REACTION, CHEMICAL. A term applied to the transformations of substances into other substances having more or less different prop erties. (Sec CHEMISTRY.) It is noteworthy that the mutual transformations of the allotropic modifications of one and the same element must be considered as chemical reactions. Thus, the transformation of yellow phosphorus into red phosphorus is a chemical reaction. In essence the two substances are identical; but they never theless differ in their chemical behavior, and under the same physical conditionethey possess different physical properties; so they must be considered as two distinct chemical individuals and their transformation into each other must be considered as a chemical reaction.
The general laws according to which chemical reactions take place include the law of the con servation of matter, the law of the conservation of the elements, the law of definite combining masses, the law of combining volumes, the law of mass action, and, of course, the law of the conservation of energy. The conservation of energy plays an important part in thermo-chem istry and electro-chemistry (qq.v.). The other laws, with the exception of that of mass action, have been considered in the general artichi CHEMISTRY (q.v.). In the present article it remains to discuss briefly the action of masses.
At first consideration the concept of mass action appears to contradict the law of definite proportions. According to the latter, while sub stances may be mixed in any proportion what ever, chemical combination only takes place be tween definite relative quantities. Thus, oxygen combines directly with hydrogen only in the proportion of eight parts (by weight) of the former to one of the latter, whether a given mixture in which the reaction is caused to take place contains the two gases in this or in any other proportion. What, then, can the masses of the reacting substances have to do with the course of the reaction? A simple example may serve to illustrate the point in question. Ordi nary alcohol and acetic acid combine in the proportion of 46 parts of the former to 60 parts of the latter. If 46 grams of the alcohol should be left in contact with 60 grams of the acid for a sufficiently long time, 30.7 grams of the alcohol would combine with 40 grams of the acid (30.7 : 40 = 46 : 60), yielding 58.7 grams of ethyl acetic ester and 12 grams of water. The rest of the alcohol (15.3 grams) and of the acid (20 grams) would remain uncom billed, side by side with the ethyl acetic ester and water, no matter how long the mixture were kept (in one experiment the mixture was ac tually kept for seventeen years). Now, if in stead of 46 grains a larger quantity of alcohol° were left in contact with GO grains of acetic acid, more of the latter would ultimately be found to have entered into combination, and con sequently less than 20 grams of it would ulti mately remain free. But the proportion of al cohol and acid combined would still be 46 parts of the former to GO parts of the latter. This example illustrates the following principles: (1) Whatever the proportion of the reacting sub stances present, chemical combination takes place between the same relative quantities; (2) whatever the proportion of the reacting sub stances present, the possible maximum of each may not enter into combination, a fraction of the several substances present refusing to com bine at all, as long as they remain in contact with the products of the reaction; (3) the amounts of the substances present determine the fraction that will enter into combination and the . fraction that will remain free. The first of these principles is nothing else than the law of definite proportions. On the other hand, the doctrine of mass action has reference to the second and third of these principles, dealing, not with the relative combining quantities, hut with the extent to which combination takes place.
A fact of the greatest importance for the theory of chemical transformations is that the course of many reactions can be reversed. For instance, we just said that ordinary alcohol and acetic acid partly combine to form ethyl acetic ester and water. But ethyl acetic ester and water, if allowed to remain mixed for a sufficient length of time, will react and produce free acetic acid and alcohol. In this transformation again
the ester and water would partly react (in accordance with the law of definite proportions) and partly remain unchanged. Quantitative ex periment would reveal the facts: (1) If we should mix SS grams of the ester with 1S grams of water (8S and IS are, respectively, the reacting weights of the two substances), then 29% grams of the former and G grams of the latter would react to form 20 grams of free acetic acid and 15?:; grams of alcohol, while the remaining 58% grams of the ester and 12 grams of water would refuse to enter into reac tion; (2) if after all change has ceased in our mixture, we should add to it a further quantity of either ester or water, then a fur/her (hut not complete) decomposition of ester into alcohol and acid would take place; (3) if, on the con trary, after all change has ceased in our mixture. we should add to it, not ester or water, but either free acetic acid or alcohol, a further change would take place, resulting in the forma tion of more ester and water. It would thus become clear that in a mixture of reacting substances with the products of their reaction. when the mixture is in a state of 'chemical equilibrium,' we may cause a reaction to take place either in one direction or in the opposite direction, by changing the relative masses of the ingredients. In other words, the masses of substances may determine the course of a chem ical reaction. The importance of this statement in chemical theory will be so much the more obvious if we consider that there is good reason for assuming that all reactions are reversible. Says Nernst: has been formerly often main tained that 'reversible reactions' are exceptional, or that two different classes of reactions must be distinguished, reversible and non-reversible. But no such definite line of demarkation exists by any means. and there is no doubt that under appropriate experimental conditions it will be possible to cause any reaction whatever to take place now in one, now in the opposite direction; that is to say, in principle every reaction is reversible." But what, then, becomes of the old notion of chemical affinity? According to that notion, a chemical change takes place solely because the components of a given substance have a greater affinity for the components of another substance than for one another: two compounds AB and CD react and yield AC and BD solely because the affinities between A and C, B and D, are greater than the affinities between A and B, C and D. if this notion were correct, the op posite reaction, viz. the transformation of AC and BD into AB and CD, would evidently he impossible. But many reversible reactions are known as a matter of fact. Therefore the old notion of chemical affinity as the sole cause of reactions must be either discarded or essentially modified. A careful study of facts leads to the following conclusion: The course of reactions and the final equilibrium to which that course leads are certainly influenced by the chemical affinities (whatever may be their ultimate na ture) ; but those affinities are not alone in deter mining the chemical phenomena which they influ ence. In studying reversible reactions it is found that the action of masses comes in as a factor in all cases. without an exception. But it is also found that if chemically equivalent masses are started with in all cases, the frac tions of those masses actually entering into reaction and the fractions remaining unchanged vary from case to case. In other words, as just stated, the equilibrium finally attained depends both on the nature of the reacting substances and on the masses present to start with. Still another factor takes part in influencing the course and end of reactions, viz. the tempera ture. Or, more exactly, the specific affinity factor at work in each case varies with the temperature. But this variation will be con sidered under THER3I0-CHEMISTRY, and may be left out of account in the present discussion, in which all reactions are assumed to take place at constant temperature.