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

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Le Chatelier's Principle: Changing Temperature

Temperatura wpływa na szybkość reakcji chemicznej, dlatego zmiana temperatury reakcji w stanie równowagi działa jak naprężenie na układ. Zasada Le Châteliera przewiduje, w jaki sposób system zareaguje, aby zminimalizować takie zakłócenia.
Zmiana temperatury zmienia wartość stałej równowagi, w przeciwieństwie do zmiany stężenia lub objętości, która przesuwa równowagę bez zmiany wartości K.
Rozważ rozkład gazowego pięciochlorku fosforu na trichlorek fosforu i chlor gazowy. W przypadku tej reakcji endotermicznej pochłonięte ciepło można traktować jako reagent.
Wzrost temperatury powoduje dodanie ciepła do systemu, podobnie jak dodanie większej ilości reagenta. W ten sposób pozycja równowagi przesuwa się w kierunku produktów i wytwarza więcej trójchlorku fosforu i chloru, aby zużyć dodatkowe ciepło, ponieważ wartość stałej równowagi, K, wzrosła.
Z drugiej strony spadek temperatury usuwa ciepło z układu, podobnie jak usunięcie reagenta. Pozycja równowagi przesuwa się w kierunku reagentów i wytwarza więcej pentachlorku fosforu do uwalniania ciepła, ponieważ wartość K spadła.
W przypadku reakcji egzotermicznej, takiej jak reakcja gazowa między dwutlenkiem siarki a tlenem w celu wytworzenia trójtlenku siarki, uwolnione ciepło można traktować jako produkt.
Wzrost temperatury jest podobny do dodawania większej ilości produktu. Powoduje to, że pozycja równowagi przesuwa się w kierunku reagentów, wytwarzając więcej dwutlenku siarki i tlenu, aby pochłonąć część dodanego ciepła, ponieważ wartość K spadła.
I odwrotnie, obniżenie temperatury tej reakcji egzotermicznej usuwa ciepło, podobnie jak usuwanie produktu. Pozycja równowagi przesuwa się w kierunku produktów i wytwarza więcej trójtlenku siarki, aby uwolnić ciepło wraz ze wzrostem K.
Tak więc wzrost temperatury sprzyja produktom w reakcji endotermicznej, podczas gdy spadek temperatury sprzyja produktom w reakcji egzotermicznej.

zasada le chateliera: zmiana temperatury

Zgodnie z prawem działania mas, równowaga naprężona przez zmianę stężenia przesunie się, aby ponownie ustanowić równowagę bez jakiejkolwiek zmiany wartości stałej równowagi K. Kiedy jednak równowaga przesuwa się w odpowiedzi na zmianę temperatury, zostaje ponownie ustalona z innym względnym składem, który wykazuje inną wartość stałej równowagi.

Aby zrozumieć to zjawisko, rozważmy reakcję elementarną:

 <img alt="Eq1" src="/files/ftp_upload/11393/11393_Equation_1.png" style="height: 50px;" />

Ponieważ jest to reakcja elementarna, prawa szybkości dla ruchu w przód i w tył można wyprowadzić bezpośrednio ze stechiometrii zrównoważonego równania:

 <img alt="Eq2" src="/files/ftp_upload/11393/11393_Equation_2.png" style="height: 65px;" />

Kiedy układ jest w równowadze,

 <img alt="Eq3" src="/files/ftp_upload/11393/11393_Equation_3.png" style="height: 50px;" />

Podstawienie praw stopy do tej równości i przestawienie daje

 <img alt="Eq4" src="/files/ftp_upload/11393/11393_Equation_4.png" style="height: 100px;" />

Stałą równowagi można wyrazić jako funkcję matematyczną stałych szybkości reakcji do przodu i do tyłu. Ponieważ stałe szybkości zmieniają się wraz z temperaturą, jak opisano za pomocą równania Arrheniusa, oczywiste jest, że stała równowagi będzie również zmieniać się wraz z temperaturą (zakładając, że zmiany temperatury wpływają w różnym stopniu na stałe szybkości). W przypadku bardziej złożonych reakcji obejmujących wieloetapowe mechanizmy reakcji istnieje podobna, ale bardziej złożona matematyczna zależność między stałą równowagi a stałymi szybkości etapów mechanizmu. Niezależnie od tego, jak złożona może być reakcja, zależność jej stałej równowagi od temperatury utrzymuje się.

Przewidywania przesunięcia równowagi w odpowiedzi na zmianę temperatury najwygodniej jest dokonać poprzez uwzględnienie zmiany entalpii reakcji. Na przykład powstawanie amoniaku w procesie Habera jest procesem egzotermicznym (wytwarzającym ciepło):

 <img alt="Eq5" src="/files/ftp_upload/11393/11393_Equation_5.png" style="height: 50px;" />

Na potrzeby stosowania zasady Le Châteliera ciepło q można postrzegać jako produkt:

 <img alt="Eq6" src="/files/ftp_upload/11393/11393_Equation_6.png" style="height: 50px;" />

Podniesienie temperatury układu jest równoznaczne ze zwiększeniem ilości produktu, a zatem równowaga przesunie się w lewo. Obniżenie temperatury układu również spowoduje przesunięcie równowagi w prawo. W procesach endotermicznych ciepło jest postrzegane jako reagent reakcji i dlatego obserwuje się odwrotną zależność temperaturową.

Zasada Le Chateliera: zmiana temperatury

Zgodnie z prawem działania masy, równowaga podkreślona przez zmianę stężenia przesunie się, aby przywrócić równowagę bez żadnych zmian w ...

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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...

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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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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