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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Le Chatelier's Principle: Changing Temperature

A temperatura afeta a taxa de uma reação química;portanto, uma alteração na temperatura para uma reação em equilíbrio atua como um stress sobre o sistema. O princípio de Le Châtelier prevê como o sistema irá responder para minimizar tais distúrbios. Uma alteração na temperatura altera o valor da constante de equilíbrio, ao contrário de uma alteração na concentração ou volume, que desloca o equilíbrio sem alterar o valor de K.Consideremos a decomposição do pentacloreto de fósforo gasoso em tricloreto de fósforo e gás de cloro.Para esta reação endotérmica, o calor absorvido pode ser considerado um reagente. Um aumento na temperatura adiciona calor ao sistema, semelhante a acrescentar mais um reagente. Assim, a posição de equilíbrio desloca-se para os produtos e produz mais tricloreto de fósforo e cloro para consumir o calor extra porque o valor do equilíbrio constante, K, aumentou.Por outro lado, uma diminuição da temperatura remove o calor do sistema, semelhante à remoção de um reagente. A posição de equilíbrio desloca-se para os reagentes e produz mais pentacloreto de fósforo para libertar calor porque o valor de K diminuiu. Para uma reação exotérmica, tais como a reação gasosa entre o dióxido de enxofre e o oxigénio para produzir trióxido de enxofre, o calor libertado pode ser considerado um produto.O aumento da temperatura é semelhante a acrescentar mais de um produto. Isto leva a que a posição de equilíbrio mude para os reagentes, produzindo mais dióxido de enxofre e oxigénio para absorver algum do calor adicionado porque o valor de K diminuiu. Inversamente, diminuir a temperatura desta reação exotérmica remove o calor, como remover um produto.A posição de equilíbrio desloca-se para os produtos e produz mais trióxido de enxofre para libertar calor uma vez que K aumentou. Assim, um aumento na temperatura favorece os produtos numa reação endotérmica, enquanto que uma diminuição na temperatura favorece os produtos numa reação exotérmica.

o princípio de le chatelier: alteração da temperatura

Em conformidade com a lei de ação das massas, um equilíbrio stressado por uma mudança de concentração irá deslocar-se para restabelecer o equilíbrio sem qualquer alteração do valor da constante de equilíbrio, K. Quando um equilíbrio muda em resposta a uma alteração de temperatura, no entanto, ele é restabelecido com uma composição relativa diferente que apresenta um valor diferente para a constante de equilíbrio.
Para entender esse fenómeno, considere a reação elementar:
 <img alt="Eq1" src="/files/ftp_upload/11393/11393_Equation_1.png" style="height: 50px;" />
Uma vez que se trata de uma reação elementar, as leis de velocidade na direção direta e inversa podem ser derivadas diretamente da estequiometria da equação equilibrada:
 <img alt="Eq2" src="/files/ftp_upload/11393/11393_Equation_2.png" style="height: 65px;" />
Quando o sistema está em equilíbrio,
 <img alt="Eq3" src="/files/ftp_upload/11393/11393_Equation_3.png" style="height: 50px;" />
Substituindo as leis de velocidade por esta igualdade e reorganizando obtemos
 <img alt="Eq4" src="/files/ftp_upload/11393/11393_Equation_4.png" style="height: 100px;" />
A constante de equilíbrio pode ser expressa como uma função matemática das constantes de velocidade para as reações direta e inversa. Uma vez que as constantes da velocidade variam com a temperatura, conforme descrito pela equação de Arrhenius, é de esperar que a constante de equilíbrio varie igualmente com a temperatura (assumindo que as constantes de velocidade são afetadas a diferentes extensões pela alteração da temperatura). Para reações mais complexas envolvendo mecanismos de reação multipassos, existe uma relação matemática semelhante, mas mais complexa, entre a constante de equilíbrio e as constantes de velocidade dos passos do mecanismo. Independentemente de quão complexa pode ser a reação, a dependência da temperatura da sua constante de equilíbrio persiste.
Prever a alteração que um equilíbrio irá experienciar em resposta a uma mudança na temperatura é realizado mais convenientemente considerando a mudança de entalpia da reação. Por exemplo, a formação de amoníaco pelo processo de Haber é um processo exotérmico (produtor de calor):
 <img alt="Eq5" src="/files/ftp_upload/11393/11393_Equation_5.png" style="height: 50px;" />
Para efeitos da aplicação do princípio de Châtelier, o calor, q, pode ser considerado como um produto:
 <img alt="Eq6" src="/files/ftp_upload/11393/11393_Equation_6.png" style="height: 50px;" />
Aumentar a temperatura do sistema é semelhante a aumentar a quantidade de um produto, e assim o equilíbrio vai mudar para a esquerda. Baixar a temperatura do sistema fará com que o equilíbrio se desloque para a direita. Para os processos endotérmicos, o calor é visto como um reagente da reação e, portanto, a dependência oposta da temperatura é observada.
Este texto foi adaptado de <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>.

O Princípio de Le Chatelier: Alteração da Temperatura

Consistente com a lei da ação das massas, um equilíbrio estressado por uma mudança na concentração mudará para restabelecer o equilíbrio sem qualquer mudança em ...

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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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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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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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