LABORATORY SYNTHESIS IN GENERAL FRESHMAN CHEMISTRY HENRY F. HOLTZCLAW, JR. University of Nebraska, Lincoln, Nebraska
DURING
the last decade, an increasing amount of attention has been given to the teachmg of theoretical principles in the freshman chemistry courses. This trend has many advantages, among them a stronger basis for understanding on the part of the student. The trend has necessitated a decreased emphasis upon descriptive chemistry and upon laboratory preparations. The author feels that the pendulum should not swing so far in the direction of theory that descriptive material is practically excluded. Theory should be supplemented liberally with descriptions of interesting portions of pertinent historical information, uses and reactions of elements and compounds, and descriptions of industrial processes. In the freshman chemistry laboratories a t the University of Nebraska, the writer has found that a few experiments requiring syntheses of compounds stimulate the interest of the students remarkably and provide them with much valuable laboratory experience. Most instructors who have assigned unknowns in general chemistry or in qualitative analysis have noticed an upsurge of interest on the part of the students when their abilities are met by the challenge of the unknown. A laboratory synthesis can bring an analogous rise of interest. The student feels that he is accomplishing something and is not simply carrying. out procedures, for the sake of learning, which he assumes have been established as aids to understanding the course. The interest generated in the student is enough to justify this type of assignment. In addition, however, much valuable experience is received by the student. First, he is on his own. Even if the entire class is assigned the same synthesis, each student is under test to produce a pure product in good yield. Second, laboratory techniques are developed by necessity in order to get good results. A variety of operations and techniques are provided that do not necessarily arise in other types of assignment, ~ h i ~ the d, student gets a small look a t the field of laboratory sYnthesis, an important phase of chemical research and chemical anrodilction. ~ ~ ~ ~ ~ ~ ~ ~ ~ The author has used laboratory preparations for several classes involving a total of several hundred students in both of the first two semesters of beginning chemistry a t the University of Nebraska. Some of the classes have been for students who have had high-school chemistry, but most of the classes have been for those who have had no chemistry training prior to university. One to two weeks (six to twelve laboratory hours) have been devoted each semester to this type of assignment. ~
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Some laboratory manuals provide a limited number of preparations. The writer first assigned to his beginning classes such preparations as sodium hexanitrocobaltate (111) and hexamminenickel (11)bromide, given in a manual that has been used in these laboratories.' The methods of preparation are very similar to those in the literat~re.~. a These two preparations include such techniques as oxidation of cobalt (11) ion to cobalt (111) ion by passage of a current of air through the solution, precipitation from 95 per cent alcohol, filtration with and without suction and both a t room temperature and in an ice bath, washing the precipitated product with alcohol and ether, drying and weighing the product, and calculation of theoretical and percentage yields. The encouraging response to this initial trial led to assignments direct from the literature. Colloidal sols have been assigned, for example, providing each student with a literature reference and requiring him to look up the reference in the library to get his procedure. An easy method of making a collodion dialysis bag and its subsequent use were explained in the lecture.' Each student made a dialysis bag and dialyzed his colloids! sol as part of the assignment. Another class was assigned preparations from "Inorganic Syntheses." Each student looked up his own procedure. Potassium trioxalatoaluminate, potassium trioxalatoferrate (111), potassium trioxalatocobaltate 'HOPKING,B. S., T. MOELLER,A N D F. B. SCMRWR,JR.. "Laboratory and Classroom Exercises in General Chemistry," D. C , ~ ~ ~co.,N~~ t hyork, 1946, pp. z19-20. a BILTZ, H., AND W. BILTZ,Taboratory Methods of Inorganic Chemistry," 2nd ed., John Wiley & Sons, Inc., New York, 1928, P P 14% a W A ~G., W., "Inorganic Syntheses," McGraw-Hill Book New Co,, 1950, Val, 111, p. 191, place a funnel in an Erlenmeyer flask of appropriate size. All equipment must be clean and dry. Pour through the funnel a small amount of collodion taking care that none dries on the walls of the flask. Upend the flslik and rotate evenly so that a uniform amount of collodion sticks to the walls of the flask and no prvt of the inside of the flask is skipped. Pour the excess calladion out. Blow a stream of air into the ask through a tube to drv the collodion. To remove the membrane from the flask when iry, fill the flask w-ith distilled water, stick a glass tube down into the flask, then slit all around between the neck of the flask and the dried collodion layer to loosen the membrane. p, the water and the bag from the flask. EII the hag with distilled water to test for leaks. I t is the author's experience that the average student can make a satisfactory bag in his first or second For dialysis, place the colloids1 sal in the oallodion bag and of distil,ed water, A arrangesuspend in a ment for contimuous changing of the distilled w&er is desirable but not essential for a beginning laboratory.
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(111), and potassium trioxalatochromate (111) have been assigned in this way.' The four preparations were divided among the members of the class. For the trioxalatoaluminat,e salt the student was asked to use one-tenth of the quantities specified in the procedure; for the other three salts, one-fifth of the specified quantities. Many other syntheses from the literature are within the scope of the average freshman student. The procedures need not be difficult ones. Indeed, care should be exercised by the instructor to select preparations within the abilities of his class. The writer reauires the student to submit a reuort on
JOURNAL OF CHEMICAL EDUCATION
each synthesis, giving the name of the compound, method of preparation, equations, theoretical yield, actual yield, percentage yield, discussion, and literature references. Along with the report the student submits his product properly bottled with a label giving his name, the name of the product, weight of the bottle and product, tare weight, weight of product, and percentage yield. No claim is made as to originality in this type of assignment. The practice of giving laboratory syntheses to freshman chemistry classes, however, does not seem to be utilized to a very significant extent. Inasmuch as the author has had gratifying results in the BAILAR, J. C., Jn., A N D E. M. JONES,"Inorganic Syntheses," use of a few such assignments each semester, he recommends the method as a valuable teaching device. McGrttw-Hill Book Co., Inc., New York, 1939, Val. I , pp. 35-8.
