Name: shifting equilibrium by changing concentration
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Le Chatelier's Principle: Changing Concentration

Un sistema è in equilibrio dinamico quando la velocità della reazione diretta è uguale alla velocità della reazione inversa. Sebbene la reazione proceda ancora in entrambe le direzioni, le concentrazioni nette di reagenti e prodotti rimangono costanti. A questo punto, se si modifica la concentrazione di un reagente o di un prodotto, per esempio aggiungendo un'ulteriore molecola A o rimuovendo qualche molecola C, l'equilibrio viene disturbato.

Tali cambiamenti aggiungono stress al sistema. Secondo il principio di Le Châtelier, quando un sistema in equilibrio viene sollecitato a causa di un cambiamento di concentrazione, di un cambiamento di volume o di pressione, o di un cambiamento di temperatura, la posizione di equilibrio si sposta in una data direzione per minimizzare lo stress e ripristinare l'equilibrio. Per comprendere la risposta di un sistema ai cambiamenti di concentrazione, considerate due vasche d'acqua collegate, A e B.Inizialmente, il livello dell'acqua nelle due vasche è uguale e il sistema è in equilibrio.

Quindi, dell'acqua viene aggiunta alla vasca B.Questo stress aumenta il livello dell'acqua nella vasca B e ne disturba l'equilibrio. Per compensare questo stress, dell'acqua fluisce nella vasca A per ripristinare l'equilibrio. Viceversa, se l'acqua viene rimossa dalla vasca B, anche l'abbassamento del livello dell'acqua innesca uno stress.

Di conseguenza, l'acqua della vasca A fluisce nella vasca B per ripristinare l'equilibrio. Per un sistema in equilibrio, il quoziente di reazione equivale alla costante di equilibrio. L'aggiunta di più reagenti alla miscela di equilibrio diminuisce Q.Secondo il principio di Le Châtelier, il sistema minimizza questo stress spostando la posizione di equilibrio verso i prodotti per usare il reagente aggiunto, e ristabilire l'equilibrio.

Allo stesso modo, la rimozione di un prodotto dalla miscela di equilibrio porterà a una risposta simile per ristabilire l'equilibrio. Al contrario, l'aggiunta di più prodotti aumenta Q.Pertanto, la posizione di equilibrio si sposta verso i reagenti per consumare il prodotto aggiunto e ripristinare l'equilibrio. Un cambiamento nella concentrazione sposta la posizione di equilibrio senza cambiare il valore di K.

principio di le chatelier: cambiare la concentrazione

Un sistema all’equilibrio è in uno stato di equilibrio dinamico, con reazioni dirette e inverse che avvengono a velocità uguali. Se un sistema in equilibrio è soggetto a un cambiamento nelle condizioni che influenza queste velocità di reazione in modo diverso (uno stress), allora le velocità non sono più uguali e il sistema non è in equilibrio. Il sistema sperimenterà successivamente una reazione netta nella direzione di un tasso maggiore (uno spostamento) che ristabilirà l’equilibrio. Questo fenomeno è riassunto dal principio di Le Châtelier: se un sistema in equilibrio è stressato, il sistema subirà uno spostamento in risposta allo stress che ristabilisce l’equilibrio.

La velocità di reazione è influenzata principalmente dalle concentrazioni, come descritto dalla legge sulla velocità della reazione, e dalla temperatura, come descritto dall’equazione di Arrhenius. Di conseguenza, i cambiamenti di concentrazione e temperatura sono i due stress che possono spostare un equilibrio.

Effetto di un cambiamento nella concentrazione

Se un sistema in equilibrio è soggetto a un cambiamento nella concentrazione di un reagente o di una specie di prodotto, la velocità della reazione diretta o inversa cambierà. Ad esempio, consideriamo la reazione di equilibrio:

<img alt="Eq1" src="/files/ftp_upload/11391/11391_Equation 1v2.png" style="text-align: center; height: 48px;" />

Quando questo sistema è in equilibrio, le velocità di reazione diretta e inversa sono uguali.

<img alt="Eq2" src="/files/ftp_upload/11391/11391_Equation 2.png" style="text-align: center; height: 50px;" />

Se il sistema viene stressato aggiungendo un reagente, N_2 o O_2, il conseguente aumento di concentrazione fa sì che la velocità della reazione diretta aumenti, superando quella della reazione inversa:

<img alt="Eq3" src="/files/ftp_upload/11391/11391_Equation 3.png" style="text-align: center; height: 50px;" />

Il sistema sperimenterà una reazione netta temporanea nella direzione in avanti per ristabilire l’equilibrio (che si sposterà a destra). Questo stesso spostamento si verificherà se una parte del prodotto NO viene rimossa dal sistema, il che diminuisce la velocità della reazione inversa, determinando nuovamente lo stesso squilibrio nelle velocità.

La stessa logica può essere utilizzata per spiegare lo spostamento a sinistra che risulta dalla rimozione di un reagente o dall'aggiunta di prodotto a un sistema di equilibrio. Entrambi questi stress determinano un aumento della velocità della reazione inversa

<img alt="Eq4" src="/files/ftp_upload/11391/11391_Equation 4.png" style="text-align: center; height: 50px;" />

e una reazione netta temporanea nella direzione opposta per ristabilire l'equilibrio.

In alternativa a questa interpretazione cinetica, l'effetto dei cambiamenti di concentrazione sugli equilibri può essere razionalizzato in termini di quozienti di reazione. Quando il sistema è in equilibrio,

<img alt="Eq5" src="/files/ftp_upload/11391/11391_Equation 5.png" style="text-align: center; height: 70px;" />

Se viene aggiunto un reagente (aumentando il denominatore del quoziente di reazione) o viene rimosso un prodotto (diminuendo il numeratore), allora Q_c < K_c e l'equilibrio si sposterà verso destra. Si noti che i tre diversi modi di indurre questo stress determinano tre diversi cambiamenti nella composizione della miscela all'equilibrio. Se viene aggiunto N_2, lo spostamento a destra consumerà O_2 e produrrà NO non appena l'equilibrio verrà ristabilito, producendo una miscela con una maggiore concentrazione di N_2 e NO e una minore concentrazione di O_2 rispetto a quella presente prima. Se viene aggiunto O_2, la nuova miscela all'equilibrio avrà concentrazioni maggiori di O_2 e NO e una concentrazione minore di N_2. Infine, se viene rimosso NO, la nuova miscela all'equilibrio avrà concentrazioni maggiori di N_2 e O_2 e una concentrazione minore di NO. Nonostante queste differenze nella composizione, il valore della costante di equilibrio sarà lo stesso dopo lo stress di prima (secondo la legge dell'azione di massa). La stessa logica può essere applicata per le sollecitazioni che comportano la rimozione di reagenti o l'aggiunta di prodotto, nel qual caso Q_c > K_c e l'equilibrio si sposterà a sinistra.

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: cambiare la concentrazione

Un sistema in equilibrio si trova in uno stato di equilibrio dinamico, con reazioni dirette e inverse che avvengono a velocità uguali. Se un sistema di equilibrio è ...

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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

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Chemical Equilibrium and Kc Rules

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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...

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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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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...

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

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Shifting Equilibrium by Changing Concentration

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