CHAPTER XX

THE RISE OF PHYSICAL CHEMISTRY

We have seen how frequently chemistry has derived great advantages from the contributions of those who could bring to its problems the equipment and point of view of the trained physicist. Such a service was performed when Boyle and his associates delivered the science from the baleful mysticism and superstition in which alchemy had enveloped it, and again when Lavoisier dispelled the fog with which the vagaries of the phlogiston theory had surrounded the phenomena of combustion. In all these cases the men who ever seek new phenomena had acquired undue influence and had given free rein to their phantasy, so that it was necessary that others who could weigh, measure and define, should control their observations and connect the facts they had discovered by relationships capable of exact mathematical expression.

The rise of modern physical chemistry marks a new movement of this kind which has exercised a dominant influence upon the science since the last decade of the nineteenth century. In one sense physical chemistry is not modern. At all periods since the time of Lavoisier certain eminent investigators have preferred to devote their attention to the borderland between the two sciences. An early instance is that of Berthollet, who at the very beginning of the nineteenth century, attempted to impress upon an inattentive world the important facts that the course of a chemical reaction depends not only upon the affinities involved, but also upon the masses of the reacting substances; that chemical reactions lead to states of equilibrium; and that the physical properties of the products, such as solubility and volatility, exert an important and sometimes a determining influence upon their course. We have seen how, unfortunately for Berthollet, he allowed his reasoning to lead him to conclusions which contradicted the law of definite proportions, with the result that not only

were his conclusions discredited but his point of view, so that discussions on similar topics remained unpopular for a long time. Much of the work of Gay-Lussac as well as of Dulong and of Regnault may be classed as physico-chemical in the best sense, and Bunsen's influence in this direction was preeminent, especially in his work on optics, on the spectroscope, and on the influence of light upon chemical reactions. It is not surprising that he was fond of saying, "Der Chemiker der kein Physiker ist, ist gar nichts!" Kopp, the great historian of the science, also did valuable work upon the physical properties of organic compounds as functions of their constitution, which received early recognition on account of its direct application to problems of structure.

Other generalizations, too, which we are in the habit of associating almost exclusively with modern physical chemistry really received attention and were accurately stated at a comparatively early day, at least so far as general principles were concerned. This applies with especial force to the law of mass action, which now ranks as one of the main foundation stones of the science.

The Law of Mass Action. In 1850 Ludwig Wilhelmy, then a docent at Heidelberg, published a brief paper on the inversion of sugar by acids in which, by means of the polariscope, he studied the progress of hydrolysis with different acids, different quantities of acid, varying temperatures and varying amounts of sugar, and worked out a mathematical expression for the velocity of the reaction, which takes account of these factors completely and correctly. He also pointed out that similar studies upon other reactions of the same type must yield equations of the same form. This proved to be the case, but Wilhelmy himself received little credit, for by the time interest in such problems had become general his work was practically forgotten. We have seen how Berthelot and Péan de St. Gilles about 1860 carried on an important investigation upon the hydrolysis of esters, and expressed their results in the form that at any moment the rate of reaction is proportional to the amount of ester remaining undecomposed. On account of the prestige of Berthelot this work came to more general notice, and about 1863 two Norwegian scientists, Guldberg and Waage, being impressed by the work of

Berthelot, gave the idea more general form, and in an extensive investigation they set forth the universal application of the law that at equilibrium the velocity of a chemical reaction is dependent upon the products of the concentrations of the reacting substances. In their principal paper published in 1867 they express this as follows, in reasoning which sounds characteristically modern:

"If we assume that the two substances A and B change by double decomposition into two new ones A' and B', and that under the same conditions A' and B' can change into A and B, then neither the formation of A' and B' nor the re-formation of A and B will be complete; but at the end of the reaction there will always be present the four substances A, B, A' and B', and the force which causes the formation of A' and B' will be held in equilibrium by that which causes the formation of A and B.

"The force which brings about the formation of A' and B' increases proportionally to the affinity coefficient of the reaction

A+BA' + B'

but it also depends upon the masses of A and B. We have concluded from our experiments that the force is proportional to the product of the active masses of the two bodies A and B. If we designate the active masses of A and B with p and q, and the affinity coefficient with k, then the force

=

k.p.q.

"If the active masses of A' and B' are p' and 'q' and the affinity coefficient of the reaction

A' + B' = A + B

equals k', then the force tending to re-form A and B equals k'.p'.q'. "This force is in equilibrium with the first and consequently

kp.q = k'p'.q'

"By experimentally determining the active masses p, q, p' and q' the relationship between the affinity coefficients can be found. On the other hand when this relationship is known the result of the reaction can be calculated in advance for any chosen proportion of the four substances at the beginning."

It is not perhaps superfluous to quote the closing paragraph of this important paper:

“Investigations in this field are doubtless more difficult, more tedious and less fruitful than those which now engage the attention of most chemists, namely the discovery of new compounds. Nevertheless it is our opinion that nothing can so soon bring chemistry into the class with the truly exact sciences as just the line of research with which this investigation deals. All our wishes would be fulfilled if we might by this piece of work direct the permanent attention of chemists toward a, branch of the science which since the beginning of the century has unquestionably been far more neglected than it deserves."

The Phase Rule.-It was much the same with the phase rule which, in recent times, has served such an excellent purpose in demonstrating the important relationships involved in heterogeneous equilibrium. In this subject also the essential underlying principles were worked out abstractly by Willard Gibbs of Yale University as early as 1876. Gibbs, however, was so indifferent to fame that he apparently did not care whether he was so much as understood by his contemporaries, so that he not only called no attention to his results, but when it came to publication he buried them in the Transactions of the Connecticut Academy. Concerning the importance of the material there concealed Ostwald has expressed himself as follows:

"To give an idea of the significance of this work it sufficies to say that a very considerable part of the laws and relationships which have in the meantime been discovered in physical chemistry and which have led to such an astonishing development of that field within the last decade1 are found in this paper more or less thoroughly developed. The questions which concern the equilibria of complex systems are here treated with unexampled comprehensiveness and completeness; and in addition the influences which are usually considered, such as temperature and pressure, there are also discussed the effects of gravity, elasticity, surface tension and electricity. Experimental research has only slowly begun to follow the paths whose goal and direction are indicated in this work, and a wealth of scientific treasures still await experimental treatment, though in many cases this would be an extremely simple matter.

"In the face of such conditions one must ask: Why did this work achieve no success commensurate with its importance? Why, immediately upon its appearance, did not those effects follow which have since been attained in another way? There are many answers. Above 1 Written in 1896.

all the blame must be laid to the uninviting form in which the author has recorded his results. In a strictly mathematical manner, and with a text so concentrated that every page requires the active coöperation of the reader, the author takes us through his 700 equations, only seldom illuminating his results with any suggestive applications."

In short this paper by Gibbs must be classed with the Statique Chimique of Berthollet which Ostwald himself once characterized as "much praised and little read."

The Theory of Electrolytic Dissociation. For similar reasons interest in the physical side of chemistry did not become widespread until the theory of electrolytic dissociation was propounded by Arrhenius in 1887. This generalization, which found the world quite unprepared when it was first announced, nevertheless rested upon important chains of evidence which had been in process of development for a long time. These had to do with several widely separated departments of the science, and it was the service of Arrhenius to trace the connection between these facts, and to weld them into a comprehensive whole. It will be well to trace the history of some of these movements in detail.

Hittorf's Work on Electrolysis.-Faraday's law rests upon the fact that whenever a current passes through an electrolyte the latter is decomposed, and for a given quantity of electricity certain definite quantities of the decomposition products appear at the electrodes in chemically equivalent proportions. Faraday rightly concluded that these components of the electrolyte are the carriers of the current and to these carriers he gave the name ions. He regarded them as formed by the current and would doubtless have explained the mechanism of the process in the terms of Grotthuss (see page 88) which were universally accepted at that time. Since the quantities of material appearing at the two electrodes are chemically equivalent it was entirely natural, at the time when he wrote, that Grotthuss should make the tacit assumption that both the anion and cation (to use words not current in his day) migrated with equal velocities. This was the universal assumption until, in 1853, Hittorf began a remarkable series of investigations in which he showed that this was not the case. He pointed out that if they wander with different velocities the fact must be susceptible of experimental proof by