Catalysis and the Elementary Chemistry Course J . A. CAMPBELL Oberlin College, Oberlin, Ohio
B .
ERZELIUS made many great contributions to chemstry, but in making one of them, he was inadvertently guilty of performing a profound disservice to future students of chemistry. This anomaly arose only because he, like most scientists, first stated a generalization and then put forth the qualifications. As has happened in many another case, abstractors have lifted the generalization and left the qualifications, to the great confounding of the aspiring student. Concerning catalysis, Berzelius, in 1835, concluded: It is then proved that several simple and compound bodies, soluble and insoluble, have the property of exercising on other bodies an action very different from chemical affinity. By means of this action they produce, in these bodies, decompositions of their elements and different recombinations of these same elements to which they themselves remain indifferent.
It is here that most quotations stop, but Berzelius went on to say: This new force, which was hitherto unknown, is common to organic and inorganic nature. I do not believe that i t is a force quite independent of the electrochemical affinities of- matter; I believe, on the contrary, that i t is only a new manifestation of the same; but, since we cannot see their connection and mutual dependence, i t will be more convenient to designate the force by a separate name. I will thereforecall this force the catalytic force, and I will call catalysis the decomposition of bodies by this force in the same way that one calls by the name analysis the decomposition of bodies by chemical affinity.
Ezperimat. Heat some potassium chlorate in a test tube until oxygen just begins to evolve from the molten salt. Remove the tube from the flame and add a pixinch of manganese dioxide. Repeat with another tube adding a small pinch of ferric oxide. Try calcium oxide, silicon dioxide, and cupric oxide. Compile from the text or a handbook a list of all the known oxides of manganese, iron, calcium, silicon, and copper and, on the basis of your results, try to interpret the reason for some of these oxides being catalytic for the release of oxygen from potassium chlorate. Try other oxides from the side shelf after jiou predict their effectiveness. (Do not use PhO2, BaOs, NanOz, or HgO since these all release oxygen as readily as does potassium chlorate.) Is your rule sufficient or only necessary? Try CulO.
In Table 1 are listed the results of the addition of various compounds to potassium chlorate which, although molten, is liberating oxygen very slowly. It may be noticed that every oxide which increases the rate of oxygen formation is of an element which can exist in various stable oxidation states, although some of the oxides which do not act as catalysts, antimony for instance, also fall into this class. The trend in Group 5 elements in the periodic table, where PIOlo and AsrOlo are catalysts whiie SblOe and BieOaare not, is in accord with the relative instability of the higher oxidation states of antimony and bismuth compared to those of phosphorus and arsenic. Other oxides than those listed also follow this oxidation-reduction criterion, but they are not generally available in elementary work. It should be mentioned that the lower oxides of the positive catalysts listed are. not trne catalysts. Their addition to 'molten potassium chlorate does hasten oxygen evolution, however, for the potassium chlorate oxidizes them to the higher oxidation state. Thus, addition of red CuzO leads to the formation of black CuO, which is then catalytic. It is therefore driven home to 'the student that in the catalysis of the decomposition of potassium chlorate by oxides a probable mechanism is the alternate oxidation and reduction of the catalyst. As a clinching experiment it might be shown that salts of the active oxides also act as catalysts, potassium dichromate for example. This indicates that it is not the crystal structure of the oxides which is the determining factor. Acatalyst is, then, a substance which, by enteringinto a reaction, affords a new mechanism for the reaction but causes no change in the equation for the net reaction or in the equilibrium concentrations. There is no net change in the chemical composition of the catalyst.
In many present-day chemistry texts (12 were examined) catalysts are defined as "substances which alter the rate of a chemical reaction but are not themselves permanently changed." In most cases little attempt is made toward any type of explanation. Such a definition is not scientificaUy correct since catalysts do undergo "permanent change," but in two of the texts-and this is truly amazing-the definite impression is created that the catalyst does not even enter into, the reaction. To these authors i t is apparently less imaginative to say that the catalyst does not enter into the reaction whose rate it changes than to say that the effect is not well understood. The fact that it is inconceivable that two substances could affect one another without reacting (a contradiction in terms) apparently escapes notice. It is small wonder that a large number of students of chemistry are M y convinced that a catalyst "speeds up a reaction but does not enter into it." The performance of a simple experiment in the laboratory quickly convinces students that, in one case a t least, the catalyst has an important role as a chemical, and not just as a bystander. For such an experiment one can turn to the classical introductory experiment on catalysis, the d e c t of catalysts on the decomposition of potassium chlorate. 582
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TABLE 1
Kclo.
KC1
+ 3/2 0, No oppolml cololysir MgO
Cao BaO
