ChemPrep was developed to be a stand-alone preparatory short-course to help students succeed in general chemistry. It is Web-based and delivered using the OWL system. Students reported that the ChemPrep materials (short information pages, parameterized questions with detailed feedback, tutorials, and answers to questions through the OWL message system) permitted them to work independently without the need for textbook or lecture. On average, students who completed ChemPrep had higher grades in the subsequent GenChem, Nursing, and Honors chemistry courses, with a greater percentage achieving a grade of C- or higher. Participation in ChemPrep was voluntary, and more women than men responded. Students in the Honors course enrolled in ChemPrep in higher percentages than students in GenChem and Nursing. SAT and departmental math placement exam scores were used as proxy measures of prior achievement and ability. Based on these, Honors chemistry ChemPrep users were on par with their peers but performed better in the course than non-users. In GenChem and Nursing chemistry courses, ChemPrep helped students of high prior achievement and ability perform better than their achievement scores would predict. Weaker or less motivated students did not respond to the voluntary offerings of ChemPrep in the same numbers as stronger or more motivated students, and we are seeking alternate ways to reach this population.

The kinetic hydrogen isotope effect, the difference observed in the rate constant of a reaction due to isotopic substitution of a hydrogen atom involved in the rate-determining step of a reaction, is mentioned in many beginning organic chemistry textbooks. The effect has been used as supporting evidence for E-2 elimination (la, 2, 3, 4a, Sa), nitration of benzene (lb, 4b,5b, 61, and oxidation of alcohols by chromic acid (7). However, while textbooks are using kinetic hydrogen isotope effects to arrive a t certain mechanisms, an examination of current beginning organic chemistry laboratory texts reveals a stark paucity of experiments dealing with the subject. In fact, a review of eight laboratory manuals yielded not one experiment using kinetic isotope effects. This lack may be due, in part, to the difficulty in preparing isotopically labeled reactants. The experiment presented here is designed to allay some of these difficulties. The isotopically labeled reactant, deutrochloric acid, is simple to prepare, and it and the other isotopically labeled compound, deuterium oxide, are relatively inexpensive. Deuterium oxide costs about $0.50/g and denterochloric acid (37 wt%) can be purchased for about $l.OO/g. The same concentration of deuterochloric acid can be prepared for less than $0.85/g.Vinyl ethers are hydrolyzed in aqueous solutions to either aldehydes or ketones and to alcohols. The reaction is catalyzed by acids, and the hydrolysis mechanism involves ratedetermining proton transfer to carbon (8,9) as the first step. For example, the mechanism for ethyl vinyl ether hydrolysis catalyzed by hydronium ion is H,C=CHOC,H~ + H~O + -H~C-C+HOC~H~ (1)The carbocation intermediate (CH&+H-OCHzCHJ is the same intermediate as that postulated for the acid-catalyzed hydrolysis of acetals and ketals.The experimentally observed kinetic hydrogen isotope effect, k~l k~. for a mechanism involving rate-determining proton transfer from hydronium ion, is a product of two terms (10) where the first term, h"Vho', represents the effect on the reaction rate due to isotopic substitution of a hydrogen atom involved in actual bond making or bond breakingin the ratedetermining step (in this case the proton being transferred from H30f). This is called the primary isotope effect. The second term, h # / k~I~, represents effects due to snbstitntious of anv other hvdroaen atoms (in this case the two . . remainingon the hy~lronnimion~ that also influence the rate of rmrtion. This is known as t h~ secundarv isoro~e effect.These isotope effects can be predicted the ore ti^&^ using the simplified approach of Bell (11) and Kreevoy (12). The maximum primary effect is calculated as 10.6 (II), and the minimum secondary effect as 0.48 (10). The maximum observed isotope effect, h~l h~, that can be expected, therefore, is 5.1 (eq 3). In practice values lower than this are generally observed-being around 3.In the present experiment ethyl vinyl ether, EVE, is the substrate, and the hydrolysis produets are acetaldehyde and ethanol. The rate of the hydrolysis re...

A discussion of pure substances at their critical m i n t is an imoortant oart of anv ohvsieal degrees of freedom are there a t the critical point?) without being able to see a substance change from liquid + vapor at equilibrium to gas above the critical point.We have constructed a cell that is safe to use from a 10-in. length of horosilicate elass that is " used for sight gauges in s c a m twilert.The tube har at) i d d 3 11 rnm and anu.d. of 13Smm. It issralcd at une end and flared at the uthcr Irefore careful annealing. A mechanical seal using a spring-loaded Teflon plunger is able to withstand COl'sP,of 72.9 atm at its T, of 31.8 ' C and d, of 0.46 g/cm3. The cell itself fits inside a clear Plexiglas jacket through which lukewarmair may be passed to warm thecell to its critical mint. The two thumb screws that hold the jacket in plnre are of such n length nnd so puiitic.nt.d that the cell may be placed safely at an angle on an overhead projector, allowing it to be viewed by an entire class. The cell and jacket are available from RB for $125 along with a set of operating instructions. The figure shows the details of construction of the apparatus; all dimensions are in inches.The cell is easily prepared for use. First it is filled with powdered dry ice. As the dry ice sublimes it purges the cell of air while cooling it. The top is then screwed tightly down and the cell placed in its jacket. Almost immediately the pressure builds to 5 atm where liquid COI forms (the m.p. is -56.6 "Cat 5.1 atm). Continued warmingin ambient air brings the cell to room temperature where a variety of methods may be used to warm the cellslowly and safely to the critical point. Near the critical point the liquid begins to get cloudy-critical opalescence, which occurs when there are long density fluctuations in the sampleand leads to muchlight scattering. As the density of the twofluidphasesapproach eachother, sheeting within the vapor is seen, muehlike wine running down thesideof agoblet, and then with further warming the meniscusdisappears. The viewing is even more dramatic upon cooling from above the critical point: sheeting is seen, then cloudiness, and then the meniscus reamears. The effect is almost ehostlv.. . .. .'l'hr npparntw arscribrd ran produce a rather dramatic demonutratim fur any sire class and makea real the existence of this wnrrimrc rlu;ivr critical point. Thr recent article by Halpcrn and Lin' describes an improved procedure for itudy~nr! the liquid-vapur phase dingram of C02 near its crit:cal temperature.' Halpern, A. M.; Lin, Min-Fa.

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