulfatesolution is placed in two wells of the 24-well and a third well is filled with a stock mixture of iodide. thiosulfate. and starch indicator. The bath is preheated/~ooledby rinsing with water above or below the temperature ultimately desired.
About 200 mL of water slightly above or below the desired temperature is poured in, depending on whether the kinetic run will be above or below room temperature. The plate is submerged carefully, making sure the bath water does not rise above the rims of the wells. The plate is held submerged by placing a thermometer in well B3 and clamping it so that the thermometer forces the base of the plate down. A photograph of the apparatus is shown below. When the appropriate temperature has been reached as indicated by the thermometer in well B3, 40 drops of the iodide solution is transferred rapidly to a sample of the peroxydisulfate solution via a calibrated Pasteur pipet. The solution is stirred with a capillary melting point tube until the blue color appears. Use of a room temperature pipet to transfer the solution does not seem to affect the calculated activation energy significantly. Time versus temperature &dies with thermometers in water-filled wells shows that the maximum time for solutions to reach thermal eauilibrium is about 10 min for a bath temperature of 35 '6 and 15 min for a bath temperat u r e of 5 ~ C These . times a r e for a b a t h not precooledheated as described above. Unclamping the thermometer and .. wntlv. ~ . u s h i n ethe plate down a few times every few mlnutes encourages water c~rculat~on and faster thermal eau~libnum.The thermometer ul well H.3 seems to give a good indication of the overall bath temperature. Duplicate kinetic runs are recommended for each set of conditions. If the two times deviate significantly from the average (+/-lo%), a third trial is required. Even with some repetition of individual trials, students can finish the experiment in a three-hour period. Results The order of the reaction with respect to peroxydisulfate is determined using five room-temperature runs of variable peroxydisulfate composition. (The order with respect to iodide is given.) Initial reaction rates are calculated from the known change in concentration of iodine with time. Rate constants &r four temperatures (5 OC, 15 T, room temperature. and 35 "C) are then calculated and a graph of the log ofthe rate constant versus the reciprocal of the Kelvin temperature is plotted. The slope of the straight line yields Emt from the equation
slope = -E,&.303R The calculated activation energy of 48.5 k J (average of three classes) agrees quite well with the reported value of 51.8 k J ( 4 ) and is near the minimum error expected for drop wise measurements (5). At the suggestion of a reviewer, the experiment was re~ e a t e du s i n ~10- bv 60-mm test tubes. a 100-mLbeaker as water hat(, and total reaction volime of 2.5 mL. Duplicate runs a t five temperatures from 20 'C to 55 'C pave an activation energy 2 48.7 k J (R = 0.9816), in excellent agreement with the microwell determined value.
a
a
Literature Cited 1. Mills, J. L.: Hampton, M. D.Miemsmle Labomtory Manual fw Ceneml Chemistry; ~~
'Presented at the 199th National Meeting of the American Chemical Society, Boston, MA, April 1990;paper CHED 246. Copies of the experimental handout given to the students can be obtained by sending a SASE to the author. A66
Journal of Chemical Education
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Random House:New York, 1988. 2. Russo, T Micm Chemistry; Kemtec Educational: Korsington, MD. 1985. 3. Thompson, S. Chomtmk: Micmscois E ~ p p n m n t afor Gmernl Chomishy: Allyn and Bacon:Needham Heights, MA, 1989. 4.
Moews,P.C.:Petmi.R.H.J
Cham. E d u r 1%,41,549.
5. E d 6 James: Fickdng, Milea. J. Chem. Educ. 1991,68,A120-122.
