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

Um sistema está em equilíbrio dinâmico quando a taxa da reação avançada é igual à taxa da reação inversa. Embora a reação ainda seja procedida em ambos os sentidos, as concentrações líquidas dos reagentes e produtos permanecem constantes. Neste momento, se um reagente ou concentração do produto for alterado, por exemplo, adicionando a molécula A adicional ou removendo alguma molécula C, o equilíbrio é perturbado.Tais mudanças são tensões sobre o sistema. De acordo com o princípio de Le Châtelier, quando um sistema está em equilíbrio é realçado devido a uma alteração na concentração, uma alteração no volume ou pressão, ou uma alteração na temperatura, a posição do equilíbrio altera-se numa direção para minimizar o stress e restabelecer o equilíbrio. Para compreender um sistema de resposta às alterações na concentração, consideremos dois tanques de água ligados, A e B.Inicialmente, o nível da água nos dois tanques é igual, e o sistema está em equilíbrio.Depois, é adiciona água ao tanque B.Este stress aumenta o nível da água no tanque B e perturba o equilíbrio. Para compensar este stress, alguma água flui para o tanque A para restabelecer o equilíbrio. Inversamente, se a água é removida do tanque B, a diminuição da água também causa stress.Consequentemente, a água do tanque A flui para o tanque B para restabelecer o equilíbrio. Para um sistema em equilíbrio, o quociente de reação iguala a constante de equilíbrio. Adicionar mais reagentes à mistura de equilíbrio diminui Q.De acordo com o princípio de Le Châtelier, o sistema minimiza este stress deslocando a posição de equilíbrio em relação aos produtos para gastar o reagente adicionado e restabelecer o equilíbrio.Da mesma forma, remover um produto a partir da mistura de equilíbrio conduzirá a uma resposta semelhante para restabelecer o equilíbrio. Em contraste, acrescentando mais produtos aumenta Q.Assim, a posição de equilíbrio desloca-se para os reagentes para gastar o produto adicionado e restabelecer o equilíbrio. Uma mudança na concentração desloca a posição de equilíbrio sem alterar o valor de K.

o princípio de le chatelier: alteração da concentração

Um sistema em equilíbrio encontra-se em um estado de equilíbrio dinâmico, com as reações direta e inversa a velocidades iguais. Se um sistema de equilíbrio for sujeito a uma mudança de condições que afete estas velocidades de reação de forma diferente (um stress), então as velocidades não são mais iguais e o sistema não está em equilíbrio. O sistema irá, subsequentemente, sofrer uma reação líquida na direção de uma velocidade superior (uma alteração) que restabelecerá o equilíbrio. Este fenómeno é resumido pelo princípio de Le Châtelier: Se um sistema de equilíbrio for sujeito a um stress, o sistema irá sofrer uma mudança em resposta ao stress que restabelece o equilíbrio.
As velocidades de reação são afetadas principalmente por concentrações, conforme descrito pela lei de velocidade de reação, e temperatura, conforme descrito pela equação de Arrhenius. Consequentemente, as mudanças na concentração e na temperatura são os dois stresses que podem deslocar um equilíbrio.
Efeito de uma Alteração na Concentração
Se um sistema de equilíbrio for sujeito a uma alteração na concentração de uma espécie de reagente ou de produto, a velocidade da reação direta ou inversa será alterada. Como exemplo, considere a reação de equilíbrio:
<img alt="Eq1" src="/files/ftp_upload/11391/11391_Equation 1v2.png" style="text-align: center; height: 48px;" />
Quando este sistema está em equilíbrio, as velocidades de reação direta e inversa são iguais.
<img alt="Eq2" src="/files/ftp_upload/11391/11391_Equation 2.png" style="text-align: center; height: 50px;" />
Se o sistema for stressado pela adição de reagentes, quer N2 quer O2, o aumento resultante da concentração provoca o aumento da velocidade de reação direta, ultrapassando a da reação inversa:
<img alt="Eq3" src="/files/ftp_upload/11391/11391_Equation 3.png" style="text-align: center; height: 50px;" />
O sistema irá experienciar uma reação direta líquida temporária para restabelecer o equilíbrio (o equilíbrio irá mudar para a direita). Esta mesma alteração irá resultar se algum produto NO for removido do sistema, o que diminui a velocidade da reação inversa, novamente resultando no mesmo desequilíbrio nas velocidades.
A mesma lógica pode ser usada para explicar a mudança para a esquerda que resulta da remoção de reagente ou da adição de produto a um sistema de equilíbrio. Ambos estes stresses resultam em uma velocidade aumentada para a reação inversa
<img alt="Eq4" src="/files/ftp_upload/11391/11391_Equation 4.png" style="text-align: center; height: 50px;" />
e uma reação inversa líquida temporária para restabelecer o equilíbrio.
Como alternativa a esta interpretação cinética, o efeito de alterações na concentração sobre os equilíbrios pode ser racionalizado em termos de quocientes de reação. Quando o sistema está em equilíbrio,
<img alt="Eq5" src="/files/ftp_upload/11391/11391_Equation 5.png" style="text-align: center; height: 70px;" />
Se for adicionado um reagente (aumentando o denominador do quociente de reação) ou se for removido produto (diminuindo o numerador) então Qc < Kc e o equilíbrio irá mudar para a direita. Note-se que as três formas diferentes de induzir este stress resultam em três mudanças diferentes na composição da mistura de equilíbrio. Se for adicionado N2, o desvio para a direita irá consumir O2 e produzir NO à medida que o equilíbrio é restabelecido, produzindo uma mistura com uma maior concentração de N2 e NO e uma menor concentração de O2 do que a que estava presente anteriormente. Se for adicionado O2, a nova mistura de equilíbrio terá maiores concentrações de O2 e NO e menor concentração de N2. Por fim, se for removido NO, a nova mistura de equilíbrio terá maiores concentrações de N2 e O2 e menor concentração de NO. Apesar dessas diferenças na composição, o valor da constante de equilíbrio após o stress será o mesmo que antes (de acordo com a lei da ação das massas). A mesma lógica pode ser aplicada para stresses que envolvam a remoção de reagentes ou adição de produto, caso em que Qc > Kc e o equilíbrio se deslocará para a esquerda.
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: LeChatelier’s Principle</a>.

O Princípio de Le Chatelier: Alteração da Concentração

Um sistema em equilíbrio está em um estado de equilíbrio dinâmico, com reações diretas e reversas ocorrendo em taxas iguais. Se um sistema de equilíbrio é ...

Education

JoVE Core

Química

Chemical Equilibrium

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KEY TERMS AND CONCEPTS

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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

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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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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Using Q to Track Equilibrium Changes

Reaction quotient Q helps track how far a reversible reaction has moved toward equilibrium. For a balanced reaction such as m A + n B ⇌ x C + y D, Q comes directly from the stoichiometric coefficients in the equation. When concentrations are used, the expression is written as Qc. For gases, a similar form uses partial pressures. These expressions use relative concentration and pressure values, not absolute values. That means reaction quotients are dimensionless and have no units. As a reaction...

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Chemical Equilibrium in Reversible Reactions

Chemical equilibrium in reversible reactions happens when a reaction moves in both directions at the same time. A reversible reaction uses a double arrow to show that products can turn back into reactants. At equilibrium, the concentrations of reactants and products stay constant over time because the forward and reverse reaction rates are equal. The video uses the reversible decomposition of colorless dinitrogen tetroxide to brown nitrogen dioxide as an example. At time zero, there is finite...

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le chatelier's principle: changing volume (pressure)

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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...

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.

key terms and concepts

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

Shifting Equilibrium by Changing Concentration | JoVE K12

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Shifting Equilibrium by Changing Concentration

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