Name: gas equilibrium under volume changes
Uploaded: 2020-09-24T15:06:51.000Z
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Le Chatelier's Principle: Changing Volume (Pressure)

El principio de Le Châtelier se puede utilizar para predecir cómo respondería un sistema en equilibrio al estrés de un cambio de volumen o presión. El volumen de un gas es inversamente proporcional a su presión;por lo tanto, para un sistema en equilibrio, una disminución de volumen aumenta la presión y perturba el equilibrio. En respuesta, la posición de equilibrio cambiará en una dirección para minimizar la tensión.La ley de los gases ideales establece que la presión de un gas es directamente proporcional al número de moles. Por lo tanto, la dirección del cambio necesaria para restablecer el equilibrio depende del número de moles de partículas de gas en cada lado de la reacción. A medida que más moles de gas dan como resultado una presión más alta, un aumento en la presión desplaza la posición de equilibrio hacia el lado con menos moles para reducir la presión.Asimismo, una disminución de la presión desplaza la posición de equilibrio hacia el lado con más moles de gas. Considere un equilibrio químico, donde un mol de pentacloruro de fósforo gaseoso se descompone en un mol de tricloruro de fósforo y un mol de cloro gaseoso, dos moles totales de producto. Si se empuja el pistón hacia abajo, el volumen del sistema de equilibrio disminuye, aumentando la presión.Esto altera el equilibrio y da como resultado que Q sea mayor que K.Por lo tanto, la posición de equilibrio se desplaza hacia los reactivos, con menos moles de partículas de gas, para disminuir la presión y restablecer el equilibrio. Por el contrario, tirar del pistón hacia arriba aumenta el volumen y disminuye la presión. En este caso, Q se vuelve menor que K.Para aumentar la presión, la posición de equilibrio se desplaza hacia los productos, el lado con más moles de gas, y se restablece el equilibrio.El aumento de la presión añadiendo un gas inerte a una mezcla de equilibrio a volumen constante no afecta el equilibrio porque las presiones parciales de los reactivos y productos gaseosos permanecen sin cambios. Para sistemas en equilibrio con igual número de moles de reactivos y productos gaseosos, como la reacción entre gas yodo y gas cloro para producir monocloruro de yodo, un cambio en el volumen del sistema no tendrá ningún efecto sobre el equilibrio.

principio de le chatelier: cambios de volumen (presión)

En el caso de los equilibrios de fase gaseosa, pueden producirse cambios en las concentraciones de los reactivos y productos con un volumen y una presión alterados. La presión parcial, P, de un gas ideal es proporcional a su concentración molar, M.
 <img alt="Eq1" src="/files/ftp_upload/11392/11392_Equation 1.png" style="text-align: center; height: 70px;" />
 Así que, los cambios en las presiones parciales de cualquier reactivo o producto son esencialmente cambios en las concentraciones; por lo tanto, estos cambios producen los mismos efectos en los equilibrios. Además de añadir o eliminar reactivos o productos, las presiones (concentraciones) de las especies en un equilibrio de fase gaseosa también pueden cambiarse cambiando el volumen ocupado por el sistema. Dado que todas las especies de un equilibrio de fase gaseosa ocupan el mismo volumen, un cambio dado en el volumen causará el mismo cambio en la concentración tanto de los reactivos como de los productos. Para discernir qué cambio inducirá esta tensión, si hay alguno, la estequiometría de la reacción debe ser considerada.
 En equilibrio, la reacción N2 (g) + O2 (g) ⇌ 2 NO (g) es descrita por el cociente de reacción
 <img alt="Eq2" src="/files/ftp_upload/11392/11392_Equation 2.png" style="text-align: center; height: 70px;" />
 Si el volumen ocupado por una mezcla de estas especies en equilibrio se reduce en un factor de 3, las presiones parciales de las tres especies se incrementarán en un factor de 3:
 <img alt="Eq3" src="/files/ftp_upload/11392/11392_Equation 3.png" style="text-align: center; height: 70px;" />
 Y así, cambiar el volumen de esta mezcla de fase gaseosa en equilibrio no produce un cambio de equilibrio.
 No obstante, un tratamiento similar de un sistema diferente, 2 SO2 (g) + O2 (g) ⇌ 2 SO3 (g), produce un resultado diferente: 
 <img alt="Eq4" src="/files/ftp_upload/11392/11392_Equation 4.png" style="text-align: center; height: 200px;" />
 En este caso, el cambio de volumen produce un cociente de reacción menor que la constante de equilibrio, por lo que el equilibrio se desplazará hacia la derecha.
 Estos resultados ilustran la relación entre la estequiometría de un equilibrio de fase gaseosa y el efecto de un cambio de presión (concentración) inducido por volumen. Si las cantidades molares totales de reactivos y productos son iguales, como en el primer ejemplo, un cambio en el volumen no cambia el equilibrio. Si las cantidades molares de reactivos y productos son diferentes, un cambio en el volumen cambiará el equilibrio en la dirección que mejor “se ajuste” al cambio de volumen. En el segundo ejemplo, tres moles de reactivo (SO2 y O2) producen dos moles de producto (SO3), y por lo tanto, la disminución del volumen del sistema hace que el equilibrio se desplace hacia la derecha, ya que la reacción de formación produce menos gas (2 mol) que la reacción de descomposición (3 mol). A la inversa, el aumento del volumen de este sistema en equilibrio daría lugar a un cambio hacia los reactantes.
 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 en los Equilibrios: Principio de LeChatelier</a>.

Principio de le Chatelier: Cambios de Volumen (Presión)

En el caso de los equilibrios en fase gaseosa, pueden producirse cambios en las concentraciones de reactivos y productos con la alteración del volumen y la presión. La presión parcial, P, de ...

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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 affects each species in the same way. To predict whether the equilibrium shifts, you must look at the reaction stoichiometry.

For the equilibrium N2(g) + O2(g) ⇌ 2 NO(g), decreasing the volume by a factor of 3 increases the partial pressures of all three gases by a factor of 3. In this case, the reaction quotient still matches the equilibrium position, so the equilibrium does not shift. The total moles of reactants and products are equal, and that is why the volume change has no effect.

A different result appears for 2 SO2(g) + O2(g) ⇌ 2 SO3(g). Here, decreasing the volume makes the reaction quotient smaller than the equilibrium constant, so the equilibrium shifts right. Three moles of reactant form two moles of product, so the forward reaction makes less gas. Increasing the volume would shift this system toward the reactants.

Gas Equilibrium Under Volume Changes

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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...

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

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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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Using Q to Track Equilibrium Changes

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

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 from an ICE Table

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)

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.

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Learn how changing volume changes pressure and concentration in gas-phase equilibrium, and why stoichiometry decides whether the system shifts left or right.

Gas Equilibrium Under Volume Changes | JoVE K12

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Gas Equilibrium Under Volume Changes

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