Man's physical universe

xanabras

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.

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