Name: gas equilibrium under volume changes
Uploaded: 2020-09-24T15:06:51.000Z
Duration: 2 min 32 s
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Chemistry

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PS.Chm.5.2

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Le Chatelier's Principle: Changing Volume (Pressure)

Das Le Châtelier-Prinzip kann verwendet werden, um vorherzusagen, wie ein System im Gleichgewicht auf die Belastung durch eine Volumen- oder Druckänderung reagieren würde.
Das Volumen eines Gases ist umgekehrt proportional zu seinem Druck, daher erhöht bei einem Gleichgewichtssystem eine Abnahme des Volumens den Druck und stört das Gleichgewicht. Als Reaktion darauf verschiebt sich die Gleichgewichtsposition in eine Richtung, um die Belastung zu minimieren.
Das ideale Gasgesetz besagt, dass der Druck eines Gases direkt proportional zur Anzahl der Mol ist. Die Richtung der Verschiebung, die zur Wiederherstellung des Gleichgewichts erforderlich ist, hängt also von der Anzahl der Mol Gasteilchen auf beiden Seiten der Reaktion ab.
Da mehr Mol Gas zu einem höheren Druck führt, verschiebt ein Druckanstieg die Gleichgewichtsposition zur Seite, mit weniger Mol, um den Druck zu senken. Ebenso verschiebt ein Druckabfall die Gleichgewichtsposition mit mehr Mol Gas zur Seite.
Stellen Sie sich ein chemisches Gleichgewicht vor, in dem sich ein Mol gasförmiges Phosphorpentachlorid in ein Mol Phosphortrichlorid und ein Mol Chlorgas zersetzt – insgesamt zwei Mol Produkt.
Wird der Kolben nach unten gedrückt, verringert sich das Volumen des Gleichgewichtssystems und der Druck steigt. Dies stört das Gleichgewicht und führt dazu, dass Q größer als K ist. Auf diese Weise verschiebt sich die Gleichgewichtsposition in Richtung der Reaktanten, mit weniger Mol Gasteilchen, um den Druck zu senken und das Gleichgewicht wiederherzustellen.
Umgekehrt erhöht das Hochziehen des Kolbens das Volumen und verringert den Druck. In diesem Fall wird Q kleiner als K. Um den Druck zu erhöhen, verschiebt sich die Gleichgewichtsposition in Richtung der Produkte, der Seite mit den meisten Mol Gas, und stellt das Gleichgewicht wieder her.
Die Erhöhung des Drucks durch Zugabe eines Inertgases zu einem Gleichgewichtsgemisch bei konstantem Volumen hat keinen Einfluss auf das Gleichgewicht, da die Partialdrücke der gasförmigen Reaktanten und Produkte unverändert bleiben.
Bei Gleichgewichtssystemen mit gleichen Molzahlen gasförmiger Reaktanten und Produkte, wie z. B. die Reaktion zwischen Jodgas und Chlorgas zur Erzeugung von Jodmonochlorid, hat eine Änderung des Volumens des Systems keinen Einfluss auf das Gleichgewicht.

das prinzip von le chatelier: volumen (druck) ändern

Bei Gasphasengleichgewichten kann es bei verändertem Volumen und Druck zu Konzentrationsänderungen der Reaktanten und Produkte kommen. Der Partialdruck P eines idealen Gases ist proportional zu seiner molaren Konzentration M. <img alt="Eq1" src="/files/ftp_upload/11392/11392_Equation 1.png" style="text-align: center; height: 70px;"> Änderungen im Partialdruck eines Reaktanten oder Produkts sind also im Wesentlichen Konzentrationsänderungen; Daher haben diese Änderungen die gleichen Auswirkungen auf Gleichgewichte. Neben dem Hinzufügen oder Entfernen von Reaktanten oder Produkten können die Drücke (Konzentrationen) von Spezies in einem Gasphasengleichgewicht auch durch Ändern des vom System eingenommenen Volumens geändert werden. Da alle Spezies eines Gasphasengleichgewichts das gleiche Volumen einnehmen, führt eine gegebene Volumenänderung zu der gleichen Konzentrationsänderung sowohl für die Reaktanten als auch für die Produkte. Um zu erkennen, welche Verschiebung, wenn überhaupt, diese Art von Stress hervorruft, muss die Stöchiometrie der Reaktion berücksichtigt werden. Im Gleichgewicht wird die Reaktion N2 (g) + O2 (g) ⇌ 2 NO (g) durch den Reaktionsquotienten beschrieben <img alt="Eq2" src="/files/ftp_upload/11392/11392_Equation 2.png" style="text-align: center; height: 70px;"> Wenn das von einer Gleichgewichtsmischung dieser Arten eingenommene Volumen um den Faktor 3 verringert wird, erhöhen sich die Partialdrücke aller drei Arten um den Faktor 3: <img alt="Eq3" src="/files/ftp_upload/11392/11392_Equation 3.png" style="text-align: center; height: 70px;"> Daher führt eine Änderung des Volumens dieses Gasphasen-Gleichgewichtsgemisches nicht zu einer Verschiebung des Gleichgewichts. Eine ähnliche Behandlung eines anderen Systems, 2 SO2 (g) + O2 (g) ⇌ 2 SO3 (g), führt jedoch zu einem anderen Ergebnis: <img alt="Eq4" src="/files/ftp_upload/11392/11392_Equation 4.png" style="text-align: center; height: 200px;"> In diesem Fall führt die Volumenänderung zu einem Reaktionsquotienten, der kleiner als die Gleichgewichtskonstante ist, und somit verschiebt sich das Gleichgewicht nach rechts. Diese Ergebnisse veranschaulichen den Zusammenhang zwischen der Stöchiometrie eines Gasphasengleichgewichts und der Auswirkung einer volumeninduzierten Druck- (Konzentrations-)Änderung. Wenn die Gesamtmolmengen der Reaktanten und Produkte gleich sind, wie im ersten Beispiel, verschiebt eine Volumenänderung das Gleichgewicht nicht. Wenn die molaren Mengen von Reaktanten und Produkten unterschiedlich sind, verschiebt eine Volumenänderung das Gleichgewicht in eine Richtung, die der Volumenänderung besser „aufpasst“. Im zweiten Beispiel ergeben drei Mol Reaktant (SO2 und O2) zwei Mol Produkt (SO3). Durch eine Verringerung des Systemvolumens verschiebt sich das Gleichgewicht nach rechts, da bei der Vorwärtsreaktion weniger Gas (2 Mol) entsteht als bei der Rückreaktion (3 Mol). Umgekehrt würde eine Vergrößerung des Volumens dieses Gleichgewichtssystems zu einer Verschiebung hin zu den Reaktanten führen. Dieser Text ist adaptiert von <a href="https://openstax.org/books/chemistry-2e/pages/13-3-shifting-equilibria-le-chateliers-principle">Openstax, Chemistry 2e, Section 13.3 Shifting Equilibria: LeChatelier’s Principle</a>

Das Prinzip von Le Chatelier: Volumen (Druck) ändern

Bei Gasphasengleichgewichten kann es bei verändertem Volumen und Druck zu Änderungen der Konzentrationen von Reaktanten und Produkten kommen. Der Partialdruck P von ...

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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

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...

le chatelier's principle: changing concentration

le-chatelier-s-principle-changing-concentration

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...

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...

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...

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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.

key terms and concepts

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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