J. I. Sleinfeld Massachusetts Institute
of Technology
I A Chemistry Projects Laboratory
Cambridge, 02139
T h i s past year, we have discontinued the customary required laboratory as part of the first-year chemistly course a t M.I.T. and have instituted in its stead a n elect,ive projects laboratory. I n this article, we tal~ulatesome of the cxperiments which wc have either originated or expanded into projects for the purpose of the course. The list is not meant t o serve as a recommended curriculum for a project laboratory; indeed, there could hardly be such a list,, since the projects offcred would depend on the interests of t,he instructors, the background of the students, the available equipment, etc. Instead, this is presented as a convenient compilation of experiments which have worked out successfully for us.
accuracy comprtred. Calibration may ho carried out eithcr by using the Joule heating effect of an electric current through a resistance coil immersed in the calorimeter, or by carrying out the acid-base neutralization reaction
The project laboratory was run on a pilot scale during the Fall of 1967, with a group of 27 students, including both freshmen and upperclassmen. This past (Spring) term, about 50 students were enrolled. A steady-state throughput of 70 to 80 students per term is anticipated. Thc text by Bell (1) was used as a source of prartical information and as a basis for several of the expcriments, but was extensively supplemented with additional materials for specific projects. One particularly valuable feature was a series of small discussion sections towards the end of each semester, in which students presented the work they had done in their individual projects.
(3) Colligotive Properties of Solutions
Project Experiments
(I)
Electromotive Force o f Chemicol Reactions
The systems used in this experiment are either the hydroquinone eqnilibrium ($) or the Cd/Hg amalgam (8).I t is particularly instructive for the students to progress in making measurement^ from a multimeter to a vacuum-tube voltmeter t o n conventional o H meter. and findlv to a null-tvne notentiometer (4); this hrives home, in a Gantitative fashion, the prohlems involved in making voltage measurements on a high-impedance source. If the studenta doing the experiment have had m y course background in thermodynamica, the temperature dopendenco of these potentials may be profitably studied, with the aid of a constant-temperature bath. AGO for the Art-hvdroauinone svstem mav be indenenduntlv cheoked by an iodbmkie t&ration.-~fa suffioiently sensitive c&orimeter is available [see Project (2)], the A H " derived from the temperature dependence of K (or &-) for this reaction may also be determined independently. IZontcs Glass Co. (Vineland, N. J.) can supply amalgam cup electrodes, standard calomel electrodes, and other useful items for this experiment.
.. .
( 2 ) Colorimetry The concept of the amount of heat liberated in a. chemical reaction can bc grasped, won though all the implicntions of the concept cannot, by someone with littlc background in thermodynamics. For this reason, a calorimetry experiment makes a very suitable short project (5). Both isothermal and adiabatic calorimeters can he constructed by the student, and their relative
232
/
Journal o f Chemicol Edumfion
H*
+ OH-
=
HzO
AHOm =
kcal - 13.36 -mole
in the calorimeter. A suitable set of reactions ta study is
++
++
2H+ = MgZ* Hz(g) Mg MgO 2H' = Mg2+ Hz0 2Mg 0% = 2Mg0
(1) (2) (3) If reaction (3) can be carried out in a homh-type combustion calorimeter ( 6 ) ,the heat of formation of water can be calculated directly from the data.
+
The isoteniscopic vapor-pressure measurement apparatus described by Bell (7) is capable of 1% accuracy or better if the following simple precautions are taken -the
pinch clamps used to regulate pressurc in tho system are replaced by inexpensive Hoke valves -an accurate thermometer is used, and stem correction made -a cathetometer is used to read the mercury levels in the manometer With these precautions, the apparatus can be used to determine phase diagrams for binary solutions. This project will be meaningful only to someone who has already had some thermodynamics, a t least to the extent of Raault's Law and its deviations. An additional precaution is necessary when working with a two-component system in the isoteniscope, and that is to avoid excessive boiling of the solution. The thermometer bulb can act as a. single-plate fractional distillation column, if thia precaution is not taken. Suitable positive-deviation systems to use are water -dioxane carbon tetrachloride-ethyl alcohol water-ethyl alcohol A suitable negative-deviation system is chloroform -acetone
( 4 ) Solution Kinefics Several different systems are suggested in which reaction rates may be studicd. Each involves the running of a "clock" reaction, supplemented by direct observation of one of the fast steps by means of a simple flow technique. In this technique, the reactants are drawn from thermostatted m a d u a t d evlinders acting as reservoirs by means of an aspirator or (suitably protected) house vacuum lines. Mixing takes place in s, threeway stopcock, which leads into a metcr or two of capillary tubing. Time resolution of several seconds may be achieved. Systems that can be used include "iodine clock" (a), i.e., H202 - I- - S20a 2- - Ht. The flow version is run a t hiah H i concentration. formaldehyde-sulfite system (9, 1.0) carbonate hydrolysis (9, 11) Arrhenius-type temporature dependence can be observed in these systems, particularly the first noted.
( 5 ) Enzyme Kinetics The inversion of sucrose, catdyred by yeast invertase, has become a regular part of the introductory laboratory curriculum
(1s). A good project can be developed out of this experiment by carrying out the temperature dependence of the initial rates, and comparing the enzymatic activation energy thus obtained with that for the acid-catdyzod inversion reaction. After the student has read several background articles dealing with the "lock-and-kcy" model of enzyme activity (13), he can construct space-filling models of the substrate and product molecules using the CPX models (14) and try t o infer something about the structure a t the active site of invcrtase. Useful precautions to take include sterilization of glassware used for preparation and storage of the enzyme in an autoclrwe, preparation of fresh sucrose solution substrate each day, and storage and handling of the fructose in a dry box, because of its extreme hygroscopicity.
(6) X-Roy Crysfollogrophy X-Ray diffraction analysis is one of the more difficult topics to get across in a lecture course, but is quite readily grasped when the students actually have a piece of film to analyae for themselves (15). We supply each student with several films made on a powder camera, which can be measured on a. small film reader. Same suitable substances for analysis, representative of vmious crystal systems, are cubic tetragonal rhombohedra1 hexagonal rhombic triclinic
alkali halides (NaCL, KCI, etc.) urea calcite graphite sulfur potassium dichromate
NaCl is a simple example through which the technique can be learned. KC1 is nearly as simple, but presents the complication of a pseudo-primitive cubic lattice due t o the equivalence of I
