Name: shifting equilibrium by changing concentration
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Le Chatelier's Principle: Changing Concentration

Un système est en équilibre dynamique lorsque la vitesse de la réaction directe équivaut à la vitesse de la réaction inverse. Bien que la réaction continue dans les deux sens, les concentrations nettes de réactifs et de produits restent constantes. À ce stade, si la concentration d'un réactif ou d'un produit est modifiée, par exemple en ajoutant une molécule A supplémentaire ou en supprimant une certaine molécule C, l'équilibre est perturbé.De tels changements sont des contraintes sur le système. Selon le principe de Le Châtelier, quand un système à l'équilibre est sollicité en raison d'un changement de concentration, un changement de volume ou de pression, ou un changement de température, la position d'équilibre se déplace dans une direction pour minimiser le stress et pour rétablir l'équilibre. Pour comprendre la réponse d'un système aux changements en concentration, considérez deux réservoirs d'eau connectés, A et B.Initialement, le niveau d'eau dans les deux réservoirs est égal et le système est en équilibre.Ensuite, de l'eau est ajoutée au réservoir B.Cette contrainte élève le niveau d'eau dans le réservoir B et perturbe l'équilibre. Pour compenser ce stress, une partie de l'eau s'écoule dans le réservoir A pour rétablir l'équilibre. Inversement, si l'eau est retirée du réservoir B, l'abaissement du niveau de l'eau déclenche également le stress.Par conséquent, l'eau du réservoir A s'écoule dans le réservoir B pour rétablir l'équilibre. Pour un système équilibré, le quotient de réaction est égale à la constante d'équilibre. L'ajout de réactifs supplémentaires au mélange d'équilibre diminue Q.Selon le principe de Le Châtelier, le système minimise ce stress en déplaçant la position de l'équilibre vers les produits pour dépenser le réactif ajouté et rétablir l'équilibre.De même, retirer un produit du mélange d'équilibre mènera à une réponse similaire dans l'ordre pour rétablir l'équilibre. En revanche, ajouter plus de produits augmente Q.Ainsi, la position d'équilibre se déplace vers les réactifs pour dépenser le produit ajouté et rétablir l'équilibre. Un changement de concentration modifie la position d'équilibre sans changer la valeur de K.

principe de le châtelier : modification de la concentration

Un système à l'équilibre est dans un état d'équilibre dynamique, avec des réactions directe et inverse se déroulant à des vitesses égales. Si un système en équilibre est soumis à des modifications des conditions qui influencent ces vitesses de réaction différemment (une contrainte), alors les vitesses ne sont plus égales et le système n'est pas à l'équilibre. Le système connaîtra ensuite une réaction nette dans le sens d'une vitesse plus grande (un déplacement) qui permettra de rétablir l'équilibre. Ce phénomène est résumé par le principe de Le Châtelier : si un système en équilibre subit une contrainte, le système sera soumis à un changement en réponse à la contrainte qui rétablit l'équilibre. 
 Les vitesses de réaction sont principalement influencées par les concentrations, décrites par la loi de vitesse de la réaction, et par la température, telle que décrite par l’équation d’Arrhénius. Par conséquent, les modifications de la concentration et de la température sont les deux contraintes qui peuvent déplacer un équilibre. 
 Effet d'une modification de la concentration 
 Si un système en équilibre est soumis à une modification de la concentration d'une espèce de réactif ou de produit, la vitesse de la réaction directe ou inverse change. Prenons l'exemple de la réaction en équilibre : 
 <img alt="Eq1" src="/files/ftp_upload/11391/11391_Equation 1v2.png" style="text-align: center; height: 48px;" />
 Lorsque ce système est à l'équilibre, les vitesses des réactions directe et inverse sont égales. 
 <img alt="Eq2" src="/files/ftp_upload/11391/11391_Equation 2.png" style="text-align: center; height: 50px;" />
 Si le système est contraint par l'ajout de réactif, N2 ou O2, l'augmentation de la concentration qui en résulte entraîne une augmentation de la vitesse da la réaction directe, dépassant celle de la réaction inverse : 
 <img alt="Eq3" src="/files/ftp_upload/11391/11391_Equation 3.png" style="text-align: center; height: 50px;" />
 Le système va subir une réaction nette temporaire dans le sens direct pour rétablir l'équilibre (l'équilibre se déplace vers la droite). Ce même déplacement se produit si un peu du produit NO est retiré du système, ce qui diminue la vitesse de la réaction inverse, entraînant à nouveau le même déséquilibre dans les vitesses. 
 La même logique peut être utilisée pour expliquer le déplacement à gauche qui résulte lorsqu'on enlève du réactif ou qu'on ajoute du produit à un système en équilibre. Ces deux contraintes entraînent une augmentation de la vitesse pour la réaction inverse 
 <img alt="Eq4" src="/files/ftp_upload/11391/11391_Equation 4.png" style="text-align: center; height: 50px;" />
 et une réaction nette temporaire dans le sens inverse pour rétablir l'équilibre. 
 Comme alternative à cette interprétation cinétique, l'effet des modifications de la concentration sur les équilibres peut être rationalisé en termes de quotients réactionnels. Lorsque le système est à l'équilibre, 
 <img alt="Eq5" src="/files/ftp_upload/11391/11391_Equation 5.png" style="text-align: center; height: 70px;" />
 Si un réactif est ajouté (augmentant le dénominateur du quotient réactionnel) ou si du produit est enlevé (diminuant le numérateur), alors Qc < Kc et l'équilibre se déplacera vers la droite. Notez que les trois façons différentes d'induire cette contrainte entraînent trois changements différents dans la composition du mélange de l'équilibre. Si N2 est ajouté, le déplacement à droite consommera de l'O2 et produira du NO au fur et à mesure que l'équilibre se rétablit, donnant un mélange avec une plus grande concentration de N2 et de NO et une plus faible concentration d'O2 que celle qui était présente auparavant. Si O2 est ajouté, le nouveau mélange de l'équilibre aura des concentrations plus élevées d'O2 et de NO et une concentration moindre de N2. Enfin, si on enlève du NO, le nouveau mélange d'équilibre aura des concentrations plus élevées de N2 et d'O2 et une concentration moindre de NO. Malgré ces différences de composition, la valeur de la constante d'équilibre sera la même après et avant la contrainte (selon la loi d'action de masse). La même logique peut être appliquée pour les contraintes impliquant la suppression de réactifs ou l'ajout de produit, auquel cas Qc > Kc et l'équilibre se déplacera vers la gauche. 
 Ce texte a été adapté d’<a href="https://openstax.org/books/chemistry-2e/pages/13-3-shifting-equilibria-le-chateliers-principle">Openstax, Chimie 2e, Section 13.3 Déplacements d'équilibres : le principe de Le Chatelier</a>.

Principe de Le Châtelier : modification de la concentration

Un système à l’équilibre est dans un état d’équilibre dynamique, avec des réactions avant et arrière se déroulant à des rythmes égaux. Si un système d’équilibre est...

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shifting equilibrium by changing concentration

Chemical equilibrium is a dynamic balance. At equilibrium, the forward and reverse reactions happen at equal rates. If the system is stressed in a way that changes one rate more than the other, it is no longer at equilibrium and will shift until balance is restored. This behavior is described by Le Châtelier’s principle.

Changes in concentration are a major stress on an equilibrium system. Reaction rates depend mainly on concentration and temperature. For that reason, changing the amount of a reactant or product can move the equilibrium in one direction or the other.

Adding a reactant, such as N2 or O2 in the example reaction, increases the forward reaction rate. The forward rate then becomes greater than the reverse rate, so the equilibrium shifts right. Removing the product NO has the same effect because it lowers the reverse reaction rate and creates the same imbalance.

The opposite changes cause a left shift. Removing reactant or adding product increases the reverse reaction rate compared with the forward rate. The system then moves in the reverse direction until equilibrium is reached again.

This shift can also be explained with the reaction quotient, Qc, and the equilibrium constant, Kc. When reactant is added, Qc becomes less than Kc. When product is removed, Qc also becomes less than Kc, so the equilibrium shifts right. If reactant is removed or product is added, then Qc is greater than Kc and the equilibrium shifts left.

Although the final composition changes, the equilibrium constant stays the same after the stress is removed and the system rebalances. If N2 is added, the new mixture has more N2 and NO and less O2. If O2 is added, the mixture has more O2 and NO and less N2. If NO is removed, the mixture ends with more N2 and O2 and less NO. The same idea applies to shifts caused by removing reactant or adding product.

Shifting Equilibrium by Changing Concentration

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Chemical Equilibrium and Kc Rules

Chemical equilibrium and the equilibrium constant Kc describe how reactants and products are balanced in a reaction. In the oxidation of sulfur dioxide, a mixture that starts with reactants only has no product at first. As the reaction moves forward toward equilibrium, reactant concentrations decrease and product concentration increases. When equilibrium is reached, the concentrations of both stay constant. A reaction can also begin with only products present. In that case, the reaction moves...

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Relating Kc and Kp in Gas Equilibria

Gas-phase equilibrium can be written using either concentration or pressure. The equilibrium constant may be shown as Kc, which uses molar concentration, or as Kp, which uses partial pressure of the reactants and products. The link between Kc and Kp comes from the ideal gas equation and the definition of molarity. Molarity is the number of moles divided by volume. In the ideal gas equation, P stands for partial pressure, V for volume, n for number of moles, R for the gas constant, and T for...

le chatelier's principle: changing concentration

Chemical equilibrium is a dynamic balance. At equilibrium, the forward and reverse reactions happen at equal rates. If the system is stressed in a way that changes one rate more than the other, it is no longer at equilibrium and will shift until balance is restored. This behavior is described by Le Châtelier’s principle. Changes in concentration are a major stress on an equilibrium system. Reaction rates depend mainly on concentration and temperature. For that reason, changing the amount of a...

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Finding Kc from Equilibrium Data

The equilibrium constant, Kc, can be found from the equilibrium concentrations of reactants and products. When those concentrations are known, the calculation is straightforward. The values are placed into the Kc expression for the reaction. One example uses nitrogen dioxide and dinitrogen tetroxide. When 0.10 mol NO2 is added to a 1.0-L flask at 25 °C, the concentrations change until equilibrium is reached. At equilibrium, [NO2] is 0.016 M and [N2O4] is 0.042 M, and these values are used to...

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Reaction quotient Q helps track how far a reversible reaction has moved toward equilibrium. For a balanced reaction such as m A + n B ⇌ x C + y D, Q comes directly from the stoichiometric coefficients in the equation. When concentrations are used, the expression is written as Qc. For gases, a similar form uses partial pressures. These expressions use relative concentration and pressure values, not absolute values. That means reaction quotients are dimensionless and have no units. As a reaction...

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Gas Equilibrium Under Volume Changes

Gas-phase equilibrium can shift when the volume of the system changes. In these reactions, a change in pressure changes concentration because the partial pressure of an ideal gas is proportional to its molar concentration. That means pressure changes affect equilibrium in the same way as concentration changes. When the volume of a gas mixture changes, the concentrations of reactants and products change together. Since every gas in the equilibrium occupies the same space, the same volume change...

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Le Chatelier's Principle in Heating and Cooling

Le Chatelier's principle helps explain how chemical equilibrium responds to heating and cooling. A system at equilibrium will shift to restore balance after a temperature change, but the equilibrium constant, K, will not stay the same. Instead, the mixture reaches a new equilibrium with a different composition. This temperature effect is different from a concentration change, which shifts equilibrium without changing K. To understand why temperature matters, the reaction can be written with...

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Estimating Equilibrium with Small Changes

A small equilibrium constant can make equilibrium calculations easier. In these reactions, the equilibrium position favors the reactants. When the initial reactant concentration is high and the Kc value is small, the change in concentration may be negligible. In that case, the equilibrium concentration is close to the initial concentration. This idea helps simplify weak acid and weak base problems. Chemists can assume that some equilibrium concentrations are equal to the initial concentrations.

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Learn how changing concentration shifts chemical equilibrium, including why adding or removing reactants or products moves the reaction left or right.

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Shifting Equilibrium by Changing Concentration

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