CHEMICAL REACTION VELOCITY The variation in the speed of chemical change and its depend ence on the nature of the reacting substances is illustrated on the one hand by the almost instantaneous change (explosive re action) which transforms a stick of dynamite into gaseous prod ucts, and on the other, by the very slow change by which a steel girder is converted into a pile of iron rust, a process which may require thousands of years. It will only be possible to explain why some changes proceed so rapidly and others with such extreme slowness when we are able to state the cause of chemical change in general. At present, we can merely conclude that this is due to the action of electrical forces, the nature of which we are begin ning to understand as the result of recent observations which have provided us with information concerning the inner structure of the atoms themselves. (See ATOM.) It usually happens, as Julius Thomsen and Marcellin P. E. Berthelot (1867) first emphasized, that rapid and energetic chemical reactions take place with the liberation of greater quantities of heat than sluggish reactions although this rule is subject to many exceptions.
A second factor determining the speed of chemical change is the concentration of the reactants. By this we mean the amounts of the different reacting substances in unit volume. Thus C. F. Wenzel, in 1777, observed that metals dissolve in moderately con centrated acids more rapidly than they do in very dilute acids. Indeed it was soon recognized, as a result of the work of Claude Louis Berthollet in 1803, that whether a given chemical change or its reverse takes place sometimes depends only on the concen trations in which the several reacting substances are brought together. In such instances the effect of concentration outweighs the effect of the nature of the reacting substances, loosely termed "chemical affinity." The earliest accurate observations of the effect of concentration on the velocity of chemical changes were made by L. Wilhelmy (185o), who showed that the rate of con version of cane sugar into glucose and fructose in the presence of an acid is at every moment very nearly proportional to the concen tration of the cane sugar. These observations constituted the first definite proof that the influence of the concentration of a reacting substance can be quantitatively stated, but did not lead to any generalized statement.
It is only in the simplest type of chemical change, in which the change may be accomplished with a single molecule of a given reactant, that the velocity of the change is proportional to the first power of the concentration of that reactant. If n molecules of a reactant are required to accomplish the chemical change the velocity of the change is proportional to the nth power of the concentration of that reactant. In many cases is will be indicated by the chemical equation for the change. Thus indicates that the formation of water from hydrogen and oxygen demands two molecules of hydrogen for every molecule of oxygen, and thus that the rate at which water is formed at any given temperature is proportional to the first power of the concentra tion of oxygen but to the square of the concentration of hydrogen. In more complicated reactions, however, the chemical equation gives no clue to the power to which the concentration must be raised in calculating reaction velocity, for the chemical equation often expresses only the total effect and final result of a series of simpler consecutive chemical changes, the slowest of which determines the speed at which the series of changes as a whole may be accomplished. Changes in pressure are without effect on the velocity of chemical change, save in so far as pressure may determine concentration, as happens especially with mixtures of gases.
A third factor which affects the velocity of chemical change is the temperature. With rare exceptions, which can be readily explained, an increase of temperature increases the rate at which a chemical change will take place. Milk quickly turns sour on a warm day; and a mixture of hydrogen and oxygen, prepared safely enough at room temperature, explodes when it is heated by an electric spark. Commonly an increase of 1° C increases the velocity of a chemical change I o% or more, and most reactions at the temperature of boiling water proceed with several hundred times the velocity observed at room temperature. This striking effect of temperature is examined mathematically later.
A fourth factor is the presence or absence of particular sub stances which may increase or diminish the velocity of the chemi cal change, without being themselves permanently altered. Thus the velocity of a reaction between substances dissolved in water is different from the velocity of the same reaction, when the solvent is alcohol or benzene. Certain chemical reactions, in cluding many of the most important in animals and plants, appear only to take place in the presence of these substances, which apparently need only be present in relatively small quantities. Changes of this sort are grouped under the head of catalytic action, or catalysis (q.v.) ; the substances essential to bring about the chemical changes between the reactants are called catalysts, or, in the case of the animal and plant ferments, enzymes (q.v.). The phenomena of catalysis and adsorption (q.v.) are closely connected with reaction velocity.
Finally, the velocity of a chemical change may be determined by the supply of energy to the reacting system from an external source. If this is light energy we have the phenomena of photo chemistry (q.v.) ; if electrical energy, the velocity of the reaction will depend on the principles discussed under electrochemistry (q.v.), especially the laws of Faraday. (See ELECTROLYSIS.) The transformations of matter which occur in radioactive changes (see RADIOACTIVITY) differ from ordinary chemical changes in proceeding at rates which are determined entirely by the inner structure of the atoms of the radioactive elements, and are thus quite independent of the states of combination of those elements and even of the temperature.