QUANTITATIVE EXPERIMENTS IN ELEMENTARY CHEMISTRY 11. A MEASURE OF CATALYSIS* The subject of catalysis i s given little attention zn most elemel~tarylaboratory courses of general chemistry. A method of measuring the catalytic effect of several substances i n the decomposition of hydrogen peroxide i s here proposed. Attention i s also given to negative catalysis, when a reaction may be affected by the presence of both a promoter and a retarder. . . . . . . The subject of catalysis in an elementary laboratory course of general chemistry is usually confined to very small proportions. In reviewing a number of laboratory manuals i t was found that in all except one, catalysis was mentioned only in couuection with the effect of manganese dioxide on the decomposition of potassium chlorate. The one exception was the excellent experiment on catalysis in the laboratory manual of Deming and Arenson (1). This is aqualitative experiment designed to illustrate six different types of catalytic action. The present article is the result of an attempt to prepare a simple quantitative experiment which will enable a student to measure catalysis. Perhaps the h-st experiment which suggests itself in this connection is the decomposition of hydrogen peroxide, on which many classic researches have been made. The action of both positive and negative catalysts can be demonstrated very clearly in this study. The method consists in allowing a definite volume of hydrogen peroxide solution to come in contact with various catalysts and then measuring the volume of oxygen evolved in a fixed time interval. Details of the experiment are as follows. Assemble a simple apparatus as shown in the diagram. The required parts include one seven-inch test tube, one 5-cc. pipet, one 500-cc. flask, one graduate (100-cc.), two pinchclamps, one foot of rubber tubing, and about two feet of glass tubing. *Presented before the Division of Chemical Education of the A. C. S. a t the Buffalo meeting, August 3lSeptember 4. 1931. 144
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Place the catalyst in the seven-inch test tube. Fill the pipet with 5 cc. of 39;; hydrogen peroxide. (The student should be told that commercial solutions of hydrogen peroxide already contain a negative catalyst such as acetanilide and sometimes sulfuric or phosphoric acids. A brown bottle is also employed to keep the solution from decomposing.) Fill the flask to the neck with distilled water and also put about 10 cc. of water in the graduate. The connecting tube between the flask and graduate is filled with water. Before starting the experiment adjust the levels of the water in the flask and graduate so that the pressure in the flask is the same as the barometric pressure. Then tighten the pinchclamp and prepare to start. Observe and record the level of the water in the graduate. Open pinchclamp A . Record time of starting, then open pinchclamp B;and quickly allow 5 cc. of hydrogen peroxide to drop from the pipet on the catalyst in the test tube. The pipet should be of the type which does not deliver its entire volume, but only between graduations. This prevents oxygen from escaping via the pipet. The oxygen which is evolved expands into the flask, thus forcing water into the graduated cylinder. The volume of oxygen delivered should be read a t the end of each half minute for a period of five minutes. Before each reading the levels must he adjusted by raising or lowering the graduates. Allowance must be made for the volume (5 CC.)dropped into the test tube. of H202 The following catalysts are suggested: solids-(a) manganese dioxide, (b) lead dioxide, (c) wood charcoal. Use separatelyfibout 0.2 g. of each of these materials. In the case of manganese dioxide and charcoal the action is perhaps purely catalytic, although in the case of lead dioxide oxygen is liberated by both compounds. Solutions (10% concentration) suggested are: (d) 2 cc. of potassium iodide solution, (e) 2 cc. of ferric chloride solution, (f)2 cc. of ferric chloride mixed with 2 cc. of copper sulfate [compare the action with that in (e)], (x) 2 cc. of silver nitrate solution, ( h ) add a small piece of zinc to the silver nitrate solution in (g) and note the enhanced reaction. To demonstrate negative catalysis, place in the test tube 0.2 g. of manganese dioxide and also 0.2 g. of sodium nitrate. Now drop 5 cc. of hydrogen peroxide and compare the volume of oxygen evolved with that obtained in (a) the case of manganese dioxide alone. While it is impossible to retard the positive effect of manganese dioxide entirely, the sodium nitrate will decrease the amount of oxygen by about one-third. A similar effect can be obtained by using wood charcoal and zinc chloride together. In this case the negative effect of zinc chloride is more marked than in the preceding case. Of course the actual study of the rates of reaction in these decompositions would be a subject for advanced workers, but a simple experiment of this
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type can be followed readily by beginners. The little-understood topic of catalysis can be thus shown capable of measurement, although its explanation may still he in dispute. Literature Cited ( 1
DEMING AND ARENSON,"ExercisesinGeneral Chemistry and QualitativeAnalysis." 2nd edition, revised, John Wiley & Sons, Inc., New York City. 1926, Exercise 73, p. 194.
