CATALYSIS. - Some substances increase the speed of certain chemical reactions when they are present in the environment in which these take place: they are called catalysts, and their action is called catalysis. It was Berzelius who proposed this term, drawing it from the Greek: καταλύειν = to loosen, to dissolve.
The following criteria allow catalysis to be distinguished from variations in speed due to other causes: a) at the end of the reaction the catalyst is found chemically unchanged: one says "chemically" because sometimes, in the case of heterogeneous catalysis, having taken part in the reaction, it is found in a physical state notably different from the initial one; b) a small quantity of catalyst is often sufficient to transform large quantities of substances; c) the catalyst does not alter the final state of equilibrium of a reversible reaction.
A decrease in speed is called negative catalysis. Sometimes it seems that the catalyst brings about reactions that would not take place without it: even in such cases, however, it is admitted that the catalyst does not bring about but only accelerates a reaction already under way, even if extremely slow or, more precisely, facilitates the transformation of a system into a false equilibrium.
Homogeneous catalysis occurs when the substances that react and the catalyst belong to the same phase: that is, they are all gaseous or dissolved in water or another solvent; heterogeneous catalysis when the solid catalyst is immersed in a mixture of gases or in a liquid where the reaction takes place. Examples of homogeneous catalysis are those provided by hydrogen ions (v. acqua), which catalyze the inversion of sucrose and the hydrolysis of saline ethers into their components, organic acid and alcohol. More important is heterogeneous catalysis, of which examples are: the formation of water from hydrogen and oxygen gases under the action of "platinum sponge," the formation of ammonia from hydrogen and nitrogen gases in the presence of iron powder, the combination of sulfur dioxide with oxygen to give sulfur trioxide in the presence of platinized asbestos.
The mechanism by which the catalyst increases the reaction speed is for the most part very complex and poorly understood: the simple schemes with which the chemist habitually represents reactions highlight only the products that react, those that are formed and the relative proportions; but, with or without a catalyst, reactions are far less simple. In recent years (Hinshelwood, 1946), for example, to explain all the peculiarities of the reaction 2H₂ + O₂ = 2H₂O, numerous intermediate reactions have had to be admitted, in which unstable molecules such as H, OH, O, HO₂ appear, unknown to elementary chemistry. The speed of these intermediate reactions is also influenced by the molecules of other apparently inert gases that may be present and by the very walls of the vessel in which the behavior of the gaseous mixture is studied.
Biological chemistry (v.) offers numerous examples of catalysis. Some of these are specific: the catalyst, which in that case is called an enzyme, favors a single very particular reaction or a narrow group of reactions; others, on the other hand, are non-specific, in that the same catalyst favors various reactions, even of a totally different type from one another. Substances in the colloidal state, whether inorganic or organic, generally have catalytic properties; and one of the reasons why the chemist cannot reproduce many reactions of living organisms lies precisely in the difficulty of constituting an artificial catalytic colloidal environment similar to that of cells.
Bibliography3 works
Cesco Toffoli