the microscale laboratory the piston is withdrawn slowly to prevent the manometer water from being forced out. Once the reaction is completed (about 5 min) the apparatus is allowed to come to room temperature before the fmal volume is recorded. The temperature and atmospheric pressure also are recorded. The partial pressure of hydrogen gas is determined using the expression from the ideal gas law equation, PV = nRT, the moles of hydrogen gas may be calculated and this value compared with the moles of magnesium used. Some typical results: 3.3 x lo-' mol Mg produced 3.5 x 104 mol hydrogen gas. B. Effect of Acid Concentrationon Readion Rate with Magnesium
The above experiment may be modified by measuring the volume of hydrogen produced every 30 s and graphing volume versus time to determine the initial reaction rate. The reaction may be repeated using different concentrations of acid to determine bow concentration of one reagent affects reaction rate. C. Rate of Decomposition of Hydrogen Peroxide l b a 25- or 50-mL Erlenmeyer flask the required volumes of 3% hydrogen peroxide, 0.1 M potassium iodide solution, and water are added (Fig. 2). To achieve steady evolution of oxygen gas, the reaction mixture must be stirred at a constant rate; if a magnetic stirrer is not available, excellent results may be achieved by adding a glass marble or several glass beads to the flask and swirling by hand. After 1 min, the stopper assembly is attached, the manometer level is adjusted, the initial volume is recorded, and thereafter the volume is recorded every 30 s for 10 minor until 10 mL of gas have evolved. The experiment is repeated using other volumes of hydrogen peroxide and potassium iodide. After graphing the volume of oxygen pmduced versus time, the rate for each reaction may he calculated from the s l o ~ of e each m a ~ hBased . on the effect of concentration on ratk, the rate'iaw for the reaction may be determined. The results for the three runs using manual swirling with a marble are shown in the graph (Fig. 2).
0. Energy of Activation for the Decomposition of Hydrogen Peroxide
The above reaction may be repeated at different temperatures (e.g., 10 'C, 30 "C, 40 "C, as well as room temperature) using the first mixture (Fig. 2). Because the reaction times are relatively short, a simple water bath (such as a beaker or margarine tub) may be used; crushed ice or hot water may be added as necessary to maintain the desired temperature. From the graphs of volume of oxygen evolved versus time, the reaction rate at each temperature may be obtained. An Arrhenius plot (Idrate) versus 1lT) may be used to calculate the activation energy for this reaction (E., = -slope x R). An activation energy of about 52 kJ mol-' was obtained for this reaction. The data from parts C and D may be analyzed conveniently using a spread sheet program to graph the results and a regression analysis to determine the rate based on the slope of each line. A102
Journal of Chemical Education
Organic QualitativeAnalysis at the Microscale Level Rhoda E. R. ~ r a i gand ' Kurt K. ~ a u f m a n ~ Kalamazoo College Kalamazoo, MI 49006
Since 1985 organic qualitative analysis using mimscale techniques has been successllly adapted to the sophomore-level laboratory course. This third-quarter course follows two quarters of techniques/synthesislaboratoryexperience with quantitative work at both the semimicro and microscale levels. This work includes hands-on experience with IR,W, NMR,and MS, as well as techniques such as gas, thin-layer, and column (gravity and flash) chromatography. Because students have acquired the essential experimental background beforehand, the organic qualitative analysis course can be operated as an open laboratory, allowing individual students to work independently at their own pace to meet submission deadlines. This environment more closely resembles a research laboratory atmosphere and has been well-received. The course enrollment has varied between 15 and 35 students. Two Different Approaches To Identifying Unknowns Two different approaches, both of which require each student to identify six unknowns during the ten-week quarter, have been used alternate years with equal success. The first approach requires the student to identify two simple unknowns (one liquid and one solid) and one four-component mixture (mixture 1)comprised of one acid, one base, and two neutrals, all ether-soluble. The second approach calls for the identification of one simple unknown (either a liquid or a solid) and two mixtures. Mixture 2 consisting of an acid, a base, and a neutral compound, and mixture 3 is made UD of two neutral bonstituents. Mixtures 1 and 2 are composed of a high boilina liauid b200 OCJ. a low meltine solid (c75 "C). and one or'twdhigh melting solids (>85 Both water-s&ble and water-insoluble compounds are included in the mixtures, and the components may be either solids or liquids. Table 1provides a partial listing of typical four-component mixtures. The neutral components of mixture 3 may be any combination of solids and liquids (boiling points s150 'C) that have R, difference great enough to allow separation by flash chromatography No duplication of compounds in either the simple unknowns or the mixtures occurs throughout the course, so students must rely on their own ability to purify compounds and to obtain and interpret spectra correctly. Compounds of more complex structure, generally polyfunctional, are assigned as the simple unknowns. ~~
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Acid and Base Components
The acid and base components of mixtures 1and 2 are isolated by standard extraction procedures for strong1 weak, water-soluble/water-insolubleacids and bases including back-extraction techniques to obtain the maxiPresented before the Micmxale Organic Laboratory Session at the 9th Biennial conference on Chemical Education, July 29, 1986. bozeman. MT. Presentation MC03. 'Author to whom inquiries should be addressed. 2ProfessorEmeritus. Kalamazoo College, Kalamazoo, MI 49006.
mum yield of water-soluble components ( I ) . (Students are graded on the isolated yields of each component.) Students are advised to work out the methodology on one-half of their sample to assess where difficulties in separation might arise. For the original mixture, accurately measured (200-mgsolid, 500-pL liquid) quantities of unknowns are dissolved in 2-3 mL ether. The Neutral Components
The neutral components of mixtures 1and 3 are separated and purified bv flash chrumatoera~hv(2).An appropriate solvent s&;n ?or flash chromatomaohv - . "is baaed on TLC results. Using TLC methodology, students independently must develop their own solvent system providingan Rf difference greater than 0.2 between the two neutral components. A number of st& solvent svstems. such as petroleum ethenethyi acetate, cyclohexandacetone, chlorofonn/methanol, of varying ratios are available in the laboratory for initial TU: testing. When students have found a suitable solvent system, the solvent-systemratio can be varied slightly to maximize the Rf separation of the two components. Geuerallv no further ~ u r i f ~ c a tion of the neutral components is reauired if the flash chromatoeraohv is -fully implemented. -
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Purified Simple Unknowns
Any isolated crude acid, base, or neutral component, if impure, must be purified prior to identification as a simple unknown. Liquids are satisfactorily purified by column chromatography, whereas solids are purified by recrystallization, sublimation, column/flash chromatography, or preparative TLC. For recrystallization, appropriate recrystallization solvents are determined by the Craig spot-plate technique (3). Purified simple unknowns then undergo ignition/Beilstein tests and physical constant measurements. For liquids, triplicate reproducible ultramicro boiling points, refractive index, and density determination using a calibrated Clemo-McQuillen pycnometer are required. For solids, duplicate (1-2 "C range) reproducible melting points and density determinations via the flotation method are included in student reports (4). Volume 72 Number 5 May 1995
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