by
John Oliver & Jim Kurtz
In the introduction to Linus Pauling's chapter on chemical equilibrium, he gives the student the following advice:
The student (or the scientist) would be wise to refrain from using the mathematical equation unless he understands the theory that it represents, and can make a statement about the theory that does not consist just in reading the equation. It is fortunate that there is a general qualitative principle, called Le Chatelier's principle, that relates to all the applications of the principles of chemical equilibrium. When you have obtained a grasp of Le Chatelier's principle, you will be able to think about any problem of chemical equilibrium that arises, and, by use of a simple argument, to make a qualitative statement about it.… Some years after you have finished your college work, you may (unless you become a chemist or work in some closely related field) have forgotten all the mathematical equations relating to chemical equilibrium. I hope, however, that you will not have forgotten Le Chatelier's principle.[1]
What is this principle which is so highly recommended, and who is its author? The nature of the principle itself is as easy to grasp as it is difficult to state succinctly, and the man himself is almost totally eclipsed by the popularity of his most famous observation.
Any system in stable chemical equilibrium, subjected to the influence of an external cause which tends to change either its temperature or its condensation (pressure, concentration, number of molecules in unit volume), either as a whole or in some of its parts, can only undergo such internal modifications as would, if produced alone, bring about a change of temperature or of condensation of opposite sign to that resulting from the external cause.[2]
This rather cumbersome expression of the Principle, in a note first published in 1884, seems to show that Le Chatelier himself was the first to experience the frustration of trying to express a generalized form of the concept in a clear and succinct manner.
Four years later, in a long article in the Annales des Mines, Le Chatelier restated the Principle in a simpler and more comprehensive form:
Every change of one of the factors of an equilibrium occasions a rearrangement of the system in such a direction that the factor in question experiences a change in a sense opposite to the original change.[3]
Subsequent authors, particularly those writing textbooks, have opted more for brevity than rigor, usually emphasizing the qualitative cause and effect nature of the idea:
If the conditions of a system, initially at equilibrium, are changed, the equilibrium will shift in such a direction as to tend to restore the original conditions.[4]
or,even more briefly,
If a stress is applied to a system at equilibrium, then the system readjusts, if possible, to reduce the stress.[5]
or,
Le Chatelier's principle is easier to illustrate than to state. Nevertheless, let us begin with this statement:
If a "stress" is applied to a system at equilibrium, the equilibrium condition is upset; a net reaction occurs in that direction which tends to relieve the "stress," and a new equilibrium is obtained.[6]
This last example voices the realization all teachers of chemistry have discovered for themselves, that the generalization is easier to teach with examples than words. In fact, the long 1888 paper contains a large number of applications to equilibria including ones involving electromotive force as well as pressure and temperature. The extension of this argument to include applications to biology and social sciences have been made by later writers and were not represented in Le Chatelier's original explanation.
One of the most promising areas of research at this time was the possibility of synthesizing ammonia from atmospheric nitrogen and hydrogen. It also presented an excellent study, then and now, in equilibrium.
N2 (g) + 3H2 (g) <===> 2NH3 (g) + heat
From the equation, we can infer that,.according to Le Chatelier's generalization, increasing the pressure should have the effect of favoring the production of ammonia, because a contraction from four volumes to two volumes occurs during the reaction. As in any exothermic reaction, the temperature would have to be limited to prevent the reverse (endothermic) reaction from being favored, but not kept so low as to seriously inhibit the rate of production. Considering these problems, and informed by the notes of Thenard who had shown complete dissociation of ammonia at 600°C in the presence of metallic iron, Le Chatelier in 1901 attempted the direct combination of the two gases at a pressure of 200 atmospheres using Thenard's temperature. The mixture of gases was forced by an air compressor into a steel Berthelot bomb, where they and the reduced iron catalyst were heated by a platinum spiral. An accidental contamination of the reaction chamber with air resulted in an explosion which blew fragments of the steel container through the floor and ceiling. Le Chatelier abandoned the project, and less than five years later, Haber and Claude were successful in producing ammonia on a commercial scale, acknowledging that the account of Le Chatelier's failed attempt had accelerated their research. Near the end of his life, Le Chatelier wrote, "I let the discovery of the ammonia synthesis slip through my hands. It was the greatest blunder of my scientific career."
The great American chemist Josiah Willard Gibbs had anticipated in a mathematical way Le Chatelier's main results, but the fact that he had published in a journal with extremely limited readership outside the United States, combined with the abstract and difficult presentation of his ideas, limited its value to the audience Le Chatelier served, the practicing chemist. Le Chatelier himself, however, was very interested in thermodynamics and was an early champion of Gibbs' work in France, being responsible for the first translation of his papers into the French language.
Le Chatelier's interest in and use of the mathematics of thermodynamics may be surprising to teachers who, informed by the standard textbook treatment of his Principle, use Le Chatelier as an example of a qualitative treatment of equilibrium. However, a more thorough look at his life and career reveals a man who consistently integrated theory with practice and whose most successful research was directed toward the problems of industry.
Born in Paris on October 8,1850 to a family of architects and engineers, Henry Louis Le Chatelier received his early training in mathematics and chemistry from his father, Louis Le Chatelier, an accomplished engineer. He assisted his father while the latter helped to create the aluminum industry in France, and thus gained much first-hand information about metallurgy. His mother was a rigid disciplinarian and a devout Catholic whose family background and love of poetry fostered in her son that appreciation of art and letters which was evident throughout his life. Le Chatelier's career was marked by a disdain for unsubstantiated speculation, coupled with an almost uncanny sense of the interrelationship between theoria and praxis in science. After finishing his formal education at the Ecole Polytechnique, the Ecole des Mines, and the College de France, his intention was to devote himself to government service as a mining engineer. After two years in the Corps des Mines at Besancon, however, he was offered, much to his surprise, a position as professor of chemistry at the Ecole des Mines. He spent the rest of his life in Paris, where he lectured at the Ecole polytechnique in 1882, was made professor at the College de France in 1883 and became professor at the Sorbonne in 1887. In the same year, he returned to the Ecole des Mines as Professor of Industrial Chemistry and Metallurgy. In 1889 he returned to the College de France where he remained until 1908 as Professor of Inorganic Chemistry.
His choice of research projects clearly reflects his appreciation of the interplay between theoretical science and its practical applications. Indeed, he consistently chose research problems of wide interest which seemed to give promise of industrial applications. His very first area of investigation, for example, brought him into the largely uncharted area of the chemistry of cements. He began by repeating the experiments of Lavoisier and Payen on the preparation of plaster of Paris. He discovered that good plaster of Paris consists of the hemihydrate of calcium sulfate, which he identified, and not of the anhydride as was previously believed. These investigations led him to a theory applicable to the setting of many kinds of cements. By this theory, when a cement comes into contact with water, a supersaturated solution is formed which deposits a less soluble hydrated material. This process of solution and solidification results, Le Chatelier explained, in the production of an interlaced, coherent mass of minute crystals.
His studies with cements also give evidence for the extraordinary practical skill with which Le Chatelier selected, modified, or in some cases actually invented the instruments needed to carry on his researches. In his work with cements, for example, he chose a method of thermal analysis devised by Regnault but little known at the time. The method would eventually prove of even greater importance in the study of steel and alloys. Methods known at the time for determining high temperatures, such as the use of gas thermometers and of thermocouples, soon proved inaccurate for Le Chatelier's work with cements, as Regnault himself had predicted. It was left to Le Chatelier to isolate the source of the erratic behavior of the instruments. This he did, and the determination of high temperatures soon became a routine procedure following the development of the platinum-platinum-rhodium couple.
As an outgrowth of his researches on cements, Le Chatelier significantly widened the range of applications thermodynamics to chemistry. He reasoned that thermodynamics should yield valuable information about those chemical phenomena which were of most concern to him in his study of cements, such as the solubility of salts and their reaction with water. The result of these investigations are seen in his well-known principles of equilibria and the displacement of equilibria to which his name is attached and for which he is certainly best known among students of chemistry. Given his keen sense for the applications of chemical theory, it is perhaps not surprising that he should have accomplished the synthesis of ammonia in 1901, anticipating Fritz Haber as was discussed above. Without his equilibrium principle, it is possible that the practical applications of the phase rule and phase law diagrams would have remained hidden for quite some time.
Another major area of research for Le Chatelier was in the combustion and explosion of gaseous mixtures. Once again, his research was closely intertwined with a practical concern for a particular problem. The specific problem that directed his attention into this area was a series of coal mine disasters in France. He attacked the problem by initiating a scientific study of the combustion of methane. Together with Mallard, a professor at the Ecole des Mines, he determined the temperature of ignition, the explosive ratio of air and methane, the speed of propagation in explosions, the explosive pressure, etc. As was often the case with Le Chatelier's scientific investigations, his pioneering work in a new field required the creative development of the appropriate apparatus. Having done this, Le Chatelier and Mallard were able to extend their techniques to the study of hydrogen, carbon monoxide, acetylene, and cyanogen. During these studies, Le Chatelier also developed devices for detecting and determining small quantities of marsh gas. The use of these instruments contributed greatly to the safety of mines, as did the development by Le Chatelier and Mallard of safer, more suitable explosives for use in the mines.
Working with Mallard on the allotropic transformations of crystalline materials, such as quartz, Le Chatelier was once again in the position of needing an instrument capable of following these transformations at temperatures above the 200 to 300 degree maximum allowed by the technique then available, polarizing microscopy. He met the challenge by contriving the so-called differential dilatometer which allowed him to follow the transformations by observing the expansion rates of a given specimen relative to the refractory material on which it rested. These investigations led to his study of the thermal expansion of glasses, to the complex reactions that take place in the production of ceramics, and then to an extended study of alloys: their thermal expansion, electrical conductivity, thermoelectric potentials, cooling and heating characteristics, tempering, and annealing. While working in the field of metallurgy he greatly improved upon the techniques then available for microscopic metallography. His improved microscope revealed the formation of compounds between iron and carbon in steel and proved the value of heat treatment in steel. This empirical data, coupled with his theoretical application of the phase rule to the allotropic transformations of a wide variety of alloys, proved to be of enormous value to the world of industry.
Still another contribution to the scientific progress of metallurgy can be seen in his creation of the Revue de Metallurgie, which was published beginning in 1904 with Le Chatelier as editor for the next ten years. Besides his many scientific papers, he wrote books on metal alloys, steel, clays and ceramics and acted as an industrial consultant in the manufacture of steel, cement and synthetic ammonia. Altogether Le Chatelier published over five hundred journal articles and books. They included not only works on chemistry and ceramics, but also numerous biographies and, toward the end of his life, articles on social welfare, the scientific management of industries ("Taylorism"), the interrelationship between pure and applied science, and the relation of science to economics.[7]
Throughout his life, Le Chatelier was a leader in progressive movements, giving much of his time and effort to causes which he believed worthwhile. In particular, he wrote and spoke extensively on educational reform, taking the lead through the example of his own highly respected career as a teacher. At the Sorbonne, where he was made professor in 1887, he directed the work of over one hundred graduate students during the period from 1908-1922. Ralph Oesper aptly describes his contribution to educational theory and practice in these words:
Le Chatelier's teaching opened a new era in chemical education. Enumeration of compounds, their properties, methods of preparation--such was the unappetizing content of the lectures usually offered to the students. He refused to follow the fashion whose effect was the alienation of the student's interest. His courses were built around general laws and principles and he presented facts only as applications of these. He stressed the value of these laws in predicting new facts and emphasized the necessity of precise measurements since these alone can lead to valid general conclusions. Never did he usurp the function of a dictionary or an encyclopedia, and his sole function was to develop the reasoning power rather than the memory of those to whom he was unfolding the beauties of chemistry.[8]
A modest man himself, Le Chatelier advised his students to be content with adding a bit to the structure of science, keeping their eyes open for the unusual in nature, but avoiding any deliberate grasping for "sensational discoveries, which do not come by merely wishing for them."[9] He expressed this and other of his lifetime ideals on the occasion of the fiftieth anniversary of his graduation from the Ecole Polytechnique, celebrated by the French Academy of Sciences in 1922. Some of what he said on that occasion has been summarized by Alexander Silverman in a paper presented before the Division of the History of Chemistry of the American Chemical Society on September 6, 1937:
He stressed the importance of discipline which his parents had imposed upon him and which was practiced at the Ecole Polytechnique while he was a student. He deplored the decreasing seriousness of study and the increasing tendency toward pleasure and even license in modern colleges and universities. He likened the irresponsible student to a bold individual who dodges vehicles in crossing a street and risks being crushed, to say nothing of the fact that he seriously ties up traffic. Of himself Le Chatelier said, '. . . throughout my scientific career I strove without any desire of the sensational, contenting myself each day with the conscientious pursuit of the task of the day. In the end I was amply rewarded.'[10]
BIBLIOGRAPHY
1. L. Pauling, College Chemistry, 3rd ed., Freeman, San Francisco, CA, 1964, pp. 437-438.
2. H. L. Le Chatelier, Comptes rendus, 99, 786 (1884).
3. H. L. Le Chatelier, Annales des Mines, 13 (2), 157, (1888).
4. L. Pauling, p. 44.
5. M. J. Sienko and R. A. Plane, Chemical Principles and Properties, 2nd ed., McGraw-Hill, New York, NY, 1974, p. 216.
6. R. H. Petrucci, General Chemistry: Principles and Modern Applications, 1st ed., Macmillan, New York, NY, 1972, p. 275.
7. "Henri Le Chatelier: His Publications," Ceram. Abs., 16, (Oct., 1937).
8. Ralph E. Oesper, "The Scientific Career of Henry Louis Le Chatelier," J. Chem. Ed., 1931, 8, p. 451.
9. R. Oesper, op . cit., p. 444.
10. Alexander Silverman, "Henri Le Chatelier: 1850 to 1936" in Aaron J. Ihde and William F. Kieffer, ed., Selected Readings in the History of Chemistry, Division of Chemical Education, American Chemical Society, Easton, PA, 1966, p. 142.