Germain Henri Hess

Germain Henri Hess was born on August 7, 1802 in Geneva, Switzerland. His family moved to Russia when his father, an artist, became a tutor for a rich family. He studied medicine at the University of Tartu, obtaining a M.D. in 1826. In school he also studied chemistry and geology and upon graduation traveled to Stockholm, Sweden to study under the chemist Jons Jakob Berzelius. Although Hess spent only one month in the lab of Berzelius, they became life-long friends and correspondents.

On his return to Russia, Hess took part in a geological expedition to the Urals. After the expedition Hess set up a medical practice in Irkutsk where he remained for two years. In 1830 Hess moved to St. Petersburg where he began teaching chemistry and doing research. Later he became a professor at the St. Petersburg Technological Institute. He remained in St. Petersburg for the remainder of his life.

Hess is most famous for the chemical law that bears his name: Hess's Law. The law is that the enthalpy of change for a chemical reaction that is carried out in a series of steps is equal to the sum of the enthalpies of change of each of the steps. Enthalpy is the heat generated or lost by a chemical reaction. Hess's law allows for the calculation of how much heat will be released or absorbed by a chemical reaction by summing up the enthalpies of each of the steps of the reaction. Hess's Law was an early law in thermochemistry, the study of energy and heat in chemical reactions.

Other discoveries made by Hess include the discovery that sugar when oxidised yields saccharic acid. The mineral Ag₂Te is named Hessite in his honor. Hess wrote the chemistry textbook that was the standard Russian chemistry text for several decades. He was forced to retire due to failing health in 1848.

Hess died in St. Petersburg on December 13, 1850.

References:

Culp, Bartlow; "Germain Henri Hess"; at chemistry.explained.com

Germain Henri Hesse Wikipedia Entry

Marcellin Berthelot

Marcellin Berthelot was born in Paris, France on October 27, 1827. He is regarded as one of the greatest chemists of all time and was responsible for the Thomsen-Berthelot principle of thermochemistry and for disproving the theory of vitalisim by synthesizing many organic compounds from inorganic substances.

The fundamental conception that underlay all of Berthelot's work was that all chemical phenomena depend on the action of physical forces which can be determined and measured. When he began his career it was assumed that all organic chemistry depended upon vital forces to produce organic compounds. Berthelot opposed this idea and in order to disprove it he synthesized many organic molecules (including hydrocarbons, and natural fats and sugars) from inorganic starting materials, thus proving that the synthesis of these molecules did not depend of vital forces and that organic chemicals obeyed the same principals as inorganic compounds.

His other major contribution is the Thomsen-Berthelot principle of thermochemistry. Berthelot and Dutch chemist Julius Thomsen both independently came up with slightly different formulations for this principle which states that all chemical changes are accompanied by the production of heat and the processes which occur will be the ones in which the most heat is produced. This postulate led to the thermal theory of affinity which postulated that the true measure of chemical affinity was the amount of heat that was produced by a particular reaction. This was later disproved by Herman von Helmholtz, who discovered that the true affinity was not the amount of heat produced by a reaction, but the maximum amount of work or free energy produced when the reaction was carried out reversibly.

For his work in disproving the theory of vitalism and pioneering work in the field of thermochemistry Marcellin Berthelot is the Dead Scientist of the Week for the Week of October 25-31, 2009.

References:

Marcellin Berthelot Wikipedia Entry

Thomsen-Berthelot principle of thermochemistry Wikipedia Entry

Marcellin Berthelot NNDB Entry