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Concept
Le Chatelier's principle
Résumé
Le Chatelier's principle (pronounced UKlə_ʃæˈtɛljeɪ or USˈʃɑːtəljeɪ), also called Chatelier's principle (or the Equilibrium Law), is a principle of chemistry used to predict the effect of a change in conditions on chemical equilibria. The principle is named after French chemist Henry Louis Le Chatelier, and sometimes also credited to Karl Ferdinand Braun, who discovered it independently. It can be stated as:
When a simple system in thermodynamic equilibrium is subjected to a change in concentration, temperature, volume, or pressure, (1) the system changes to a new equilibrium, and (2) this change partly counteracts the applied change.
Phenomena in apparent contradiction to Le Chatelier's principle can also arise in systems of simultaneous equilibrium (see response reactions).
Le Chatelier's principle is sometimes alluded to in discussions of topics other than thermodynamics.
Introduction The following thermodynamic statement is abstract, in general terms, but, for introduction, it may help the reader to bear in mind a simple example: A body of gas in a cylinder with a piston has its pressure externally controlled through pressure on the piston. The body of gas starts in a state of internal thermodynamic equilibrium by setting its own volume. The externally controlled pressure is then 'perturbed' by a controlled finite amount, say and the body of gas sets its own volume again through a change in volume If the piston and cylinder are insulated so that the gas cannot gain or lose energy as heat, then the Le Chatelier–Braun principle has nothing to say, because, though the temperature of the gas may change, there is available no possibility of 'moderation' or 'feedback'. But if the piston and cylinder walls conduct heat between the body of gas and a closed external 'heat reservoir' of externally controlled temperature and internal energy, then 'moderation' or 'feedback' can occur, or be investigated, through temperature change in, or heat transfer to or from, the body of gas; and the Le Chatelier–Braun principle tells about it.
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