In chemistry, Le Châtelier's principle, also called the Chatelier's principle, can be used to predict the effect of a change in conditions on a chemical equilibrium. The principle is named after Henry Louis Le Chatelier and Karl Ferdinand Braun who discovered it independently. It can be summarized as:
If a chemical system at equilibrium experiences a change in concentration, temperature, volume, or partial pressure, then the equilibrium shifts to counteract the imposed change and a new equilibrium is established.
This principle has a variety of names, depending upon the discipline using it. See for example Lenz's law and homeostasis. It is common to take Le Châtelier's principle to be a more general observation, roughly stated:
Any change in status quo prompts an opposing reaction in the responding system.
In chemistry, the principle is used to manipulate the outcomes of reversible reactions, often to increase the yield of reactions. In pharmacology, the binding of ligands to the receptor may shift the equilibrium according to Le Châtelier's principle thereby explaining the diverse phenomena of receptor activation and desensitization. In economics, the principle has been generalized to help explain the price equilibrium of efficient economic systems. In simultaneous equilibrium systems, phenomena can occur which are in apparent contradiction to Le Châtelier principle; these can be resolved by the theory of Response reactions.
Status as a physical law
Le Chatelier's principle qualitatively describes systems of non-instantaneous change; the duration of adjustment depends on the strength of the negative feedback to the initial shock. Where a shock initially induces positive feedback (such as thermal runaway) the new equilibrium can be far from the old one, and can take a long time to reach. In some dynamic systems the end-state cannot be determined from the shock. The principle is typically used to describe closed negative-feedback systems, but applies generally in nature, since the second law of thermodynamics ensures that the disequilibrium caused by an instantaneous shock must have a finite half-life. The principle has analogs throughout Systems theory.
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