Name: shifting equilibrium by changing concentration
Uploaded: 2020-09-24T15:06:50.000Z
Duration: 2 min 27 s
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Chemistry

Matter and its Interactions

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Le Chatelier's Principle: Changing Concentration

Un sistema está en equilibrio dinámico cuando la velocidad de la reacción directa es igual a la velocidad de la reacción inversa. Aunque la reacción sigue avanzando en ambas direcciones, las concentraciones netas de reactivos y productos permanecen constantes. En este punto, si se cambia la concentración de un reactivo o producto por ejemplo, agregando una molécula A adicional o eliminando alguna molécula C, el equilibrio se altera.Tales cambios son tensiones en el sistema. De acuerdo con el principio de Le Châtelier, cuando un sistema en equilibrio se somete a tensión debido a un cambio de concentración, un cambio de volumen o presión, o un cambio de temperatura, la posición de equilibrio cambia en una dirección para minimizar la tensión y restablecer el equilibrio. Para entender la respuesta de un sistema a los cambios de concentración, considere dos tanques de agua conectados, A y B.Inicialmente, el nivel de agua en los dos tanques es igual y el sistema está en equilibrio.Luego, se agrega agua al tanque B.Este estrés eleva el nivel del agua en el tanque B y perturba el equilibrio. Para compensar esta tensión, parte del agua fluye hacia el tanque A para restablecer el equilibrio. Por el contrario, si se extrae agua del tanque B, la disminución del nivel de agua también provoca estrés.En consecuencia, el agua del tanque A fluye hacia el tanque B para restablecer el equilibrio. Para un sistema en equilibrio, el cociente de reacción es igual a la constante de equilibrio. Agregar más reactivos a la mezcla de equilibrio disminuye a Q.De acuerdo con el principio de Le Châtelier, el sistema minimiza esta tensión desplazando la posición de equilibrio hacia los productos para consumir el reactivo añadido y restablecer el equilibrio.Asimismo, eliminar un producto de la mezcla de equilibrio conducirá a una respuesta similar para restaurar el equilibrio. En contraste, agregar más productos aumenta Q.Por lo tanto, la posición de equilibrio se desplaza hacia los reactivos para gastar el producto agregado y restablecer el equilibrio. Un cambio en la concentración cambia la posición de equilibrio sin cambiar el valor de K.

principio de le chatelier: cambios de concentración

Un sistema en equilibrio se encuentra en un estado de equilibrio dinámico, con reacciones hacia delante y hacia atrás que tienen lugar a velocidades iguales. Si un sistema en equilibrio es sometido a un cambio en las condiciones que afecta a estas velocidades de reacción de forma diferente (una tensión), las velocidades ya no son iguales y el sistema no está en equilibrio. El sistema experimentará posteriormente una reacción neta en la dirección de una velocidad mayor (un cambio) que restablecerá el equilibrio. Este fenómeno se resume en el principio de Le Châtelier: Si un sistema en equilibrio es sometido a una tensión, el sistema experimentará un cambio en respuesta a la tensión que restablece el equilibrio.
 Las velocidades de reacción se ven afectadas principalmente por las concentraciones, como se describe en la ley de velocidad de reacción, y la temperatura, como se describe en la ecuación de Arrhenius. En consecuencia, los cambios en la concentración y la temperatura son los dos estresores que pueden desplazar un equilibrio.
 Efecto de un cambio en la concentración
 Si un sistema en equilibrio es sometido a un cambio en la concentración de un reactante o de un producto, la velocidad de la reacción directa o inversa cambiará. Por ejemplo, considere la reacción en equilibrio:
 <img alt="Eq1" src="/files/ftp_upload/11391/11391_Equation 1v2.png" style="text-align: center; height: 48px;" />
 Cuando este sistema está en equilibrio, las velocidades de reacción hacia delante y hacia atrás son iguales.
 <img alt="Eq2" src="/files/ftp_upload/11391/11391_Equation 2.png" style="text-align: center; height: 50px;" />
 Si el sistema se estresa añadiendo reactivo, ya sea N2 u O2, el aumento de concentración resultante hace que aumente la velocidad de la reacción hacia delante, superando la de la reacción inversa:
 <img alt="Eq3" src="/files/ftp_upload/11391/11391_Equation 3.png" style="text-align: center; height: 50px;" />
 El sistema experimentará una reacción neta temporal en la dirección de avance para restablecer el equilibrio (el equilibrio cambiará hacia la derecha). Este mismo cambio se producirá si se elimina alguna cantidad de producto, NO, del sistema, lo que disminuye la velocidad de la reacción inversa, lo que de nuevo dará lugar al mismo desequilibrio en las velocidades.
 La misma lógica se puede utilizar para explicar el cambio hacia la izquierda que resulta de eliminar el reactivo o añadir producto a un sistema en equilibrio. Ambas tensiones dan como resultado un aumento de la velocidad de la reacción inversa
 <img alt="Eq4" src="/files/ftp_upload/11391/11391_Equation 4.png" style="text-align: center; height: 50px;" />
 y una reacción neta temporal en la dirección inversa para restablecer el equilibrio.
 Como alternativa a esta interpretación cinética, el efecto de los cambios en la concentración sobre los equilibrios puede racionalizarse en términos del cociente de reacción. Cuando el sistema está en equilibrio,
 <img alt="Eq5" src="/files/ftp_upload/11391/11391_Equation 5.png" style="text-align: center; height: 70px;" />
 Si se añade un reactivo (aumentando el denominador del cociente de la reacción) o se elimina el producto (disminuyendo el numerador), Qc < Kc y el equilibrio se desplazará hacia la derecha. Tenga en cuenta que las tres formas diferentes de inducir esta tensión producen tres cambios diferentes en la composición de la mezcla en equilibrio. Si se añade N2, el cambio hacia la derecha consumirá O2 y producirá NO a medida que se restablece el equilibrio, produciendo una mezcla con una concentración mayor de N2 y NO y una concentración de O2 menor que la que estaba presente previamente. Si se añade O2, la nueva mezcla en equilibrio tendrá mayores concentraciones de O2 y NO y una menor concentración de N2. Finalmente, si se elimina NO, la nueva mezcla en equilibrio tendrá mayores concentraciones de N2 y O2 y una menor concentración de NO. A pesar de estas diferencias en la composición, el valor de la constante de equilibrio será el mismo antes y después de la tensión (según la ley de acción de masas). Se puede aplicar la misma lógica para las tensiones que implican la eliminación de reactivos o la adición de productos, en cuyo caso Qc > Kc y el equilibrio se desplazará hacia la izquierda.
 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: Principio de LeChatelier</a>.

Principio de le Chatelier: Cambios de Concentración

Un sistema en equilibrio se encuentra en un estado de equilibrio dinámico, con reacciones hacia adelante y hacia atrás que tienen lugar a velocidades iguales. Si un sistema de equilibrio es ...

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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

shifting-equilibrium-by-changing-concentration

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...

homogeneous equilibria for gaseous reactions

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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...

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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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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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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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

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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...

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

Shifting Equilibrium by Changing Concentration | JoVE K12

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Shifting Equilibrium by Changing Concentration

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