Name: le chatelier's principle in heating and cooling
Uploaded: 2020-09-24T15:06:51.000Z
Duration: 2 min 19 s
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Matter and its Interactions

PS.Chm.5.2

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Le Chatelier's Principle: Changing Temperature

La temperatura influisce sulla velocità di una reazione chimica;pertanto, un cambiamento di temperatura per una reazione in equilibrio agisce come uno stress sul sistema. Il principio di Le Châtelier prevede come risponderà il sistema per ridurre al minimo tali disturbi. Una variazione di temperatura cambia il valore della costante di equilibrio, a differenza di una variazione della concentrazione o del volume, che sposta l'equilibrio senza modificare il valore di K.Considerate la decomposizione del pentacloruro di fosforo gassoso in tricloruro di fosforo e gas di cloro.

Per questa reazione endotermica, il calore assorbito può essere pensato come un reagente. Un aumento della temperatura aggiunge calore al sistema, in modo simile all'aggiunta di più reagenti. Pertanto, la posizione di equilibrio si sposta verso i prodotti e genera più tricloruro di fosforo e cloro per consumare il calore extra perché il valore della costante di equilibrio, K, è aumentato.

D'altra parte, una diminuzione della temperatura rimuove il calore dal sistema, in modo simile alla rimozione di un reagente. La posizione di equilibrio si sposta verso i reagenti e produce più pentacloruro di fosforo per rilasciare calore, poiché il valore di K è diminuito. Per una reazione esotermica, come la reazione gassosa fra anidride solforosa e ossigeno per produrre anidride solforica, il calore rilasciato può essere pensato come un prodotto.

Un aumento della temperatura è simile all'aggiunta di più di un prodotto. Ciò fa sì che la posizione di equilibrio si sposti verso i reagenti, producendo più anidride solforosa e ossigeno per assorbire parte del calore aggiunto, poiché il valore di K è diminuito. Al contrario, la diminuzione della temperatura di questa reazione esotermica rimuove il calore, come la rimozione di un prodotto.

La posizione di equilibrio si sposta verso i prodotti e produce più anidride solforica per rilasciare calore all'aumentare di K.Pertanto, un aumento della temperatura favorisce i prodotti in una reazione endotermica, mentre una diminuzione della temperatura favorisce i prodotti in una reazione esotermica.

principio di le chatelier: variazione della temperatura

Coerentemente con la legge dell’azione di massa, un equilibrio sollecitato da un cambiamento di concentrazione si sposterà per ristabilire l’equilibrio senza alcun cambiamento nel valore della costante di equilibrio, K. Quando un equilibrio si sposta in risposta a un cambiamento di temperatura, tuttavia, si ristabilisce con una diversa composizione relativa che presenta un diverso valore della costante di equilibrio.

Per comprendere questo fenomeno, consideriamo la reazione elementare:

 <img alt="Eq1" src="/files/ftp_upload/11393/11393_Equation_1.png" style="height: 50px;" />

Poiché si tratta di una reazione elementare, le leggi sui tassi per la direzione diretta e inversa possono essere derivate direttamente dalla stechiometria dell’equazione bilanciata:

 <img alt="Eq2" src="/files/ftp_upload/11393/11393_Equation_2.png" style="height: 65px;" />

Quando il sistema è in equilibrio,

 <img alt="Eq3" src="/files/ftp_upload/11393/11393_Equation_3.png" style="height: 50px;" />

Sostituendo le leggi tariffarie in questa uguaglianza e riorganizzando i dati

 <img alt="Eq4" src="/files/ftp_upload/11393/11393_Equation_4.png" style="height: 100px;" />

La costante di equilibrio può essere espressa come funzione matematica delle costanti di velocità per le reazioni dirette e inverse. Poiché le costanti di velocità variano con la temperatura come descritto dall'equazione di Arrhenius, è ovvio che anche la costante di equilibrio varierà con la temperatura (assumendo che le costanti di velocità siano influenzate in misura diversa dalla variazione di temperatura). Per reazioni più complesse che coinvolgono meccanismi di reazione a più fasi, esiste una relazione matematica simile ma più complessa tra la costante di equilibrio e le costanti di velocità delle fasi del meccanismo. Indipendentemente da quanto complessa possa essere la reazione, la dipendenza della sua costante di equilibrio dalla temperatura persiste.

È più conveniente prevedere lo spostamento che un equilibrio subirà in risposta a un cambiamento di temperatura considerando la variazione di entalpia della reazione. Ad esempio, la formazione di ammoniaca mediante il processo di Haber è un processo esotermico (che produce calore):

 <img alt="Eq5" src="/files/ftp_upload/11393/11393_Equation_5.png" style="height: 50px;" />

Ai fini dell’applicazione del principio di Le Châtelier, il calore, q, può essere visto come un prodotto:

 <img alt="Eq6" src="/files/ftp_upload/11393/11393_Equation_6.png" style="height: 50px;" />

Aumentare la temperatura del sistema è come aumentare la quantità di un prodotto, quindi l’equilibrio si sposterà a sinistra. Allo stesso modo, l’abbassamento della temperatura del sistema farà sì che l’equilibrio si sposti verso destra. Per i processi endotermici, il calore è visto come un reagente della reazione e quindi si osserva la dipendenza opposta dalla temperatura.

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: Le </a><a href="https://openstax.org/books/chemistry-2e/pages/13-3-shifting-equilibria-le-chateliers-principle">Ch</a>â<a href="https://openstax.org/books/chemistry-2e/pages/13-3-shifting-equilibria-le-chateliers-principle">t</a><a href="https://openstax.org/books/chemistry-2e/pages/13-3-shifting-equilibria-le-chateliers-principle">elier’s Principle</a>

Principio di Le Chatelier: variazione della temperatura

Coerentemente con la legge dell'azione di massa, un equilibrio stressato da un cambiamento di concentrazione si sposterà per ristabilire l'equilibrio senza alcun cambiamento in ...

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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 forward and reverse rate laws based on the balanced equation. At equilibrium, the forward and reverse reaction rates are equal. When those rate laws are combined and rearranged, the equilibrium constant can be expressed as a function of the forward and reverse rate constants.

Because rate constants change with temperature, as described by the Arrhenius equation, the equilibrium constant can also change with temperature. This idea applies to both elementary reactions and more complex multistep mechanisms. In multistep reactions, the math is more complicated, but the temperature dependence of the equilibrium constant still remains.

A practical way to predict the direction of the shift is to use the reaction’s enthalpy change. In the Haber process, ammonia formation is exothermic, which means it produces heat. For Le Chatelier's principle, heat can be treated like a product. Raising the temperature adds heat, so the equilibrium shifts left. Lowering the temperature removes heat, so the equilibrium shifts right. For endothermic processes, heat acts like a reactant, so the temperature effect is opposite.

Le Chatelier's Principle in Heating and Cooling

le-chateliers-principle-heating-and-cooling

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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...

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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...

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le chatelier's principle: changing volume (pressure)

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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

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 heating and cooling change chemical equilibrium, why K changes with temperature, and how exothermic and endothermic reactions shift.

Le Chatelier's Principle in Heating and Cooling | JoVE K12

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Le Chatelier's Principle in Heating and Cooling

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