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Le Chatelier's principle (pronounced UK: /lə ʃæˈtɛljeɪ/ or US: /ˈʃɑːtəljeɪ/), also called Chatelier's principle (or the Equilibrium Law),^[1][2] is a principle of chemistry used to predict the effect of a change in conditions on chemical equilibrium.^[3] 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 defined as:

If the equilibrium of a system is disturbed by a change in one or more of the determining factors (as temperature, pressure, or concentration) the system tends to adjust itself to a new equilibrium by counteracting as far as possible the effect of the change

— Le Chatelier's principle, Merriam-Webster Dictionary

In scenarios outside thermodynamic equilibrium, there can arise phenomena in contradiction to an over-general statement of Le Chatelier's principle.

Le Chatelier's principle is sometimes alluded to in discussions of topics other than thermodynamics.

Thermodynamic statement

General scenario The Le Chatelier–Braun principle analyzes the qualitative behaviour of a thermodynamic system when a particular one of its externally controlled state variables, say ${\displaystyle L,}$ changes by an amount ${\displaystyle \Delta L,}$ the 'driving change', causing a change ${\displaystyle \delta _{\mathrm {i} }M,}$ the 'response of prime interest', in its conjugate state variable ${\displaystyle M,}$ all other externally controlled state variables remaining constant. The response illustrates 'moderation' in ways evident in two related thermodynamic equilibria. Obviously, one of ${\displaystyle L,}$ ${\displaystyle M}$ has to be intensive, the other extensive. Also as a necessary part of the scenario, there is some particular auxiliary 'moderating' state variable ${\displaystyle X}$, with its conjugate state variable ${\displaystyle Y.}$ For this to be of interest, the 'moderating' variable ${\displaystyle X}$ must undergo a change ${\displaystyle \Delta X\neq 0}$ or ${\displaystyle \delta X\neq 0}$ in some part of the experimental protocol; this can be either by imposition of a change ${\displaystyle \Delta Y}$, or with the holding of ${\displaystyle Y}$ constant, written ${\displaystyle \delta Y=0.}$ For the principle to hold with full generality, ${\displaystyle X}$ must be extensive or intensive accordingly as ${\displaystyle M}$ is so. Obviously, to give this scenario physical meaning, the 'driving' variable and the 'moderating' variable must be subject to separate independent experimental controls and measurements.

Explicit statement

The principle can be stated in two ways, formally different, but substantially equivalent, and, in a sense, mutually 'reciprocal'. The two ways illustrate the Maxwell relations, and the stability of thermodynamic equilibrium according to the second law of thermodynamics, evident as the spread of energy amongst the state variables of the system in response to an imposed change.

The two ways of statement share an 'index' experimental protocol (denoted ${\displaystyle {\mathcal {P}}_{\mathrm {i} }),}$ that may be described as 'changed driver, moderation permitted'. Along with the driver change ${\displaystyle \Delta L,}$ it imposes a constant ${\displaystyle Y,}$ with ${\displaystyle \delta _{\mathrm {i} }Y=0,}$ and allows the uncontrolled 'moderating' variable response ${\displaystyle \delta _{\mathrm {i} }X,}$ along with the 'index' response of interest ${\displaystyle \delta _{\mathrm {i} }M.}$

The two ways of statement differ in their respective compared protocols. One way posits a 'changed driver, no moderation' protocol (denoted ${\displaystyle {\mathcal {P}}_{\mathrm {n} }).}$ The other way posits a 'fixed driver, imposed moderation' protocol (denoted ${\displaystyle {\mathcal {P}}_{\mathrm {f} }.}$)

'Driving' variable forced to change, 'moderating' variable allowed to respond; compared with 'driving' variable forced to change, 'moderating' variable forced not to change

This way compares ${\displaystyle {\mathcal {P}}_{\mathrm {i} }}$ with ${\displaystyle {\mathcal {P}}_{\mathrm {n} },}$ to compare the effects of the imposed the change ${\displaystyle \Delta L}$ with and without moderation. The protocol ${\displaystyle {\mathcal {P}}_{\mathrm {n} }}$ prevents 'moderation' by enforcing that ${\displaystyle \Delta X=0}$ through an adjustment ${\displaystyle \Delta Y,}$ and it observes the 'no-moderation' response ${\displaystyle \Delta M.}$ Provided that the observed response is indeed that ${\displaystyle \delta _{\mathrm {i} }X\neq 0,}$ then the principle states that ${\displaystyle |\delta _{\mathrm {i} }M|<|\Delta M|}$.

In other words, change in the 'moderating' state variable ${\displaystyle X}$ moderates the effect of the driving change in ${\displaystyle L}$ on the responding conjugate variable ${\displaystyle M.}$^[4][5]

'Driving' variable forced to change, 'moderating' variable allowed to respond; compared with 'driving' variable forced not to change, 'moderating' variable forced to change

This way also uses two experimental protocols, ${\displaystyle {\mathcal {P}}_{\mathrm {i} }}$ and ${\displaystyle {\mathcal {P}}_{\mathrm {f} }}$, to compare the index effect ${\displaystyle \delta _{\mathrm {i} }M}$ with the effect ${\displaystyle \delta _{\mathrm {f} }M}$ of 'moderation' alone. The 'index' protocol ${\displaystyle {\mathcal {P}}_{\mathrm {i} }}$ is executed first; the response of prime interest, ${\displaystyle \delta _{\mathrm {i} }M,}$ is observed, and the response ${\displaystyle \Delta X}$ of the 'moderating' variable is also measured. With that knowledge, then the 'fixed driver, moderation imposed' protocol ${\displaystyle {\mathcal {P}}_{\mathrm {f} }}$ enforces that ${\displaystyle \mathrm{\Delta <!--\; \Delta \; -->}L=0,}$Zdroj:https://en.wikipedia.org?pojem=Le_chatelier's_principle
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