LE CHATELIER, HENRY-LOUIS

born Oct. 8, 1850, Paris, France died Sept. 17, 1936, Miribel-les-chelles French chemist who is best known for the principle of Le Chatelier, which makes it possible to predict the effect a change of conditions (temperature, pressure, and concentration of reaction components) will have on a chemical reaction. This principle proved invaluable in the chemical industry for developing the most efficient chemical processes. Le Chatelier was educated at the Collge Rollin, cole Polytechnique, and cole des Mines, Paris. After working two years as a mining engineer, he was named professor of chemistry at the cole des Mines in 1877. He became an authority on metallurgy, cements, glasses, fuels, and explosives, and his interests turned to the study of heat. He developed a platinum and rhodium thermocouple for measuring high temperatures and an optical pyrometer, which measures intense heat by analyzing the light from the heat source. Le Chatelier first enunciated his principle in 1884 and dealt with the effect of changing pressures and other conditions in his Loi de stabilit de l'equilibre chimique (1888; Law of Stability of Chemical Equilibrium). He was unaware for some time that his conclusions had been anticipated in part by the American physicist J. Willard Gibbs, whose works Le Chatelier later translated into French. At Le Chatelier's suggestion (1895) the oxyacetylene torch was developed for use in welding and cutting metal. In 1908 he became professor of chemistry at the University of Paris, and from 1914 to 1918 he worked for the Ministry of Armaments. The Le Chatelier principle states that if a system (a substance or a collection of substances) in a balanced, or equilibrium, state is disturbed, the system will readjust in such a way as to neutralize the disturbance and restore equilibrium. The principle may be illustrated by the behaviour of a reversible chemical reaction such as that of hydrogen and nitrogen to form ammonia. The reaction is represented by the equation: 3H2 + N2 2NH3. The double arrows indicate that while ammonia is being formed it is also being decomposed. The formation reaction is accompanied by evolution of heat while the decomposition reaction requires absorption of heat. At equilibrium, i.e., when the velocity by which ammonia is being formed is just balanced by the velocity by which it is being decomposed, a definite amount of ammonia as well as hydrogen and nitrogen will be present. According to the Le Chatelier principle, if the mixture is heated, the equilibrium condition will be displaced in such a way that the resultant increase in temperature will be minimized, that is, in a direction that consumes heat energy. Thus, the addition of heat energy will favour the decomposition of ammonia, this being the reaction that absorbs heat, until finally a new equilibrium condition point is reached. In this new equilibrium state there will be less ammonia present and more hydrogen and nitrogen. The principle therefore predicts that at a higher temperature the reaction will yield less ammonia than at a lower temperature. This conclusion is the basis for the use of low temperature in view of the high pressure employed in the industrial manufacture of ammonia. The Le Chatelier principle applies not only to reversible chemical reactions but equally to reversible physical processes, such as evaporation or crystallization.