ROGERR. FESTA Normeast Missouri State University Kirksvllle, MO 63501
profile/ in chemiftry J. H. van't Hoff W. A.
E. McBryde
Universiy of Waterloo Waterloo, ON, Canada N2L 3 0 1
The first Nobel Prize in chemistry was awarded in 1901 to Jacobus Henricus van't Hoff, a Dutch-born scientist then living and working in Berlin. He was a t that time described as the greatest living physical chemist, hut by today some of the things upon which his reputation was based have through familiarity lost their remarkableness, and others have by assimilation into other more comprehensive theories and principles lost their identification with their originator. This brief memoir shall recall some of van't Hoff's achievements and try to set these into relationship with other important chemical ideas that were emerging in the 19th century. Van't Hoff, known as Henri to his friends, was horn in Rotterdam on August 30,1852, the third of seven children. His father was a physician and was able to provide a comfortable and intellectually stimulating home life for his family. Henri completed high school before he was 17, spent the next two years completing the three-year program at the Polytechnic School in Delft, from which he emerged a t the top of his class. His ensuing university experience was acquired, as was not uncommon a t the time, in several institutions. The first year a t Leiden he concentrated in mathematics; the second year he spent a t Bonn in KekulB's laboratory learning the theory and practice of organic chemistry; the third year, encouraged by Kekulb, he went to Paris to study under the latter's former teacher C. A. Wurtz. During that third year he took time toreturn to the Netherlands to write the qualifying examinations for the doctorate at the University of Utrecht. Back in the Netherlands in 1874 he wrote his first scientific paper (see below) and then polished off the work for a doctoral thesis on a rather innocuous hit of organic chemistry in time to receive his degree before Christmas! In spite of this impressive academic performance van't Hoff had difficulty in securing a job, and i t was not until 1876 that he obtained a lectureship in physics at the State Veterinary School in Utrecht. The following year he was appointed lecturer in chemistry at the University of Amsterdam where he remained for 18 years, eventually becoming head of the chemistry department. In his first publication (1874) van't Hoff introduced the third dimension into the chemists' description of molecules. We must recall that the concept of valency, especially as it applied to the atoms in organic and organometallic compounds, had only become evident and generally accepted following the publication of classic papers by Kekuli. and Couper in 1858. In these the basis of molecular structure in organic compounds was the quadrivalent carbon atom and its ability to link with other carbon atoms into chains. Following Couper, organic chemists began to represent organic compounds by structural formulae, and could thereby devise pictorially unique representations for isomeric compounds. However, such representations could not account for another type of isomerism identified first by Pasteur in 1848 in the optically active forms of tartaric acid, and found also in 1872 for lactic acid by Wislicenus. With insight probably cultivat-
Hoff. 1852-191 1. Photograph courtesy of Jan van der Elsken, Physical Chemistry Laboratory. University of Amsterdam.
J~CO~U Henricu~ S van't
ed by his considerable study of mathematics, van't Hoff proposed that the valence links of the carbon atom were directed toward the corners of a regular tetrahedron. By this three-dimensional representation he could then demonstrate the ~ossibilitiesof stereoisomerism. These included not only the chiral molecules in which a carbon atom was linked to four dissimilar atoms or groups hut also the cistrans isomerism of which unexplained instances had by that time been recognized. In one of those surprising hut not so rare coincidences that the history of chemistry reveals, a second paper outlining more or less the same ideas was published only two months later; the author of this, J. A. LeBel, had been a fellow student with van't Hoff in Paris only a few months before, and yet there is no evidence of the two men having ever discussed their common interest in this problem. With his appointment a t Amsterdam van't Hoff resumed exoerimental investigations in organic chemistry but soon shifted his emphasis'to study aspects of chemical reactions that were ~eculiarlvcharacteristic of organic chemistry, namely that the reaitions were often slow and often incomplete. In other words he took up the study of kinetics and equilibrium. A few earlier investigations on reaction rates had appeared between 1850 and 1866, and two Norwegian scientists (Guldhers between 1864 ~ and 1867 . ~ ~Waaee) ~ ~ ~ ,~~ - and drew upon this worYk and their;& to formulate the law of mass action as the fundamental ~ r i n c i ~ofl echemical kinetics. Much of these authors' wori, studies on Chemical Affinities. is concerned with cases of reactions kineticallv balanced at equilibrium. Unfortunately, through geographical ~
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Volume 64
Number 7
July 1987
573
isolation and the means chosen for its publication, this Norweeian work remained almost unknown throuehout Eurone unyil 1879. Mrimwhile, in 1877 van't off had'kIependvntIs stated the law of mass action and hv 1881. in an extended treatise entitled Studies of ~hemic'l ~ y n k m i c s ,had laid out the systematic basis on which reaction kinetics have been studied ever since. From measurements of rates in controlled experiments he was able to establish the "order" of reactions with respect to particular reagents, and therehy to infer the sequence of events composing.the "mechanism" of a reaction. By his clear and inspiring treatment of the subject of chemical kinetics and of the disturbanceswhich are found to influence the rate of a reaction; by hisdevelopment of experimentalmethodsof investigatingreaction velocities; by his discussion of the influence of temperature on the velocity or reaction coefficient and of the conditions of equilibrium in heterogeneous as well as in homogeneous systems, van't Hoff brought the whole subject of chemical dynamics vividly before the minds of chemists and aroused a widspread interest in a field of investigation which had, until then, been hut scantily tilled.. . .' Within the aforementioned treatise appeared the familiar relationship, known as the van't v off isochor, between the tem~eraturecoefficient of an ecluilibrium constant and the ent6alpy change accompanying the reaction d In kldT = AHIRP
This relationship shows not only that the equilibrium constant increases with increase in temperature for an endothermic reaction (which was incorporated into Le Chatelier's principle a sear later) but also bv how much. In 1885, in amither important puhlkati(m, Lous of Equilibritrm in Dilute Gusrous or Dissolved S.wtemr, van? Hoff examined some of the characteristics of osmotic pressure of aqueous solutions. The German botanist Wilhelm Pfeffer had shown in 1877 that for solutions of cane sugar and other substances the osmotic pressure was proportional to the concentration of a solution and that it increased as the temperaturr increased. Van'r Huff was able to drmonitrate with the aid of Pfeffer's and other ouhlished data that the oimotic pressure P for dilute solutions of many substances that todav are called nonelectrolvtes could he expressed hv an equaiion identical to the ideal gas equation (even to- the value of R): P V = nRT
Here nlV represented the molar concentration of the solution. He was thereby led to believe that there was a fundamental identitv between the gaseous and the dissolved state and so applied to solutions'a number of thermodyhamic relationships that had been established for Eases. One obvious inference from thr torrgoing rquation is applicability of Avogadro's law to dilute solutions, and hence the deduction thatisotonic solutions (having the same osmotic pressure) have the same molar concentration. Also, reasoning from thermodynamic cycles, van't Hoff was able to show that isotonic solutions should have the same vapor pressures and freezine point lowerines. This cave access to crvosco~icand other &ys of determ&ing moiar masses, which until then, had onlv been measured from vaDor densities. Electrolvtes did not quite conform to the foregoing relationships, but, by the introduction of a correcting factor i (a small number) in the form P V = inRT, the data could be fitted. The factor i, which did depend on the particular substance and t o some
extent on its concentration, was seized upon by Arrhenius to help validate his still suspect t h e o ~ yof electrolytic dissociation. He inferred that i signified the total number of particles (ions or molecules) generated by the dissociation of each molecule of solute. Such ideas, though superseded in the 1920's hv concepts of interionic attraction and activitv. .. served aH an important bridge to understanding the nature of solution. I t was for this part of van't Hoff s researches and his work in chemical dynamics that he was cited for the Nobel Prize. In 1896 van't Hoff accepted a research professorship in Berlin, a t the invitation of the Prussian Academy of Sciences. There he devoted most of his time to a btudy of crystallization of salts from multiconstituent solutions; in this he sought to apply the still comparatively little understood phase rule of Willard Gihhs to the essentially geochemical problem of the origin and composition of salt and potash beds. In van't Hoffs lifetime, chemistry and the way chemists thought about matter and reactions changed dramatically, and he must be regarded as one of the prime movers in bringingabout thesechanges. He told the story of how, when he went to school in Rotterdam, the lower forms were tauaht that the formula of water was HO, hut word filtered do-wn from the upper forms that a different formula, H20, might be gaining acceptance. So much for the state of the art in the early 1860's. Van't Hoff brought to bear upon his work a well-developed ability in mathematics, so that he was able to grasp and utilize the somewhat abstract reasoning of thermodynamics as applied to chemical processes, and to formulate the quantitative laws governing reaction velocities and the effects of temperature on these and upon equilibrium constants. In his disposition to use mathematics to interpret chemical observations he was much influenced by the writings of the French mathematician and philsopher Auguste Comte, under the spell of whose writing van't Hoff fell as a 19-year-old student at Delft. He appears to have shown no outstanding qualities either as an experimenter or as a lecturer, hut he did possess a nimble imagination, and it was by the use of this that he was so successful in formulating workable solutions to chemical problems. At the age of 23 he was turned down for a job as a school teacher in Breda; the director of the school reported to his superior after an interview with van't Hoff that the latter was a daydreamer. Throughout his mature life, van't Hoff was in freauent association with Svante Arrhenius in Sweden and ifh helm Ostwald in Germany. These three men created and wielded tremendous influence in a new branch of chemistry called physical chemistry, which dealt more in fundamental principles governing chemical processes (especially equilibrium and rates of reaction) than in the accumulation of descriptive facts (synthesis and properties). Ostwald and van't Hoff founded a new journal (the Zeitschrift fur physikalische Chemie) by which these new approaches to chemistry could be disseminated. Van't Hoff appears to have been blessed with a warm and agreeable persoda~ity,which assured the loyalty of students and colleagues. Personal descriptions of the man include phases such as "a kindliness of heart and quiet simplicity of character" or "simple and homelv". At the time of publication, his first pap& on the tetrahedral carbon atbm was subjected to a blistering attack by the aging but influential German chemist, Hermann Kolbe. By the exercise of his ever-present good humor, van't Hoff made light of the matter and ultimatelv turned it to his own advantage. - - These -----personal characteristics, no less than his many significant contributions to chemical principles, made him a very popular choice to receive the first Nobel Prize in chemistry. Van't Hoff died of tuberculosis in Berlin in 1911. ~
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Findlay, A,; Williams, T. One Hundred Years of Chemishy;Methuen: London, 1966; pp 64-70,87-112.
574
Journal of Chemical Education
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