VOL.6, NO. 11
QUALITATIVE ANALYSISIN GENERAL CHEMISTRY
1949
THE TEACHING OF QUALITATIVE ANALYSIS IN THE GENERAL CHEMISTRY COURSE* PAUL H. FALL, HIRAM COLLEGE, HIRAM, OHIO The inclusion of elementary qualitative analysis in the latter half of a course in general chemistry is apparently beyond the "swaddling clothes" stage. However, the motives for such a course and the methods of presentation may vary widely. Certainly the main motive is not to make expert or even semi-expert analysts of the students, but it should be t o train them in the scientific method and to aid them in a very effective way in learning chemistry. For example, there is no easier way to learn about the solubilities of the chlorides of the two dozen common metals than to observe a row of twenty-four test tubes, each containing a solution of a nitrate of a metal, to which some hydrochloric acid has been added. By this method the student can easily pick out the metals (silver, lead, andmercury) whose chlorides are insoluble in water or dilute acids. It is no great tax on the brain cells to remember these three, and, by process of elimination, it is a very simple matter to learn and remember that the other twenty-one metallic chlorides must be soluble. In addition to simplification of the learning of some chemical and physical properties of a fairly large number of compounds, a course in qualitative analysis constantly forces u p o n i h ~ a applif cations of the principles of (1) equilibrium, (2) solubility product, (3) repression of ionization, (4) law of molecular concentration, etc. But how shall the course be presented? Shall we use the orthodox method that begins with some preliminary discussion of the principles just mentioned, proceeds to give a table depicting the "scheme" of qualitative analysis-with the metals listed in their respective groups+-and then follows with very specific,detailed, matter-of-fact directionsfor analyzinga "known" or "unknown?" Or shall we eliminate such slavish "cook-book" work (which such a method inevitably develops and encourages) by choosing a method of approach that leads the student to develop and to work out a general "scheme" of analysis for himself; a method which helps him to work out for himself the details involved in the separation and identification of the different metals in any of the groups? The orthodox method most assuredly enables the student to analyze a larger number of "unknowns" in the time allotted to the work. But of what real value is this accomplishment if the result has been reached by the "cook-book" route? The quality of qualitative work ought to be of more importance than the quantity. * Paper read before the Division of Chemical Education at the 77th Meeting of the A. C. S., April 29-May 3, 1929, at Columbus, Ohio.
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The writer has used the orthodox method five years and, for the last four years, the method to be described. As a result of critical observation and comparison of the two methods, enthusiasm for the following method is increasing. By oral instruction and mimeograph sheets, a review is given of equilibrium and the law of molecular cotuentration. Experiments that help to make these principles real and comprehensible are given, but not in a form such as this: "Add A to B and explain the result." Instead, some simple directions are given and then questions are asked that lead the student to discover for himself certain facts and principles. With this knowledge as a background he is asked questions that involve prophesying or determining what will happen if he does so and so. Then the solubility product principle is plainly and simply discussed, its universal application in all precipitations is emphasized (as well as the solubility of precipitates), the simple mathematics involved is explained and illustrated and some experiments involving the applications of the principles are given. Here again, after requiring the student to make some simple mathematical calculations, in which he determines the numerical value of the solubility product of a certain salt and the ion product obtained by mixing certain volumes of solutions of known concentration, he is asked to predict whether or not he should obtain a precipitate by makiig such a mixture. This adds zest and enthusiasm to the work, and does eliminate "cook-book" work. Later he is directed to try the experiment and to state whether or not his prediction has been fulfilled and to give the evidence. For the beginniig of strictly qualitative analysis, suitable racks, containing test tubes of various solutions, are placed in convenient locations for the students' inspection. The first rack contains twenty-five test tubes of solutions, each containing a metal nitrate and each labeled accord'mg to the cation present. A placard is placed on the rack labeled "Metal ions treated with HC1." The students are directed to examine, to observe which metals yield precipitates (chlorides), and to list these metals as Group I , calling them the "Hydrochloric Acid Group." They are also asked to list the color and the formula of each precipitate. A second rack labeled, "Solutions of the metals, except those found in Group I, to which H2S has been added," contains twenty-one labeled test tubes. The student is directed to observe as in the first case and to list as Group 11, "The Hydrogen Sulfide Group," the metals whose chlorides are soluble in water or dilute acids but whose sulfides are insoluble. In a similar manner the rest of the metals are classified into their respective groups, but--and this is the important thing-the student classifies the metals himself. Then he is asked to outlme a general "scheme" of analysis. i m in which question marks are used To guide him a skeleton table is given h instead of metal ions or compounds. From the group division he has just
made for himself he can fill in the proper metals or precipitating agents in place of the question marks. This method consumes considerably more time than would be required to have him gaze upon a properly prepared printed chart or "scheme" of analysis but the value of such a method and the benefits derived therefrom seem to the writer to outweigh by far the extra time consumed. Furthermore, the student realizes that certain metals fall into certain groups because of the insolubility of their chlorides or sulfides, etc., and not because some author or committee thought such a division would be a nice scheme. An instructor inspects the tables thus prepared by the student before he is permitted t o proceed with a subsequent experiment. I n our course in general chemistry a study of the metals does not precede our laboratory course in qualitative analysis but it constitutes an integral part of the qualitative work, since one purpose of the course is to aid the student in learning chemistry and also to aid him in applying what he is learning. After the metals have been classified and a general "scheme" of analysis has been worked out by the student himself, we take up individual experiments on the properties of silver and its salts, getting the metal into solution and also finding solvents for some of the salts of silver. In a similar manner mercury and lead and their salts are studied. All the while the method of giving directions for experiments to be performed and asking questions on the results obtained is such that will lead the student to see how he can make use of certain properties of a substance, or certain reagents, so that he can separate a particular metal from others and obtain a confirmatory test for it. Then follows an experiment such as this: "From what you have learned about the properties of certain salts of mercury and silver in Exps. 14 and 15, explain how you would proceed to separate a mixture of the metal ions and obtain confirmatory tests for each. Try the separation and tests by taking a mixture containing about 4 cc. of a solution of each salt (AgN03 and HgN03)." After lead is studied, the student is directed to outline a method for the separation and identification of lead, silver, and mercury in a mixture of the three. He is then required to analyze "Known No. 1" according to the method which he himself has just outlined. A special form of "known analysis" sheet is furnished him, having five vertical columns labeled, respectively, Procedure Number; Substance; Treatment; Result; Equation Expressing the Reaction. This sheet is inspected by the instructor and the student is questioned concerning the procedure and the method he would use in analyzing an "unknown" that might contain only the metals of Group I. When the instructor is satisfied that the student knows "how," "what," and "why," an "unknown" is issued him with a special form sheet for recording his operations and conclusions.
By the method of filling in the above "known analysis" sheet the student demonstrates whether or not he really understands what it is all about. Our experience of the last four years has been that a very high percentage of the students really do gain a comprehensive knowledge of "how," "what," and "why." Furthermore, the students relish the work because they realize that the methods of analyses that they outline are the result of their own labors and their own thinking and logical conclusions. They feel that it is of far more value than a "cook-book" procedure. After the "knowns" and "unknowns" for Group I have been analyzed, there follow experiments and questions on metals in subsequent groups, and in each case the student works out his own method of analysis, guided aright, of course, by reactions that will lead to the desired end and by questions that involve applications of the information thus obtained. Thus in each case the student develops his own methods for the separation and identification of the metals within each group. Naturally this method consumes much more time (hence fewer "unknowns" can be analyzed) than if the student is required to follow printed specific details of procedure for separation and identification of the metals. What evidence is there that such a method as here described is of sufficient value to warrant the time consumed? In the first place, there is no question but that the student gains a better understanding of what it is all about. And if this were all that were to be gained, the results would justify the slower method. But perhaps the best criterion is the judgment and enthusiasm of the students themselves. With only a very few exceptions the students have been very enthusiastic about the course and since it comes in the second semester it acts as a good spring tonic. More than once good students who had planned to major in some other department have decided decision reached very largely as a result of interest to major in chemistand enthusiasm that was aroused in the course of qualitative analysis as now conducted. But most important of all, this method gives the student the research spirit and makes h i feel that he too can apply the scientific method.
