656 CREATIVE CHEMISTRY The role which insignificant amounts of mineral matter play in reactions of this kind is indicated in the familiar trick of setting fire to a lump of sugar. The lump will not ignite on application of a lighted match, but if a speck of tobacco ash be added, the sugar will readily inflame.' Moisture is a common catalyst; thus a dry mixture of carbon monoxide and oxygen will not explode even when in contact with a red-hot platinum wire, but if even a minute trace of moisture is present an explosion will take place at once. Catalysis May Be Negative as Well as Positive. As a general rule one is interested in increasing the rate of chemical reactions, but occasionally he desires to decrease the rate instead. The knock in an automobile engine is produced by the premature explosion of the gasoline in the cylinder. This knock can be remedied by adding an antiknock substance to the gasoline which decreases the rate of combustion of the gasoline; lead tetraethyl used in ethyl gasoline is a negative catalyst. So effective is nicotine in retarding the oxidation of sodium sulfite that even a puff of tobacco smoke will produce a noticeable retardation. The Nature of Catalysis Is Not Entirely Understood. It is characteristic of many catalysts that they must be in an extremely finely divided condition to be active. Inasmuch as colloidal materials present a very great surface, one is not surprised to learn that many colloids serve as excellent catalysts. It is suspected that such processes are catalyzed by the adsorption of the reactants on the surface of the catalyst, which thus increases the effective concentration of the reactants. It has been learned, however, that catalysts are specific, i.e., only certain reactions are catalyzed by a given substance. Inasmuch as the action of the catalyst depends so intimately upon its chemical nature, one cannot escape the conclusion that the catalyst actually enters into temporary combination with one of the reactants and thus activates it. If this combination should be permanent, the reaction would soon cease. There is good evidence in some cases of catalysis for the formation of such an intermediate compound, which then reacts with other molecules, thus liberating the original catalyst. The catalyst acts as a sort of chemical parson joining elements in the bonds of matrimony. It is typical of surface catalysis that the catalyst is easily poisoned; thus mere traces of impurities, such as arsenic or phosphorus, will poison the nickel used to catalyze the hydrogenation of oils and thereby > News Edition, American Chemical Society, Vol. 17, No. 23, Dec. 10, 1939, p. 736.
AIR AND WATER AS RAW MATERIALS 657 effectively stop the reactions. In this case it is probable that the surface of the catalyst is covered with a layer of the poisonous substance to a depth of one atom or molecule, which thus renders it unable to adsorb other substances. The activity of catalysts may also be increased by the use of small amounts of foreign substances called promoters. The intermediate-compound and adsorption theories mentioned above fail to explain some types of catalysis, and no satisfactory general definition has yet been found. We have a great deal more to learn about catalysis before we can hope to enter into successful competition with nature in the synthesis of many natural products. Catalysis Plays an Important Part in Life Processes. Microorganisms, sometimes called nature's chemists, bring about a host of chemical reactions by a secretion of catalysts called enzymes. Similar enzymes are produced in the higher plants and animals. It is the enzymes produced in plants that cause fruits to ripen. The digestion of foods by animals is greatly facilitated by the enzymes produced in the mouth, stomach, and intestines. Vitamins contained in foods act as physiological catalysts in animals, and it has been adequately demonstrated that many important reactions in the human body will not take place in their absence. The human body also secretes important catalysts, called hormones, which control the efficiency of the utilization of food, the rate and extent of growth, sexual development and activities, and many other functions. Some of man's greatest conquests of diseases have been accomplished by the analysis, synthesis, and controlled use of these organic catalysts. These physiological catalysts will be discussed in more detail in Unit X. Nitrogen Fixation Is an Excellent Example of Sjnithesis Which Requires Catalysts. The maintenance of the nitrogen supply of the soil is probably the most difficult problem involved in keeping up the fertility of the soil because nitrogen compounds are expensive and easily lost from the soil. Nitrogen was formerly obtained from manures, and in some sections of the world, such as China, manures are still the chief source of nitrogen. Nitrogen may be added to the soil by legume crop rotation, but this process takes the land out of service, although it may even then pay; for example, 1533 pounds of seed cotton were obtained from an acre of ground after rotation with cowpeas, whereas only 837 pounds of cotton were obtained when grown year after year without a rotation of crops.