Name: le chatelier's principle in heating and cooling
Uploaded: 2020-09-24T15:06:51.000Z
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Chemistry

Matter and its Interactions

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Le Chatelier's Principle: Changing Temperature

La temperatura afecta la velocidad de una reacción química;por lo tanto, un cambio de temperatura para una reacción en equilibrio actúa como una tensión en el sistema. El principio de Le Châtelier predice cómo responderá el sistema para minimizar tales perturbaciones. Un cambio en la temperatura cambia el valor de la constante de equilibrio, a diferencia de un cambio en la concentración o el volumen, que cambia el equilibrio sin cambiar el valor de K.Considere la descomposición del pentacloruro de fósforo gaseoso en tricloruro de fósforo y cloro gaseoso.Para esta reacción endotérmica, el calor absorbido se puede considerar como un reactivo. Un aumento de temperatura agrega calor al sistema, similar a agregar más de un reactivo. Por lo tanto, la posición de equilibrio se desplaza hacia los productos y produce más tricloruro de fósforo y cloro para consumir el calor adicional porque el valor de la constante de equilibrio, K, ha aumentado.Por otro lado, una disminución de la temperatura elimina el calor del sistema, similar a la eliminación de un reactivo. La posición de equilibrio se desplaza hacia los reactivos y produce más pentacloruro de fósforo para liberar calor porque el valor de K ha disminuido. Para una reacción exotérmica, como la reacción gaseosa entre el dióxido de azufre y el oxígeno para producir trióxido de azufre, el calor liberado se puede considerar como un producto.Un aumento de temperatura es similar a agregar más de un producto. Esto hace que la posición de equilibrio se desplace hacia los reactivos, produciendo más dióxido de azufre y oxígeno para absorber parte del calor agregado porque el valor de K ha disminuido. Por el contrario, la disminución de la temperatura de esta reacción exotérmica elimina el calor, como elimina un producto.La posición de equilibrio se desplaza hacia los productos y produce más trióxido de azufre para liberar calor a medida que aumenta K.Así, un aumento de temperatura favorece los productos en una reacción endotérmica, mientras que una disminución de temperatura favorece los productos en una reacción exotérmica.

principio de le chatelier: cambios de temperatura

De acuerdo con la ley de acción de masas, un equilibrio estresado por un cambio en la concentración, cambiará para restablecer el equilibrio sin que se produzca ningún cambio en el valor de la constante de equilibrio, K. Cuando un equilibrio cambia en respuesta a un cambio de temperatura, sin embargo, se restablece con una composición relativa diferente que muestra un valor diferente para la constante de equilibrio.
 Para entender este fenómeno, considere la reacción elemental:
  <img alt="Eq1" src="/files/ftp_upload/11393/11393_Equation_1.png" style="height: 50px;" />
 Dado que se trata de una reacción elemental, las leyes de velocidad para el avance y el retroceso pueden derivarse directamente de la estequiometría de la ecuación balanceada:
  <img alt="Eq2" src="/files/ftp_upload/11393/11393_Equation_2.png" style="height: 65px;" />
 Cuando el sistema está en equilibrio,
  <img alt="Eq3" src="/files/ftp_upload/11393/11393_Equation_3.png" style="height: 50px;" />
 Substituyendo las leyes de velocidad en esta igualdad y reorganizando se obtiene
  <img alt="Eq4" src="/files/ftp_upload/11393/11393_Equation_4.png" style="height: 100px;" />
 La constante de equilibrio se puede expresar como una función matemática de las constantes de velocidad para las reacciones de avance y retroceso. Dado que las constantes de velocidad varían con la temperatura, tal como se describe en la ecuación de Arrhenius, es lógico que la constante de equilibrio también varíe con la temperatura (suponiendo que las constantes de velocidad se vean afectadas en diferentes grados por el cambio de temperatura). Para reacciones más complejas que implican mecanismos de reacción de varios pasos, existe una relación matemática similar pero más compleja entre la constante de equilibrio y las constantes de velocidad de los pasos en el mecanismo. Independientemente de lo compleja que sea la reacción, persiste la dependencia entre la constante de equilibrio y la temperatura.
 Predecir el cambio que un equilibrio experimentará en respuesta a un cambio en la temperatura se logra más convenientemente considerando el cambio de entalpía de la reacción. Por ejemplo, la formación de amoníaco por el proceso de Haber es un proceso exotérmico (que produce calor):
  <img alt="Eq5" src="/files/ftp_upload/11393/11393_Equation_5.png" style="height: 50px;" />
 Para aplicar el principio de Le Châtelier, el calor,q, puede considerarse un producto:
  <img alt="Eq6" src="/files/ftp_upload/11393/11393_Equation_6.png" style="height: 50px;" />
 Elevar la temperatura del sistema es similar a aumentar la cantidad de un producto, y así el equilibrio se desplazará hacia la izquierda. Al bajar la temperatura del sistema, el equilibrio se desplazará hacia la derecha. Para los procesos endotérmicos, el calor se considera como un reactivo de la reacción y, por lo tanto, se observa la dependencia opuesta de la temperatura.
 Este texto ha sido adaptado de<a href="https://openstax.org/books/chemistry-2e/pages/13-3-shifting-equilibria-le-chateliers-principle"> Openstax, Química 2e, Sección 13.3 Cambios de los Equilibrios:</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"> Principio de Le Châtelier</a>.

Principio de le Chatelier: Cambios de Temperatura

De acuerdo con la ley de la acción de las masas, un equilibrio estresado por un cambio en la concentración cambiará para restablecer el equilibrio sin ningún cambio en ...

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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...

calculating the equilibrium constant

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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