Name: gas equilibrium under volume changes
Uploaded: 2020-09-24T15:06:51.000Z
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Chemistry

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PS.Chm.5.2

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Le Chatelier's Principle: Changing Volume (Pressure)

Il principio di Le Châtelier può venire usato per prevedere come un sistema in equilibrio risponderebbe allo stress di un cambiamento di volume o pressione. Il volume di un gas è inversamente proporzionale alla sua pressione;dunque, per un sistema in equilibrio, una diminuzione di volume aumenta la pressione e disturba l'equilibrio. In risposta, la posizione di equilibrio si sposterà in una direzione per ridurre al minimo lo stress.

La legge dei gas ideali sostiene che la pressione di un gas è direttamente proporzionale al numero di moli. Pertanto, la direzione dello spostamento, necessario per ripristinare l'equilibrio, dipende dal numero di moli delle particelle di gas su ambo i lati della reazione. Poiché un maggior numero di moli di gas si traduce in una pressione più alta, un aumento della pressione sposta la posizione di equilibrio sul lato con meno moli per abbassare la pressione.

Allo stesso modo, una diminuzione della pressione sposta la posizione di equilibrio dal lato con più moli di gas. Considerate un equilibrio chimico, in cui una mole di pentacloruro di fosforo gassoso si decompone in una mole di tricloruro di fosforo e una mole di cloro gassoso due moli totali di prodotto. Se il pistone viene spinto verso il basso, il volume del sistema di equilibrio diminuisce, aumentando la pressione.

Questo disturba l'equilibrio e si traduce in Q maggiore di K.Pertanto, la posizione di equilibrio si sposta verso i reagenti, con meno moli di particelle di gas, al fine di abbassare la pressione e ripristinare l'equilibrio. Al contrario, tirando il pistone verso l'alto il volume aumenta e la pressione diminuisce. In questo caso, Q diventa minore di K.Per aumentare la pressione, la posizione di equilibrio si sposta verso i prodotti, il lato con la maggior parte delle moli di gas, e ripristina l'equilibrio.

L'aumento della pressione aggiungendo un gas inerte ad una miscela di equilibrio a volume costante, non influisce sull'equilibrio perché le pressioni parziali dei reagenti gassosi e dei prodotti rimangono invariate. Per sistemi in equilibrio con un numero uguale di moli di reagenti e di prodotti gassosi, come la reazione fra gas iodio e gas cloro per produrre monocloruro di iodio, un cambiamento nel volume del sistema non avrà alcun effetto sull'equilibrio.

principio di le chatelier: variazione del volume (pressione)

Per gli equilibri in fase gassosa, i cambiamenti nelle concentrazioni di reagenti e prodotti possono verificarsi con volume e pressione alterati. La pressione parziale P di un gas ideale è proporzionale alla sua concentrazione molare M.

<img alt="Eq1" src="/files/ftp_upload/11392/11392_Equation 1.png" style="text-align: center; height: 70px;" />

Quindi i cambiamenti nelle pressioni parziali di qualsiasi reagente o prodotto sono essenzialmente cambiamenti nelle concentrazioni; pertanto, questi cambiamenti producono gli stessi effetti sugli equilibri. Oltre ad aggiungere o rimuovere reagenti o prodotti, le pressioni (concentrazioni) delle specie in un equilibrio in fase gassosa possono essere modificate anche cambiando il volume occupato dal sistema. Poiché tutte le specie di un equilibrio in fase gassosa occupano lo stesso volume, una data variazione di volume causerà la stessa variazione di concentrazione sia per i reagenti che per i prodotti. Per discernere quale spostamento, se presente, questo tipo di stress indurrà, è necessario considerare la stechiometria della reazione.

All'equilibrio, la reazione N_2 (g) + O_2 (g) ⇌ 2 NO (g) è descritta dal quoziente di reazione

<img alt="Eq2" src="/files/ftp_upload/11392/11392_Equation 2.png" style="text-align: center; height: 70px;" />

Se il volume occupato da una miscela in equilibrio di queste specie diminuisce di un fattore 3, le pressioni parziali di tutte e tre le specie aumenteranno di un fattore 3:

<img alt="Eq3" src="/files/ftp_upload/11392/11392_Equation 3.png" style="text-align: center; height: 70px;" />

E quindi, la modifica del volume di questa miscela di equilibrio in fase gassosa non comporta uno spostamento dell’equilibrio.

Un trattamento simile di un sistema diverso, 2 SO_2 (g) + O_2 (g) ⇌ 2 SO_3 (g), tuttavia, produce un risultato diverso:

<img alt="Eq4" src="/files/ftp_upload/11392/11392_Equation 4.png" style="text-align: center; height: 200px;" />

In questo caso, la variazione di volume risulta in un quoziente di reazione inferiore alla costante di equilibrio, e quindi l’equilibrio si sposterà verso destra.

Questi risultati illustrano la relazione tra la stechiometria di un equilibrio in fase gassosa e l'effetto di una variazione di pressione (concentrazione) indotta dal volume. Se le quantità molari totali di reagenti e prodotti sono uguali, come nel primo esempio, una variazione di volume non sposta l'equilibrio. Se le quantità molari di reagenti e prodotti sono diverse, una variazione di volume sposterà l’equilibrio in una direzione che “accomoda” meglio la variazione di volume. Nel secondo esempio, tre moli di reagente (SO_2 e O_2) producono due moli di prodotto (SO_3), quindi diminuendo il volume del sistema si sposta l'equilibrio verso destra poiché la reazione diretta produce meno gas (2 mol) rispetto alla reazione inversa (3 mol). Al contrario, l’aumento del volume di questo sistema di equilibrio comporterebbe uno spostamento verso i reagenti.

Questo testo è adattato da <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>

Principio di Le Chatelier: variazione del volume (pressione)

Per gli equilibri in fase gassosa, le variazioni delle concentrazioni dei reagenti e dei prodotti possono verificarsi con volume e pressione alterati. La pressione parziale, P, di ...

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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

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Relating Kc and Kp in Gas Equilibria

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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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Using Q to Track Equilibrium Changes

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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.

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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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