Le Châtelier's principle

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Le Châtelier's principle,

chemical principle that states that if a system in equilibrium is disturbed by changes in determining factors, such as temperature, pressure, and concentration of components, the system will tend to shift its equilibrium position so as to counteract the effect of the disturbance (see chemical equilibriumchemical equilibrium,
state of balance in which two opposing reversible chemical reactions proceed at constant equal rates with no net change in the system. For example, when hydrogen gas, H2, and iodine gas, I2
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). For example, at a given temperature a covered beaker partly filled with water constitutes a system in which the liquid water is in equilibrium with the water vapor that forms above the surface of the liquid. While some molecules of liquid are absorbing heat and evaporating to become vapor, an equal number of vapor molecules are giving up heat and condensing to become liquid. If stress is put on the system by raising the temperature, then according to Le Châtelier's principle the rate of evaporation will exceed the rate of condensation until a new equilibrium is established. At the new equilibrium point a greater proportion of molecules will exist in the vapor phase. Le Châtelier's principle is evident in chemical systems, as in the common-ion effectcommon-ion effect,
decrease in solubility of an ionic salt, i.e., one that dissociates in solution into its ions, caused by the presence in solution of another solute that contains one of the same ions as the salt. The common-ion effect is an example of chemical equilibrium.
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 and in bufferbuffer,
solution that can keep its relative acidity or alkalinity constant, i.e., keep its pH constant, despite the addition of strong acids or strong bases. Buffer solutions are frequently solutions that contain either a weak acid and one of its salts or a weak base and
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 solutions (see also separate article on pH). Le Châtelier's principle can be used to encourage formation of a desired product in chemical reactions. In the Haber processHaber process
, commercial process for the synthesis of ammonia, NH3. Pure hydrogen and nitrogen gases are mixed in the appropriate proportion, heated to between 450°C; and 600°C;, compressed to about 1,000 atmospheres pressure, and passed over a catalyst.
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 for the industrial synthesis of ammonia, nitrogen gas and hydrogen gas react to form ammonia gas in the reaction N2+3H2→2NH3 ; the process is exothermic, i.e., one that gives off heat. Since four molecules—three of hydrogen and one of nitrogen—react to form two molecules of ammonia, the reactants have a higher gas pressure than the products. When the reaction is run under high external pressure, up to 1000 atmospheres, and relatively low temperature, about 500°C; (932°F;), the system favors formation of the substance that will result in a lower total number of molecules, i.e., the ammonia. Running the reaction at relatively low temperature causes it to go far to completion, although if the temperature is too much below 500°C; the rate of reaction is too slow.
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The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Le Châtelier’s Principle


(known in Russian as Le Châtelier-Braun principle), a principle stating that an external influence causing a shift in the thermodynamic equilibrium of a system brings about processes in the system tending to counteract the effect of that influence. Thus, the heating of a system in equilibrium produces internal changes (for example, chemical reactions) that are accompanied by heat absorption, and the cooling of a given system produces changes that are accompanied by the evolvement of heat. The shift in equilibrium under increased pressure is connected with a reduction in the overall volume of a system; a decrease in pressure is accompanied by physical or chemical processes leading to an increase in volume.

In 1884, J. Van’t Hoff formulated the principle of shift in equilibrium as a function of temperature. The given principle was generalized by H. Le Châtelier in 1884, and C. Braun gave his thermodynamical substantiation in 1887. Le Châtelier’s principle was formulated by analogy with the Lenz law of induction; the principle is rigorously derived from the general condition of thermodynamic equilibrium (maximum entropy). Le Châtelier’s principle makes it possible to determine the direction of the shift in equilibrium of a thermodynamic system without a detailed analysis of the equilibrium conditions.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

Le Chatelier's principle

[lə′shäd·əl‚yāz ‚prin·sə·pəl]
The principle that when an external force is applied to a system at equilibrium, the system adjusts so as to minimize the effect of the applied force.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
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