James E. Finholt Carleton Colleae Northfield, Minnesota ~
~~
I 1
A Laboratory- Program in Chemical Kinetics
Three years ago a new course entitled chemical kinetics was offered for the first time at Carleton College. The course was offered in the senior year to students who had already takeh courses in organic chemistry, analytical chemistry, and thermodynamics. I t is the second in a series of three, oneterm physical chemistry courses which replaces a one year physical chemistry course. The laboratory work in this course was designed to be something of a bridge between the student-planned research of graduate school and the teacher-planned laboratory exercises typical of much undergraduate laboratory work. It was felt that students should be forced to assume much of the responsibility for planning as well as executing their experiments. Students should he challenged by the task of interpreting raw data without having a handy instruction sheet to tell them what equation to use and when to use it. Above all it was hoped that somehow during the course every student would feel the thrill that is often accompanied by the exclamation "A ha! I have discovered something." I t was also exoected that the laboratorv work would reinforce the ideas developed in the lecture part of the course and provide an opportunity for the students to become familiar with a wide variety of chemical instruments. The detailed plans for the laboratory work were based on some assumptions or intuitive rules of thumb. Since real experimentation was to he encouraged, ample time for mistakes or rambles down blind alleys should be provided. Unless a student has the freedom to make mistakes, he really has no freedom a t all. This concept means that an average experiment will Presented aa part of the Symposium on Kinetics in the Undergaduate Curriculum before t,he Division of Chemied Education ~t the 144th Meeting of the American chemied Society, LOS Angeles, California, April, 1063.
require at least two afternoons of work. I n order to ease the shock of wehning the students from teacher planned exercises, a gradual transition should be made starting with fairly detailed instructions and ending with assignments having almost no specific instructions. The emphasis in each experiment should be placed on the discovery of the form of the rate law and its mechanistic implications, rather than just the determination of the numerical value of a rate constant. This means that a student never knows the form of the rate law for a given reaction before he begins experimenting with the reaction. This approach was expected to provide an opportunity for students to learn general methods of data analysis suitable for a variety of kinetic situations as well as to stimulate student interest. Assigned Experiments
The first reaction studied in the course is the familiar alkaline hydrolysis of ethyl acetate. CHaCOOC.Hs
+ 08- = CHICOO- + C2HsOH
Considerable help in the way of detailed instruction is provided the student in this experiment. For example it is suggested that he first check to see if the reaction follows a one term rate law of the form k[OH-lm[EtAc1"
-=
He is further instructed to run his first experiment with stoichiometric quantities of reactants so the rate law will be simplified to the form dt
-,--- ,
TO make life really easy for the student, a detailed procedure for carrying out this first, stiochiometric
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experiment is provided. This procedure includes the amounts and concentrations of all reagents, sampling procedures, and the analytical method to be used in determining the hydroxide concentration in each aliquot. There are still a few details left for the student to work out. He must decide what mathematical technique should be used to analyze the results of the stoichiometric experiment and so determine k and n. He might decide to use the the quantity m technique developed by Powell of plotting per cent reaction versus log 1 (I), or he might check each of the integrated forms of the rate law graphically or analytically. The student is also asked to design and carry out a second experiment to allow the determiuation of m and n individually. This might be done by changing the concentration of only one of the reactants in the second experiment and comparing the initial rates of the first and second experiments. Or, an experiment could be designed in which the general second order integrated rate law could be used to analyze the data. The actual experimental procedure to be followed in the second reaction need only be slightly different from that used in the first experiment. Two afternoons of laboratory work are required by a team of two students for the completion of this problem. Stock solutions of all reagents as well as standardized solutions of all analytical reageuts are provided for the experiment. This policy is followed as far as is possible for all experiments in the course. The hydrolysis of ethyl acetate is an appropriate reaction to begin the course for several reasons. It is simple and straightforward from a kinetic point of view, the analytical procedure used to follow the reaction is simple and familiar, and the species involved in the reaction are familiar. These considerations of familiarity allow the student to concentrate on understanding the kinetic techniques and ideas. The second experiment is a study of the followiug reaction (2).
+
+ 2 0-H =
02N-&CI 2,4-dinitrochlorohemene
O
N
piperidine
-
+
2,4dinitrophenyl piperidine
o 2 + c l piperidiniurn chloride
Again the student is given a considerabIe amount of help in planning the experiment, but a little more effort is demanded from him than in the ethyl acetate experiment. It is suggested to the student that the technique of flooding be used in his first experiment with the initial concentration of the piperidine much greater than twice the initial concentration of the 2,4dinitrochlorobenzene. If the reaction can be described by a one term rate law, its rate law should have the form rate = k [2,4dinitrochlorobemenelm[piperidineI"
which simplifies under conditions of excess piperidine to rrtte = k,. [2,4dinitrochlorobenzenelm
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where k,, is a pseudo rate constant related to the true rate constant by the following equation. k,. = k [piperidine]"
Complete details for carrying out an experiment under conditions of excess piperidine are provided for the student. These iustructions provide for a spectrophotometric determination of 2,4dinitrophenylpiperidine in quenched aliquots. As in the hydrolysis experiment the student is asked to decide what mathematical approach should be used to analyze the data from the experiment with the excess piperidme. He is then asked to design and conduct a second experiment which mill allow the determination of the order with respect to the piperidine. Most students choose one of three approaches. Some attempt to carry out a reaction with the 2,4dinitrochlombenzene in excess and they immediately run into trouble because it is not soluble euough in the ethanol used as the solvent for the reaction. Others choose to use stoichiometric conditions and have trouble with too intense coloration of the product and sometimes too rapid a reaction. The best choice is to keep the piperidine in excess hut change its initial concentration from that of the first experiment. This allows determination of a second pseudo rate constant, k . The following two simultaneous equations can be set up and solved for k and n. k,.
=
klpiperidine]"
ktn. = klpiperidine]'"
The students' woes are not over. He is now asked to design and carry out an experiment to verify the results he has obtained so far; but this is to be done by taking aliquots, quenching, and determining the chloride concentration in each aliquot by a potentiometric titration. In order to get aiough chloride for accurate analysis, completely diflerent conditions must be used than were used in the first part of the experiment. As a final task, the student is asked to design and carry out an experiment which will allow t,he calculation of AH1 and AS$. In the interpretation of his results the student is asked to propose a mechanism which would account for the fact that the rate law is first order in each reactant while the stoichiometry of the equation calls for two molecules of piperidine to react with each molecule of 2,4-dinitrochlorobenzene. This reaction is highly satisfactory from a number of points of view. I t allows the very important technique of flooding to be used, it can he studied by a variety of analytical methods, it illustrates that the form of the rate law cannot he predicted from the stoichiometry, and it goes to completion. Besides all of this, it works well. As an exercise in thinking and as a relief from the demands of careful, quantitative experimental work, the students have usually been asked to spend an afternoon with test tubes and graduated cylinders studying the iodate-hydrogen sulfite clock reaction (3). The reaction is carried out in acid solution in the presence of starch with an excess of iodate. I t proceeds according to the following equation until all of the hydrogen sulfite has reacted. IOa' + 3HSOI- = I- 3 S O P 3H+
+
+
After all the hydrogen sulfite has reacted, Ioappearsand is detected by the abrupt appearance of a blue-black color caused by the formation of the starch-iodine complex. Students never cease to be amazed to discover that increasing the hydrogen sulfite concentration decreases the time required for all of the hydrogen sulfite to react. The solution to this riddle makes an excellent problem for seniors, but the existence of the riddle makes this a poor reaction for freshman chemistry demonstrations. After the clock reaction, different experiments have been done during each of the three years that the course has been offered. These ekperiments have included the following: a polarographic study of the hydrolysis of hexachloroautimonate(V) (4), a conductometric study of the alkaline hydrolysis of methylbenzoate and related compounds as an illustration of the use of the Hammett equation ( 5 ) , the determination and interpretation of AH$ and AS$ for the reaction of bromoacetate and thiosulfate (6), and a test of the Br@nsted equation for the prediction of ionic strength effects on the oxidation of hexaaquochromium(111) by peroxydisulfate (7). Xot more than one or two of these experiments have been done by any single pair of students. I t is hoped that in future years experiments can be developed which will deal with very fast reactions, gas phase reactions, and the use of radioactive tracer techniques. Independent Work
The last part of the course is used for an independent study in which the students must plan all aspects of the work with very little help from the instructor. Each team of students is expected to choose a general experimental approach, decide on the analytical methods to he used, work out the proper concentration range so that the reaction is neither too fast nor too slow, and take care of all the other details which must he considered in planning a kinetic investigation. The study of the reaction of hydrogen peroxide and iodide in acid solution is an excellent subject for such work (8). 2H+
+ H,02 + 31-
=
1,-
+ 2H.O
The kinetics of this reaction can be studied in several ways using different analytical procedures. The rate law for the reaction has two terms so it is sufficiently puzzling to provide a challenge to the best of students. As an alternative to working on this prohlem students have sometimes worked on unsolved problems of their own choosing. These problems have usually developed from suggestions made by the instructor. They have ranged from an attempt to determine the mechanism for the decomposition of
time, and indeed, it is not very often that this is done. It is important that the students do not become so bogged down with severe analytical problems, or hopelessly complex kinetic situations that no progress a t all can be made toward discovering the rate law. Student reaction to this course has been most gratifying, especially after the course has been completed. None of the students who have taken the course have avoided periods of frustration and exhaustion, but on the other hand, all of them have experienced moments of elation when all the pieces of the puzzle seemed to fit together. The achievement of being able to plan an experiment and work out the problems involved in its execution gave almost all of the students a feeling of real satisfaction. The basic purpose of this laboratory program is to provide an opportunity for advanced chemistry students to become involved with the planning and interpretation of experimental work. Chemical kinetics is a particularly appropriate setting for such work for two reasons. First, the experimental work can be conducted without the use of difficult or time consuming experimental methods. Secondly, a minimal theoretical background is required to design and interpret the kind of Emetic experiments discussed here. When a student is asked to do laboratory work requiring considerable theoretical background or difficult experimental techniques, he has all he can do to understand his instructions much less plan new work in the field himself. I n summary, the laboratory program described here consists of a series of four or five experiments. I n all experiments the student is faced with the problem of determining a rate law. I n the first experiment, the hydrolysis of ethyl acetate, the student does only a small amount of the planning. In the second experiment, the reaction of piperidine and 2,4-dinitrochlorobenzene, he must do more planning; and finally, he is asked to study the kinetics of a reaction with no help at all. The experiments done in the course are not distinctive and are nndoubtably done a t many colleges. We feel that the involvement of the students in experimental design is somewhat distinctive and has resulted in a stimulating and interesting course. Acknowledgement
Dr. William Child of Carleton College has made important contributions in the development of this course. He taught the course during the past year and designed several of the experiments. Literature Cited
FROST, ARTHURA., A N D PEARSON, RALPHG., "Kineti~sand Mechanism," 2nd edition, John Wiley and Sons, Inc., New York, 1961, p. 14. BUNNETT, J. F., A N D CROCKFORD, 11. D., J. C I ~ M EDUC., . 33,552 (1956).
in acid solution to the development of an experiment suitable for use in the freshman chemistry course based I on the reduction of thiocyanatoiron(II1) by tiu(I1). Care must he exercised in choosing problems for this kind of independent work. It is not necessary that the prohlem be completely worked out in this period of
SORUM, C. H., C H A R L ~F.N ,S., NEPTUNE, J. A., A N D EDWARDS, JOHN O., J. dm. Cham. Soc., 74,219 (1952). NEUMANN, H. M., A N D RAMETTE, R. W.,J . Am. Chem. Soc., 78.1848 11956).
(8)
LIEBHAFBKY, H. A,, AND MOHAMMED, A,, J. Am. Chem. Soe., 55, 3977 (1933).
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