Germain Henri Hess
Germain Henri Hess is noted today for two fundamental principles of thermochemistry: the law of constant summation of heat (known simply as Hess's law) and the law of thermoneutrality. These discoveries were remarkable in that they were postulated without any supporting theoretical framework and took place in a field of study almost totally neglected by his contemporaries. Hess's law is of immense practical importance and is used to this day to determine heats of reaction when their direct measurements are difficult or impossible.
Hess was born on August 8, 1802, in Geneva, Switzerland. At the age of three, he moved with his family to Russia when his father, an artist, became a tutor to a rich family. Hess began his medical studies in 1822 and obtained an M.D. in 1826. While in school, he became interested in chemistry and geology, and upon graduation traveled to Stockholm, Sweden, to study with Jöns Jakob Berzelius, the most renowned chemist of the period. Although he spent barely a month in Berzelius's laboratories, Hess was strongly influenced by him in his later career, and they remained lifelong friends and correspondents. After practicing medicine in Irkutsk for two years, Hess returned to St. Petersburg, where he remained a member of the academic establishment for the remainder of his life.
Although Hess, like most of his colleagues, was involved with the discovery and analysis of new substances, he became interested in the more theoretical aspects of chemistry. His investigations into the nature of chemical affinity—why atoms are attracted to each other—led him to study the amounts of heat generated by chemical reactions. His experiments, carried out on the various hydrates of sulfuric acid, showed that the heat evolved in their formation was always the same, whether the reactions proceeded directly or stepwise through intermediates . Although this can be seen in hindsight as a specific example of the law of the conservation of energy, Hess developed it two years before Julius Robert von Mayer elucidated the more general principle in 1842.
Hess was immediately aware of the significance of his discovery and continued his studies in the same realm. In 1842 he proposed the law of thermoneutrality, which stated that no heat is evolved by the exchange reactions of neutral salts in aqueous solutions . He was not able to fully explain these observations, and it was not until forty-five years later that the process of electrolytic dissociation was more completely elucidated by the Swedish physicist and chemist Svante Arrhenius.
Although his research activity diminished after these two major discoveries, Hess remained influential in the development of chemistry in Russia. His textbook Fundamentals of Pure Chemistry saw seven editions and remained the standard Russian text in chemistry until 1861. He remained active in teaching and mentoring younger scientists, until declining health forced his retirement in 1848. He died on December 13, 1850, at the relatively young age of forty-eight.
Leicester, Henry M. (1951). "Germain Henri Hess and the Foundation of Thermo-chemistry." Journal of Chemical Education 28:581–583.