Henry Louis Le-Chatelier


       Henri Louis Le Châtelier (8 October 1850 – 17 September 1936) was an influential Frenchchemist of the late 19th and early 20th centuries. He is most famous for devising Le Chatelier’s principle, used by chemists to predict the effect a changing condition has on a system in chemical equilibrium.Le Chatelier was born on 8 October 1850 in Paris and was the son of French materials engineer Louis Le Chatelier and Louise Durand. His father was an influential figure who played important roles in the birth of the French aluminium industry, the introduction of the Martin- Siemens processes into the iron and steel industries, and the rise of railway transportation. As a child, Le Châtelier attended the college rollin in Paris. At the age of 19, after only one year of instruction in specialized engineering, he followed in his father’s footsteps by enrolling in the Ecole polytechnique on 25 October 1869. Like all the pupils of la polytechnique, in September 1870, Le Châtelier was named second lieutenant and later took part in the Siege of Paris.After serving as an army lieutenant in the Franco-Prussian War of 1870-71,After brilliant successes in his technical schooling, he entered the Ecole des mines in Paris in 1871.Le Châtelier applied his earlier work on gas explosions to the chemical reactions occurring in blast furnaces, which are used to manufacture steel, and determined why certain gases were unexpectedly present in the furnace exhaust. By explaining blast furnace chemistry, Le Châtelier enabled industrial engineers to develop furnaces that could reach higher temperatures by preheating the combustion air with hot exhaust gases. Le Chatelier finished college at the Ècole Polytechnique, earning a degree in science and engineering in 1875. Two years later, he became a chemistry professor at the Ecole des Mines, where he began research on cement, ceramics, and glass. Some of his experiments required the measurement of very high temperatures, for which the equipment available at the time was inadequate. To measure these temperatures more accurately, Le Châtelier developed the thermocouple, an instrument consisting of a platinum wire and a platinum-rhodium alloy wire that measures temperature as a function of the difference in voltage between the two wires. He also introduced the use of known boiling and melting points as standards for calibrating thermocouples. Around the same time, Le Châtelier developed an optical pyrometer, another temperature measurement device. Although other methods have replaced the pyrometer, Le Châtelier’s equipment was useful at the time, and scientists continue to employ thermocouples in high-temperature research.

Le Châtelier’s scientific experience culminated in the discovery for which he is best known today- Le Châtelier’s principle. Announced in 1884, the principal states that when a system is in equilibrium and one of the factors affecting it is changed, the system will respond by minimizing the effect of the change. Essentially, the principle predicts the direction that a chemical reaction will take when pressure, temperature, or any other condition is altered. By employing Le Châtelier’s principle, scientists were able to maximize the efficiency of chemical processes. For example, Fritz Haber (1868-1934) utilized the principle to develop a practical process for synthesizing ammonia from nitrogen and hydrogen. Le Châtelier himself had tried this but was unsuccessful.Le Châtelier married Geneviève Nicolas, a friend of the family and sister of four fellow students of la polytechnique. They had seven children, four girls and three boys, five of whom entered scientific fields; two were lost preceding Le Châtelier’s death.
For the rest of his career, Le Châtelier continued teaching. In addition to his position at the Ecole des Mines, he held posts at the College de France and at the Sorbonne. After working for the French government during World War I, Le Châtelier retired from the Ecole des Mines in 1919 at age 69.He died on 17 September 1936.

Le Châtelier’s Principle states that a system always acts to oppose changes in chemical equilibrium; to restore equilibrium, the system will favor a chemical pathway to reduce or eliminate the disturbance so as to restabilize at thermodynamic equilibrium.If a chemical system at equilibrium experiences a change in concentration, temperature or total pressure, the equilibrium will shift in order to minimize that change.This qualitative law enables one to envision the displacement of equilibrium of a chemical reaction.

For example:
A change in concentration of a reaction in equilibrium for the following equation:
N2(g) + 3H2(g) ⇌ 2NH3(g)
1vol         3vol            2vol
        4 vol                   2 vol
                                   ◘ Decrease in volume takes place  
If one increases the pressure of the reactants (Nitrogen, N2 and Hydrogen, H2) the reaction will tend to move towards the products to decrease the pressure of the reaction.
Another example: In the Contace Process for the production of sulphuric acid, the second stage is a reversible reaction,the oxidation of SO:
2SO2(g) + O2(g) ⇌ 2SO3(g)
2vol              1vol           2vol
        3vol                        2vol
                                       ◘ Decrease in volume takes place
The forward reaction is exothermic and the reverse reaction is endothermic. If the temperature were increased, the larger amount of thermal energy in the system would favor the endothermic reverse reaction, as this would absorb the increased energy; in other words the equilibrium would shift to the reactants in order to remove the stress of added heat. For similar reasons, lower temperatures would favor the exothermic forward reaction, and produce more products.

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