Name: le chatelier's principle in heating and cooling
Uploaded: 2020-09-24T15:06:51.000Z
Duration: 2 min 19 s
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Le Chatelier's Principle: Changing Temperature

La température affecte la vitesse d'une réaction chimique;donc, un changement de température donc une réaction à l'équilibre agit comme une contrainte sur le système. Le principe de le Châtelier prévoit comment le système va réagir pour minimiser ces perturbations. Un changement de température change la valeur de la constante d'équilibre, contrairement à un changement de concentration ou de volume, qui déplace l'équilibre sans changer la valeur de K.Considérons la décomposition du pentachlorure de phosphore gazeux en trichlorure de phosphore et chlore gazeux.Pour cette réaction endothermique, la chaleur absorbée peut être considérée comme un réactif. Une augmentation de la température ajoute de la chaleur au système, similaire à l'ajout d'un réactif supplémentaire. Ainsi, la position d'équilibre se déplace vers les produits et produit plus de trichlorure de phosphore et de chlore pour consommer la chaleur supplémentaire car la valeur de la constante d'équilibre, K, a augmenté.D'autre part, une diminution de la température élimine la chaleur du système, de la même manière que l'élimination d'un réactif. La position d'équilibre se déplace vers les réactifs et produit plus de pentachlorure de phosphore pour libérer de la chaleur parce que la valeur de K a diminué. Pour une réaction exothermique, telle que la réaction gazeuse entre le dioxyde de soufre et l'oxygène pour produire du trioxyde de soufre, la chaleur dégagée peut être considérée comme un produit.Une augmentation de la température est similaire à l'ajout plus d'un produit. Cela provoque la position d'équilibre se déplacer vers les réactifs, produisant plus de dioxyde de soufre et de l'oxygène pour absorber une partie de la chaleur ajoutée parce que la valeur de K a diminué. Inversement, diminuer la température de cette réaction exothermique élimine la chaleur, comme on élimine un produit.La position d'équilibre se déplace vers les produits et produit plus de trioxyde de soufre pour libérer de la chaleur puisque K a augmenté. Ainsi, une augmentation de température favorise les produits dans une réaction endothermique, alors qu'une diminution en température favorise les produits dans une réaction exothermique.

principe de le châtelier : modification de la température

Conformément à la loi d'action de masse, un équilibre contraint par une modification de la concentration se déplace pour rétablir l'équilibre sans modification de la valeur de la constante d'équilibre, K. Lorsqu'un équilibre se déplace en réponse à un changement de température, cependant, il est rétabli avec une composition relative différente qui présente une valeur différente de la constante d'équilibre. 
 Pour comprendre ce phénomène, considérons la réaction élémentaire : 
  <img alt="Eq1" src="/files/ftp_upload/11393/11393_Equation_1.png" style="height: 50px;" />
 Comme il s'agit d'une réaction élémentaire, les lois de vitesse pour le sens direct et le sens inverse peuvent être déduites directement à partir de la stœchiométrie de l'équation équilibrée : 
  <img alt="Eq2" src="/files/ftp_upload/11393/11393_Equation_2.png" style="height: 65px;" />
 Lorsque le système est à l'équilibre, 
  <img alt="Eq3" src="/files/ftp_upload/11393/11393_Equation_3.png" style="height: 50px;" />
 Le remplacement des lois de vitesse dans cette égalité et la réorganisation donne 
  <img alt="Eq4" src="/files/ftp_upload/11393/11393_Equation_4.png" style="height: 100px;" />
 La constante d'équilibre peut être exprimée comme une fonction mathématique des constantes de vitesse pour les réactions directes et inverses. Puisque les constantes de vitesse varient avec la température comme le décrit l'équation d'Arrhénius, il est logique que la constante d'équilibre varie également avec la température (en supposant que les constantes de vitesse sont impactées à des degrés différents par le changement de température). Pour les réactions plus complexes impliquant des mécanismes de réaction en plusieurs étapes, il existe une relation mathématique similaire mais plus complexe entre la constante d'équilibre et les constantes de vitesse des étapes du mécanisme. Quelle que soit la complexité de la réaction, la dépendance de sa constante d'équilibre envers la température persiste. 
 Prédire le déplacement qu'un équilibre subira en réponse à une modification de la température est réalisé plus aisément en considérant la variation d'enthalpie de la réaction. Par exemple, la formation de l’ammoniac par le procédé Haber est un processus exothermique (produisant de la chaleur) : 
  <img alt="Eq5" src="/files/ftp_upload/11393/11393_Equation_5.png" style="height: 50px;" />
 Afin d’appliquer le principe de Le Châtelier, la chaleur, q, peut être considérée comme un produit : 
  <img alt="Eq6" src="/files/ftp_upload/11393/11393_Equation_6.png" style="height: 50px;" />
 L'élévation de la température du système revient à augmenter la quantité d'un produit et l'équilibre se déplace donc vers la gauche. L'abaissement de la température du système entraîne également un déplacement vers la droite de l'équilibre. Pour les processus endothermiques, la chaleur est considérée comme un réactif de la réaction et donc on observe la dépendance opposée envers la température. 
 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 Déplacement d’équilibres :</a><a href="https://openstax.org/books/chemistry-2e/pages/13-3-shifting-equilibria-le-chateliers-principle"></a><a href="https://openstax.org/books/chemistry-2e/pages/13-3-shifting-equilibria-le-chateliers-principle"></a><a href="https://openstax.org/books/chemistry-2e/pages/13-3-shifting-equilibria-le-chateliers-principle">Le principe de Le Châtelier</a>.

Principe de le Châtelier : modification de la température

Conformément à la loi de l’action de masse, un équilibre souligné par un changement de concentration se déplacera pour rétablir l’équilibre sans aucun changement de ...

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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 forward and reverse rate laws based on the balanced equation. At equilibrium, the forward and reverse reaction rates are equal. When those rate laws are combined and rearranged, the equilibrium constant can be expressed as a function of the forward and reverse rate constants.

Because rate constants change with temperature, as described by the Arrhenius equation, the equilibrium constant can also change with temperature. This idea applies to both elementary reactions and more complex multistep mechanisms. In multistep reactions, the math is more complicated, but the temperature dependence of the equilibrium constant still remains.

A practical way to predict the direction of the shift is to use the reaction’s enthalpy change. In the Haber process, ammonia formation is exothermic, which means it produces heat. For Le Chatelier's principle, heat can be treated like a product. Raising the temperature adds heat, so the equilibrium shifts left. Lowering the temperature removes heat, so the equilibrium shifts right. For endothermic processes, heat acts like a reactant, so the temperature effect is opposite.

Le Chatelier's Principle in Heating and Cooling

le-chateliers-principle-heating-and-cooling

the equilibrium constant

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

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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...

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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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Chemical Equilibrium in Reversible Reactions

Chemical equilibrium in reversible reactions happens when a reaction moves in both directions at the same time. A reversible reaction uses a double arrow to show that products can turn back into reactants. At equilibrium, the concentrations of reactants and products stay constant over time because the forward and reverse reaction rates are equal. The video uses the reversible decomposition of colorless dinitrogen tetroxide to brown nitrogen dioxide as an example. At time zero, there is finite...

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Equilibrium concentrations can be found from initial concentrations and an equilibrium constant. This skill matters in many areas of science and technology, including pharmaceutical product formulation and dosing. After a drug is ingested or injected, it may take part in several chemical equilibria that change its final concentration in the body system of interest. Scientists use these quantitative equilibrium relationships to help calculate a dosage that will produce the desired therapeutic...

le chatelier's principle: changing volume (pressure)

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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...

le chatelier's principle: changing temperature

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...

the small x assumption

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.

key terms and concepts

Learn how heating and cooling change chemical equilibrium, why K changes with temperature, and how exothermic and endothermic reactions shift.

Le Chatelier's Principle in Heating and Cooling | JoVE K12

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Le Chatelier's Principle in Heating and Cooling

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