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4h California Association of Chemistry Teachers
Robert Condon, Jerrett Rollins, Paul Merrick, Jay Polon, and Bruce Whipperman
Sequoia Union High School District Redwood City, California
Chemical Theory from Laboratory Data A new high school chemistry course
The authors of this course take the premise that sciace information and science understanding are different. The former deals with what is found out while the latter emphasizes how it is found out. High school chemistry has tended more and more toward the "what" aspect of the science as more information becomes available. The authors believe that understanding is the primary objective, and that a laboratory course stressing the quantitative nature of chemistry produces maximum growth in this direction. To that end, essentially all class activities are hut variations of the two basic themes, data collection and data interpretation. I n a limited sense, students need not go into the laboratory for data. Measurements have been made for years, and the information can be made available on data sheets for student evaluation. A small fraction of the data for the course is even handled this way; however, common sense requires us to capitalize on the high level of interest in laboratory work. Apparently there is no data quite like that a student collects for himself. Students are a t their best when defending their own speculations or conclusions based on their own original measurements. Data from the class as a group deserve consideration, too. Two or three measurements each, by 25 or 30 students, generally produce a very respectable average value. The central theme that gives the course direction is the derivation of relative weights. In a standard high school chemistry program, one sees the periodic table as a finished product during the first week or so. Students in this course spend most of the year collecting data and discovering the relationships that evolve to periodicity. The essential theme is a laboratory approach. No attempt will be made t o discuss the entire content of the course. A few specific problems have been chosen for detailed treatment to illustrate t,he "flavor" of the approach. Quant,itative experimentation in the high srhool laboratory has always had inhcrent problems. These have caused many teachers to curtail sharply, or to avoid completely, experiments requiring accuratc measurement. When large numbers of students are using equipment in the necessarily "hurried" environment of the 50-minute laboratory period, the degenerat,ion and maladjustment of balances and ot,her calibrated instruments become the limit,ing factors in the success of experiments. It is not economically feasible to provide these in large numbers; yet good laboratory management demands that students not
have t,o wait idly for one or two special instruments t o become available for their individual use. Shortcircuiting the maintenance problem by providing large numbers of simple, cheap, and effective measuring devices became a major issue in the successful development of this course. The production of individual apparatus has allowed the "take home" experiment to become a reality. The general philosophy dictates that students he encouraged to make weighings and other simple measurements outside of class, thus freeing school laboratory time for those portions of experiments which cannot he done elsewhere and for the all-important class discussions leading t o interpretations. Measurement and the Search for Relationships
In any quantitative experiment the measurement itself becomes the basic problem. At present, most eleventh grade students entering high school chemistry classes have little skill in actually measuring such simple quantities as weight and volume. Their experiences in mathematics have led them to look for perfection where numbers are involved. The concept of error is foreign t o most. Interpretation of scattered data on a graph seems inconsistent with the straight line required by a formula. Students sample the many-sided problem of collecting and analyzing data in a brief pre-chemistry measurement unit. They develop concepts that will be useful later on, but mainly they learn how to look for relationships in data that they collect. The balance (Fig. 1) is a basic t,ool in the course.
Figure I. Student b o l m c e and typic01 objectr weighed. rtoples or unit weights.
Note use of
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Details of its construction will be provided interested readers on request. It is sensitive enough to detect the hydrogen gas in a 50-ml plastic bottle as compared to the weight of the evacuated container. I t can he carried about in a shoebox and set up for weighing in about one minute. Studentas eventually develop reasonable skill in handling their own balances in a variety of weighing situations, and learn a little physics besides. At the outset, ordinary wire staples are used as units of weight. Figure 1 depicts their calibration t o gram values a t the appropriate time by weighing known volumes of water in a plastic container. Weight and volume determinations are made for various sizes of lead, iron, and aluminum samples. Maximumminimum measurement notions and interpolations are introduced by having the students make weighings to the nearest whole staple. The concept of per cent of error follows logically, once the limits of error have been estimated. The weight and volume measurements for the metal samples provide excellent data for the introduction of graphing techniques. All students graph their own data after they have made measurements on a sufficient number of samples of one of the metals. Great stress is given t o the expression of range of measurement. The resulting graph introduces classic inductive reasoning, which is so basic t o the interpretation of quantitative laboratory data. Graphing of weight and volume measurements for the other metals gives the student the opportunity to make numerical comparisons. See Figure 2. Interpolation and extrapolation (prediction), proportionality, and the density concept are natural byproducts of this simple weightvolume exercise.
measurement and student laboratory work provides the quantita, tive relationship between volume of gas released and weight of metal used. Hydrochlorio acid plus magnesium, and hydrcchloric acid plus calcium provide a pair of reactions that is almost ideal. A graph of Volume of Gas Released (uncorrected data) versus Weight of Metal Used is made for eaoh metal. The data dso are plotted together. The composite graph has limited meaning unless the same gas is given off from eaoh reaction. It is usually not necessary to probe very hard before this eomment arises from the class. In a subsequent period the metals are reacted with acid again and the gases identified as the same.
Replacement data is collected for iron and added to the magnesium and calcium combination graph (see Fig. 3). One can read across such a graph and determine weights of metal that are equivalent-equivalent in at least one sense; they release equal amounts of hydrogen (whatever that might mean at this early date). Class discussion can hardly begin before the first question is asked, "Does the kind of acid used make any difference in the reaction?"
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2 4 6 8 Weight of Metal Reocted (in stapler)
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Figure 3. Volume of go. d l e c t e d (in ccl versus weight of metal reocted = calcium, and 0 = iron. (in rtoples); 0 = magnesium,
A
From this point at least two pathways for further study are apparent: extension to include more acids and more metals or closer attention t o the other snbstances in reactions already considered. Deftly guided class discussion leads to both possibilities, but emphasis narrows the discussion temporarily t o the latter as a point of departure. Weight and Stoichiomelry
0.5 1.0 Volume of Metal Sample (in cc)
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Figure 2. Weight of metal romple (in staples) versus volume of metal = aluminum, A = iron, and = lead (areo of recsample (in cc); tangle shows vncerlainty of measurement).
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Weight and Equivalence The search for relationships is soon extended to include the reaction of acid8 and metals. First a qualitative demonstration by the teacher is done to indicate the range of the problem. Class discussion points out the quantities that are subject to
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Adding calcium t o sulfuric acid provides a means of considering products other than hydrogen in metal-acid reactions. The white, insoluble material on the surface of the calcium metal grows in amount if left overnight and experiment confirms the suggestion that the amount of this material is also proportional t o the amount of metal used. Figure 4 shows the result of adding increasing amounts of calcium t o equal amounts of dilute sulfuric acid present in excess. One need hardly draw a graph t o view these tubes in a quantitative way. "How much metal can acid 'dissolve' ('eat up,' etc.)?" Figure 5 summarizes the results of an experiment to probe this problem. The two strips
Figure 4.
Demonrtmting the oddition of increasing weights of colcivm to
H1504.
of magnesium were the same length until one of them was pushed slowly into a tube containing hydrochloric acid. The test tubes, left t o right, show: the muriatic acid, a similar tube after reaction "stopped" (note excess magnesium ribbon sticking out), and a similar tube after reaction and after heating t o boil off water and excess hydrogen chloride. The balance is needed to weigh the white material so it can be compared wit,h original magnesium weight. The hydrogen was not collected this t,ime. The volume is known, however, through the proportionality between the hydrogen and magnesium. Students soon learn t o look for such rdationships which are certain t,o come up on snbsequent tests. This approach of building upon and extending the data of previous experiments becomes st,andard procedure. Soon it is carried forward t o include some decomposition reactions and, lat,er, to a considcration of metal-sulfur reactions. This invest,. ment of time furnishes some experimental foundation on which to build further notions of proportionality, percentage composition, and multiple proportions. Approach to Atomic Weight
A thorough study of the gas laws gives reason to question the qualit,y of the metal-acid replacement, data gathered earlier. Instead of returning en masse to repeat all the hydrogen collections, students try out the new refinements on the data from a lithium-water reaction. Most of the class can generally agree on modifying the old data by some average correction,
about lo%, to approximate the conversion from room conditions t,ostandard conditions. The die-hard experimenters are allowed to go hack and try again. Figure 6 shows some of the equipment students use t,o react sodium with alcohol. In addition t o presentinp another problem in refining gas volume, the experiment provides new dat,a t,o establish the gram equivalent weight concept. By this time, students have derived molecular and at,omic weights for gases along with dual equivalent weights for some of the metals. The extension of at,omic weights t o include metals seems hopeless without some estimate of particle number or a method for examining equal numbers of particles, as was done with gases. Indeed, an extension of the heat capacity-particle number relationship in gases to inclnde metals in the solid state is called for on a trial basis. Could samples of metal with the same heat capacity contain the same number of particles? This is used as a kind of hopeful guess t o be tried out (the Law of DuLong and Petit). One sees the possibility of comparing equivalent weight,s of metals with the
Figure 6.
Equipment for studying the reaction of sodium with alcohol.
weights of the metals that contain equal numbers of particles. This is what is needed t o unscramble the combining factor (valence) problem. After several exploratory graphs, students plot the volume of hydrogen released by metal samples versus weight of samples that show a temperature change of 1°C per calorie. Even if metal samples with the same heat capacity contain only approximately the same number of particles, the atomic weight is forthcoming since it must he a whole number (combining factor or valence) multiple of the equivalent weight. The number (or numbers) associated with a particular metal can be seen as the relative effectiveness of t,he sample in producing hydrogen. Thus, the aluminum sample is t,hree (nearest whole number) t,imes as effective in producing hydrogen as the sodium sample, even though t,hey both contain approximately t,he same number of atoms. Finally, t,he student is givcn a sample of a met,al unfamiliar to him (indium or tin, for example); his task is t,o go into t,he laboratory and make the necessary measurements to determine its atomic weight. Testing
Figure 5. "When mognerium reacts with hydrochloric m i d , whot d o you get, and how much?'
Quizzes, tests, and examinations set the tone for a course. To the student they represent what the teacher Volume 39, Number 4, April 1962
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feels is most important. Memory-type questions characterize the approach where science information is held to be most important. Where science undcrstanding is desired, reasoning and interpretive type questions seem most appropriate. In the attempt to ~mphasizoscience understanding, testing proceeds on the following basis: Tests are all open book and laboratory centered. Students are encouraged to use the mimeographed Iaboratory sheets, worksheets, graphs, summary sheets, and previous tests. By necessity, memory-type questions are eliminated. Typically the information on which questions are based i s presented in the .form of a graph. A lead paragraph introduces the graph as data resulting from an experiment closely paralleling current laboratory work. The graph is never quite the same as the ones the students have drawn in interpreting their own results, hut the same kind of reasoning process is required. Data points are plotted, errors and all, just as they might have come from the laboratory. Development of concepts i s built into the puestioning. First, the basic relationships are explored at the "confidence-building" level. Students soon learn to expect further questions a t the interpretation level. Finally a few well-directed questions are injected to encourage speculations or intuitive judgments leading to further theory. Probably no other activity focuses more combined class concentration than testing. It has the student's fullest attention and thus represents the best possible climate for learning. It cannot be wasted on evaluation alone, it must do double duty. Conditions are perfect for broadening the data base by providing information for substances and reactions similar to those considered in laboratory experiments. The framework of the test gives perspective to what has been accomplished and direction to what follows. In short, examinations become an integral part of the presentation of subject matter in the course. Conclusion
One wonders, in fact,, if a comparison between this and a traditional course in high school chemistry is fair to either, since the goals of each are apparently different. The traditional course at,tempts to give students a background in chemistry through a survey of the theories and laws that govern chemical changes (accompanied by laboratory experiences that illustrate the points to be made). This course is dedicated to forcing students to decide what can and cannot be said about chemical changes based on data collected. As an example of the sciences, chemistry shonld be portrayed as a collection of ideas that correlate a great number of observations. The observations, not the laws in a book, are the final authority. Since chemistry is too big to learn in a year, one can only sample it. The intent of this course is to sample vertically; i.e. students find out how to establish a law or theory by collecting and correlating data. A major effort is made to foster arguments that arise in class discussion between students with different interpretations of the same data. Though the subject matter of any course must evolve, the upper limit seems to be dictated by the kind of 216
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laboratory experiences a high school chemistry student can have. The teacher of traditional chemistry will look to this course in vain for some old favorites. The assignment of electrons to their proper orbit, for example, is absent; yet the basis for valence, the disparity between equivalent weight,s and corresponding numbers of atoms, is a cornerstone. The course is fun to teach, if only because of the extra involvement and responsibility one feels. One thing, a t least, has been demonstrated: Something can be done about a chemistry curriculum at the high school level by high school teachers exclusive of any but local support. Acknowledgment
The authors acknowledge the support of Dr. Rex Turner, District Superintendent, and E. F. "Scotty" Elson, long-suffering Director of Curriculum. Walter Cottle and Ruth Hull (Carlmont High School) and Lewis Karcher (Menlo Atherton High School) have joined the authors in teaching the course. Data plotted in Figures 2 and 3 were from Ruth Hull's chemistry students.
