MORE EFFECTIVE INDIVIDUAL LABORATORY INSTRUCTION IN GENERAL INORGANIC CHEMISTRY 111. THE USE OF AN ELASTIC TASK* W. E. BRADT,Tm STATE COLLEGE OP WASHINGTON. PULLMAN, WASHINGTON, AND R. D. G E X ~ EUtnvsnsrn . OF CINCINNATI, CINCINNATI, OHIO
The use of a n elastic task, i n general inorganic chemistry laborntory instruction for students with varying prenrious chemical training, i s described. The task presented i n each experiment attempts to challer ge the ability of the better student while, at the same time, permitting the weak but conscientious student to accomplish a n acceptable minimum requirement. Two typical experiments are rmimed and their quantitative features are described. B y means of these quantitative features, comparisons are made between the students who had, and who had not, taken chemistry courses in hizh school. The results conridered show no significant differences between these two groups of students. I t i s concluded that the elastic task i s a desirable instructional feature .for classes containing students of varying previous chemical training.
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The senior author has developed a method for teaching general inorganic chemistry laboratory in order to permit efficient individual laboratory instruction of students with varying previous chemical experience. One important phase of this method is the presentation to the student of an elastic task which will challenge the ability of the better student while presenting an acceptable minimum which is within the reach of the weak hut conscientious student. This and other outstandinp features,** together with the instructional procedure of this method, have been discussed in the first paper (1) of this series. In the second paper (2) the application of this method is presented by means of specific illustrations taken from a typical experiment. The purpose of this paper is to demonstrate that students who have had no previous chemical training can work in the same laboratory sections with students who have had high-school courses in chemistry without being handicapped by this situation. The elastic task consists of an assignment which will offer to the more advanced students a field for indefinitely thorough study while permitting the beginning students to study the same subject matter in a more elementary way. In order to permit this, the subject matter of the course deals with the more fundamental phases of inorganic chemistry. Some of the experiments will be encountered later, in advanced chemistry courses. * Presented before the Division of Chemical Education of the A. C. S. at the Indianapolis meeting, March 30-April 3, 1931. **Other outstanding features are: (a) the required use of several references in the laboratory, ( b ) systematic reading in popular chemistty, (c) the length of the experiments, each requiring from twelve to eighteen laboratory hours, ( d ) written quizzes, (e) more instructional time for assistants and full instructional time for the instructor. 1574
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The use of the study topics, the topics covered, and the required references makes possible the performance of the assignment in a manner limited only by the student's ability. Students having had no previous chemistry are asked to study only a specified pair of the required references. Other students are expected to read more than this and are encouraged to read in the more advanced texts. As a result of this requirement, some students will study each item of the topics covered and the study topics in advanced and elementary references, while those who have had no previous chemistry will refer only to the two assigned elementary references. It will be seen that this task is, therefore, very elastic. Beginning students are not discouraged by their apparent failure to use all references because they are not expected to do so. More advanced students cover the same material, but in a manner limited only by their own capacity. Experiments I11 and I V of the laboratory manual will be briefly presented, followed by a comparison of the achievements of those students who had had no previous chemistry with the achievements of those who had had earlier training in the subject. These two experiments were chosen because the writers anticipated that the greatest difference between the two groups of students would become evident a t that time. Content of "Experiment 111" The general content of the first of these experiments, Experiment 111, is shown by the following outline: Title: The ap@oximate determination qf atomic weights by thc use of thc law of Dulong and Petit. Object: An illustration of the law of Dulong and Petit and the determination of the atomic weight of a solid element. Topics Covered: Atoms. Molecules. Atomic weights. Atomic theory. Law of Dulong and Petit. Properties of solids. Crystal structure. Gram atomic weight. Specific heat. Use of the metric system. Use of logarithms. Graphic representation. Use of balances. References: "Optional" or semi-popular library references and "required" references, which are available in the laboratory. The use of these references has been fully discussed in previous papers (I), (Z), and constitutes one of the phases of the elastic task. Students, who have had no previous chemistry, study only specified references, while other students are urged to study topics more thoroughly in additional texts. Apparatus and Materials: A list. Procedure: 1. The dclermination of the atomic weight of copper. (a) Theoretical discussion by the student preceding his execution of the experimental work. (6) Experimental determination of the specific heat of copper. (c) Calculation of the atomic weight of copper, using 6.4 as its atomic heat. ( d ) Recalculation of the atomic heat of copper by using its correct atomic weight and the determined specific heat. 2. The determination o j the atomic weight of an "unknown" metal according to tkc above Wocedure.
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3. The preparation of time-kperature curves from expnimen!ul resdts. Exercises: Ten problems to be solved outside of laboratory hours and handed to the "Record Assistant" on a specified date. Study Topics: Fifty topics taken from the "Topics Covered," the "Procedure," and a specified pair of the "Required References." These are listed in topic form to further student studv prior to auizzinn. .. - This also constitutes an im~ortantphase of the elastic task. Notes: Written in report form and handed to the "Record Assistant" prior to each quiz. The notes include ell quantitative data, such as temperature readings. observed weights and volumes, calculations, etc. Quiz: A written examination (thirty minutes) of a t least five questions based on the "Procedure. Study Topics, Exercises," andtbe twospecified"Required References." Record: The following record is kept by the "Record Assistant" for each student. He also records all data reported by students and all "unknowns" issued.
1. Grade (%) on reported atomic weight of copper, based on a comparison with the correct value. 2. Grade (yo)on reported atomic weight of copper based on the accuracy of the calculation from the reported specific heat. 3. Grade (%) on reported atomic weight of "unknown." 4. Grade (% X 2) on plotted data. 5. Grade (% X 4) on notes. 6. Grade (%) on exercises. 7. Grade (% X 5) on written quiz. 8. Grade (%) on laboratory application and technic. 9. Total (%) points on "Experiment 111." (1600 possible.) 10. Average grade (0/,) far "Experiment 111."
This experiment, including the written notes, the thirty-minute quiz, and the written exercises, required five laboratory periods of three hours each for its completion. The laboratory notes are compiled by the student during laboratory hours as he performs the experimental work, and a "Progress" check, obtained from an assistant a t the end of each period, marks the daily progress. The exercises represent the only work required outside of laboratory hours. The required use of reference books in the laboratory prevents the more superficial student from skimming through the experiment. The rapid workers are encouraged to study the more advanced phases of the experiment in advanced texts. Quantitative Aspects of "Experiment 111" One of the features of this method of instruction is the emphasis placed upon the quantitative technic. In Experiment I11 there are five quantitative values which the student determines experimentally. (1) Specific heat of copper. (2) Atomic weight of copper (assuming its atomic heat to be 6.4). (3) Calculation of Dulong and Petit's constant from the specific heat of copper. (4) Specific heat of an "unknown" metal. (5) Atomic weight of the "unknown" metal. An average of student values (determined experimentally for copper
by the use of Dulong and Petit's law) shows 72 to he the atomic weight of copper. This high value is caused by errors due to the simplicity of the apparatus and could have been offset by using a higher value for the Dulong and Petit constant. An error of this type cannot justly he considered a measure of the students' ability. I t was found that 44.4% of Group A (36 students who had had no previous chemical experience) obtained values for the atomic weight of copper between 68 and 74 and that 66.0% of them obtained values between 64 and 74. Further, 35.7% of Group B (137 students who had had a previous secondary course in chemistry) ohtained values between 68 and 74, and 61.3% obtained atomic weight values for copper between 64 and 74. These data show that the results of Group A are slightly superior to those of Group B. Consequently we can conclude, a t least, that this quantitative requirement did not work a hardship upon the students who had had no chemistry course in high school. The average atomic weight values obtained by the entire class on a series of "unknown" samples show an average difference between two successive years of only 1.8yo. Since these "unknowns" were actually alloys instead of pure metals, there was no possibility for the student to guess the correct value. This demonstrates that a satisfactory degree of accuracy is possible. Seventy-nine per cent ofthe 1929-30 students in this chemistry course had had a previous course in secondary school chemistry. An average grade was computed from the average per cent grades on Experiment 111 for the students who had had no previous chemistry course in high school. A like grade was determined for the students who had had a previous highschool course. This average for the %on-high-school" group was 76.6y0, while that of the latter group was 76.8y0, showing a non-significant difference in favor of the group which had had a previous course in chemistry. These grades are based upon eight items, including notes, preparation of graphs, solutions of problems, successful observance of quantitative data, and the writing of answers for the questions of a well-balanced and comprehensive quiz. Consequently, the writers feel that if the students of Group A (no high-school chemistry course) were deficient after the completion of the experiment, these grades would demonstrate this fact. Consequently, one is forced to conclude either that the tasks in Experiment 111have enabled the students in Group A to overcome their handicap due to the lack of previous chemical experience, or that these tasks have not presented an opportunity for the students of Group B to take advantage of their previous secondary chemistry course. This will he discussed more fully later. At all events, the fact is obvious that the students with no previous chemistry have equalled the grades of the students who had had previous chemical experience.
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Content of "Experiment IV" The general content of Experiment I V is shown by the following outline: Title: Combining proportions by weight and the accurate determination of atomic weights. Object: An investigation of chemical weight relations and the experimental derivation of atomic weights. Topics Covered: Conservation of mass. Law of definite proportions, Combining proportions. Atoms. Molecules. Atomic weights. Dulong and Petit's Law. Symbds. Formulas. Derivation of formulas. Use of beam balance. Quantitative synthesis of compounds. Per cent composition. References: "Optional" and "Required." Apparatus and Materials: A list. Procedure: 1. Combining proportions by weight. (a) The quantitative preparation of the sulfide of a known metal (copper). (6) Calculation of the percentage composition of the known sulfide. (c) The quantitative preparation of the sulfide of an unknown metal, "M." ( d ) Calculation of the percentage composition of the unknown sulfide. (e) The quantitative preparation of a known (copper) oxide by heating the metal in air. V) The preparation of the oxide of an unknown metal "N" by means of nitric acid. 2 . Atomic weights. (a) The calculation of the atomic weight of copper, sulfur, "M," and "N." (The value of the specific heat of each particular unknown is obtained from the Record Assistant.) (b) The calculation of the formula for each compound prepared. Exercises: Ten problems. Study Topics: A list of review topics. Notes: In report form. Quiz: Thirty minutes. Written. Record: The following record is kept for "Experiment IV" by the Record Assistant. He also records all data reported by students and all "unknowns" issued t o students. 1. Grade (%) based on per cent of copper in cuprous sulfide. 2. Grade (%) based on the per cent of "M" in the compound formed. 3. Grade (%) based on the per cent of copper found in copper oxide. 4. Grade (%) based on per cent of " N in the compound formed. 5, 6, 7, 8. Grades (%) based, respectively, on reported atomic weights of Cu, S, ',M" and "N," 9. Grade (% X 4) on notes. 10. Grade (yo) on exercises. 11. Grade (% X 5) on thirty-minute written quiz. 12. Grade (%) on laboratory application and technic (obtained from the Floor Assistants). 13. Total number of per cent points. (Maximum of 2000 points.) 14. Average (%) grade on "Experiment IV."
The time required for the completion of this experiment was six laboratory periods of three hours each. During this time all notes were prepared, required references were studied, the study topics were reviewed and the quiz was written.
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Quantitative Aspects of "Experiment IVY' In "Experiment IV" the student determines four quantitative values. (1) The per cent Cu in Cuts. (2) The per cent of metal in an unknown sulfide. (3) The per cent Cu in CuO. (4) The per cent metal in an unknown oxide. From these data and the assumption that the atomic weight of oxygen is sixteen, a possible atomic weight of copper is calculated. This is checked against the value obtained in Experiment I11 from the determined specific heat value. I n the same way a value for the atomic weight of sulfur is calculated. That for the two unknown metals is calculated from the student's data and checked against specific heat values which are obtained from the "Record Assistant." Unknown metals used for the quantitative preparation of oxides and sulfides are Cu, Ni,P b , Sn, Fe, and A1 (oxide only). In the quantitative preparation of cuprous sulfide, the average value as determined by 125 students of Group B (those students having had previous chemistry courses) for the per cent of copper in their CuzSwas 76.91, showing an error of -2.92%. The average value reported by 38 students of Group A (no previous chemistry) was 77.79% Cu, showing an error of -2.04yo. It will be seen that in this instance the difference, although not significant, favors Group A. A similar conclusion is made obvious by a consideration of the results obtained in the quantitative preparation of cupric oxide. In this case, 34 Group A students reported with an average error of 2.70y0, while 122 Group B students made an average error of 2.94y0. Calculation of an average grade from the "average per cent grade on "Experiment IV" for 31 students of Group A (no previous chemistry) who completed the experiment gave a value of 73.7%. A similar grade for the 118 students of Group B who completed this experiment was 72.5%. It was thought possible that a distorted conclusion was possible because only students who completed "Experiment IV" were considered. Consequently similar grades were calculated for all students who started the experiment and who failed to complete it. The average grade for 36 students of Group A was 70.0%, while that for 125 students of Group B was 71.4%. This again is a non-significant difference. Further, since several of the withdrawals were due to extra-scholastic causes, this last comparison is considered to be less reliable than the one first listed. In the early part of this paper it was suggested that the tasks of "Experiment 111" were not designed to enable students to take advantage of previous chemical experience. This can hardly be said to be the case in "Experiment IV." The quantitative syntheses, the theoretical bases for the calculations, and the calculations themselves are difficult. A student with previous training in chemistry will he forced t o utilize it as much as possible.
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Conclusions
1. The authors have demonstrated that, in each item considered, the students who have taken previous secondaly courses in chemistry have failed to show better technic or grades than the students who have had no previous chemical experience. 2. This has been made possible by the use of a previously described ( I ) , (2) method of instruction which permits more efficient individual instruction. 3. An important phase of this method is the use of an elastic task which is suitable for students who have had varying previous chemical experience. Summary 1. A brief discussion of two experiments occurring early in the semester is presented, showing their adaptability for use in the instruction of large general inorganic chemistry laboratory classes of students with varied previous chemical experience. 2. These large experiments represent an elastic task which taxes the ability of the better student and yet makes it possible for the weak but conscientious student to perform thoroughly an acceptable minimum requirement. 3. The average per cent grades, both for "Experiment 111" and "Experiment IV" show no significant differencesfavoring those students who have had high-school courses in chemistry. Literature Cited SMOKER, "More Effective Individual Laboratory Instruction in General Inorganic Chemistry. I," J. &EM. EDUC., 6, 1933-9 (Nov., 1929).
(I) B m r
AND
(2) BRADT,"More Effective Individual Laboratory Instruction in General Inorganic Chemistry. 11. An Experiment on Molecular Weights," ibid., 8, 1 5 2 9 (Jan., 1931).
