Van't Hoff, Jacobus Hendricus

van't Hoff, Jacobus Hendricus

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COFOUNDER OF MODERN STEREOCHEMISTRY AND PHYSICAL CHEMISTRY
1852–1911

Jacobus Hendricus van't Hoff, born in Rotterdam, Netherlands, was in his youth, along with the French chemist Joseph Achille Le Bel, the cofounder of modern stereochemistry. He then became one of the fathers of modern physical chemistry. In 1901 he was awarded the first Nobel Prize in chemistry for his work on chemical dynamics and the osmotic pressure in solutions.

As is the case with many creative minds, van't Hoff's career initially encountered many roadblocks, mostly related to the ignorance of the scientific establishment. Unemployed for two years after receiving his doctorate, he eventually took a teaching job in physics at the veterinary school in Utrecht. Success finally came in 1877 when the newly founded University of Amsterdam offered him a lectureship; the next year he became a professor of chemistry at the same institute. Although his international reputation led to the formation of a new chemical laboratory at the Amsterdam school, which was completed in 1891, he moved to Berlin in 1896 as both a university professor and member of the prestigious Prussian Academy of Science.

Shortly before he submitted an only average doctoral thesis in synthetic organic chemistry to the University of Utrecht, the twenty-two-year-old van't Hoff had printed and distributed a twelve-page pamphlet at his own expense that, although ignored for many years, essentially outlined the foundation of modern stereochemistry. Inspired by the earlier ideas of the German chemist Johannes Wislicenus to extend chemical structure theory from constitutional chemical formulas to representations in three-dimensional space, van't Hoff suggested a structural distinction between optical isomers that had been represented up until that time by the same formula. By screening known substances for optical activity (i.e., the rotation of the plane of polarized light), he found that all their constitutional formulas contained at least one carbon atom that combined with four different atomic groups. If the latter were placed at the corners of a tetrahedron in three-dimensional space, with the carbon atom at its center, there were exactly two possible tetrahedra with asymmetric carbon atoms, each being the mirror image of the other, that could account for the pairs of optical isomers. This structural theory was further supported by his observation that for every known chemical transformation where the optical activity of a reactant disappeared, there was no more asymmetric carbon atom in the structural representation of the reaction product. Independently and virtually simultaneously, Le Bel arrived at the same theory in Paris.

Due to his early interest in philosophy, particularly the ideas of French philosopher Auguste Comte, van't Hoff's chemical research strove for general and theoretical insight into chemistry and thus gradually shifted from organic to physical chemistry. In 1884 he published his well-known Études de dynamique chimique (Studies in chemical dynamics); it contained many important ideas on chemical kinetics and thermodynamics that confirm van't Hoff's status as a true pioneer in the field. Based on numerous measurements of organic reaction rates, he classified chemical reactions according to different orders and molecularities and formulated the temperature dependence of reaction rates, now known as the "Arrhenius equation." Following through on earlier ideas, he interpreted chemical equilibria as dynamic states where backward and forward reaction rates were equal, which he represented by a double arrow. He further formulated the temperature dependence of the equilibrium constant, known as the "van't Hoff isochore," and suggested what later came to be known as "Le Chatelier's principle." Most important, he established the foundation of chemical thermodynamics by expressing the relationship between what is now called the free energy of a reaction and its equilibrium constant.

During the late 1880s van't Hoff turned the seemingly exotic phenomenon of osmotic pressure into a crucial part of the new physical chemistry of solutions. He discovered that the osmotic pressure π increased with the concentration of the solute, c = n /V, and the absolute temperature T according to the equation

which is equivalent to the ideal gas law if one replaces the osmotic pressure π, with the gas pressure, p :

with R being the gas constant in both cases. The surprising correspondence between the two suggested that the osmotic pressure depended only on the number of solute molecules and not their chemical nature, like other so-called colligative properties such as vapor pressure lowering and freezing point depression. Van't Hoff did not simply advance all these phenomena on a common thermodynamic basis; he also successfully explained apparent anomalies with the Arrhenius theory of electrolytic dissociation.

In his later years, van't Hoff applied chemical thermodynamics to the formation of marine salt deposits, which made him one of the pioneers in the science of petrology.

see also Thermodynamics.

Joachim Schummer

Bibliography

Hornix, Willem J., and Mannaerts, S. H. W. M., eds. (2001). Van't Hoff and the Emergence of Chemical Thermodynamics: Centenary of the First Nobel Prize for Chemistry, 1901–2001. Delft, Netherlands: DUP Science.

Snelders, H. A. M. (1976). "Van't Hoff, Jacobus Hendricus." In Dictionary of Scientific Biography, Vol. XIII, ed. Charles C. Gillispie. New York: Scribner.