Name: gas equilibrium under volume changes
Uploaded: 2020-09-24T15:06:51.000Z
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Le Chatelier's Principle: Changing Volume (Pressure)

O princípio de Le Châtelier pode ser utilizado para prever como um sistema em equilíbrio responderia ao stress de uma alteração no volume ou pressão. O volume de um gás é inversamente proporcional à sua pressão;portanto, para um sistema em equilíbrio, uma diminuição no volume aumenta a pressão e perturba o equilíbrio. Em resposta, a posição de equilíbrio irá mudar numa direção para minimizar o stress.A lei do gás ideal estabelece que a pressão de um gás é diretamente proporcional ao número de mols. Assim, a direção da alteração necessária para restabelecer o equilíbrio está dependente do número de mols de partículas de gás de ambos os lados da reação. Como mais mols de gás resultam numa pressão mais elevada, um aumento da pressão altera a posição de equilíbrio para o lado com menos mols para baixar a pressão.Da mesma forma, uma diminuição da pressão desloca a posição de equilíbrio para o lado com mais mols de gás. Consideremos um equilíbrio químico, onde um mol de pentacloreto de fósforo gasoso decompõe-se num mol de tricloreto de fósforo e um mol de cloro gasoso um total de dois mols de produto. Se o pistão for empurrado para baixo, o volume do sistema de equilíbrio decresce, aumentando a pressão.Isto perturba o equilíbrio e leva a que Q seja maior do que K.Assim, a posição de equilíbrio desloca-se para os reagentes, com menos mols de partículas gasosas, para baixar a pressão e restabelecer o equilíbrio. Inversamente, puxando o pistão para cima aumenta o volume e diminui a pressão. Neste caso, Q torna-se menor do que K.Para aumentar a pressão, a posição de equilíbrio desloca-se para os produtos, o lado com o maior número de mols de gás, e restabelece o equilíbrio.Aumentar a pressão adicionando um gás inerte a uma mistura de equilíbrio num volume constante, não afeta o equilíbrio porque as pressões parciais dos reagentes gasosos e os produtos permanecem inalterados. Para sistemas de equilíbrio com números iguais de mols de reagentes e produtos gasosos, tais como a reação entre o gás de iodo e gás de cloro para produzir monocloreto de iodo, uma alteração no volume do sistema não terá qualquer efeito sobre o equilíbrio.

o princípio de le chatelier: alteração do volume (pressão)

Para equilíbrios de fase gasosa, podem ocorrer alterações nas concentrações de reagentes e produtos com volume e pressão alterados. A pressão parcial, P, de um gás ideal é proporcional à sua concentração molar, M.
<img alt="Eq1" src="/files/ftp_upload/11392/11392_Equation 1.png" style="text-align: center; height: 70px;" />
Assim, as alterações nas pressões parciais de qualquer reagente ou produto são essencialmente alterações nas concentrações; portanto, essas alterações produzem os mesmos efeitos nos equilíbrios. Além da adição ou remoção de reagentes ou produtos, as pressões (concentrações) das espécies de fase gasosa em equilíbrio também podem ser alteradas alterando o volume ocupado pelo sistema. Uma vez que todas as espécies de fase gasosa em equilíbrio ocupam o mesmo volume, uma certa mudança de volume irá causar a mesma alteração de concentração tanto para os reagentes como para os produtos. A fim de discernir que deslocamento, se houver, esse tipo de stress irá induz, a estequiometria da reação deve ser considerada.
No equilíbrio, a reação N2 (g) + O2 (g) ⇌ 2 NO (g) é descrita pelo quociente de reação
<img alt="Eq2" src="/files/ftp_upload/11392/11392_Equation 2.png" style="text-align: center; height: 70px;" />
Se o volume ocupado por uma mistura em equilíbrio destas espécies for diminuído por um factor de 3, as pressões parciais das três espécies serão aumentadas por um factor de 3:
<img alt="Eq3" src="/files/ftp_upload/11392/11392_Equation 3.png" style="text-align: center; height: 70px;" />
Assim, a alteração do volume desta mistura de fase gasosa em equilíbrio não resulta em uma alteração do equilíbrio.
Um tratamento semelhante de um sistema diferente, 2 SO2 (g) + O2 (g) ⇌ 2 SO3 (g), no entanto, produz um resultado diferente:
<img alt="Eq4" src="/files/ftp_upload/11392/11392_Equation 4.png" style="text-align: center; height: 200px;" />
Neste caso, a mudança de volume resulta em um quociente de reação menor do que a constante de equilíbrio, e assim o equilíbrio vai mudar para a direita.
Estes resultados ilustram a relação entre a estequiometria de um equilíbrio de fase gasosa e o efeito de uma alteração da pressão induzida por volume (concentração). Se as quantidades molares totais de reagentes e produtos forem iguais, como no primeiro exemplo, uma mudança de volume não altera o equilíbrio. Se as quantidades molares de reagentes e produtos forem diferentes, uma alteração no volume deslocará o equilíbrio em uma direção que melhor “se adapte” à alteração do volume. No segundo exemplo, três moles de reagente (SO2 e O2) produzem dois moles de produto (SO3), pelo que a diminuição do volume do sistema faz com que o equilíbrio se mova para a direita, uma vez que a reação nessa direção produz menos gás (2 mol) do que a reação inversa (3 mol). Por outro lado, aumentar o volume deste sistema de equilíbrio resultaria em uma mudança na direção dos reagentes.
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 do Volume (Pressão)

Para equilíbrios em fase gasosa, mudanças nas concentrações de reagentes e produtos podem ocorrer com volume e pressão alterados. A pressão parcial, P, de ...

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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 affects each species in the same way. To predict whether the equilibrium shifts, you must look at the reaction stoichiometry.

For the equilibrium N2(g) + O2(g) ⇌ 2 NO(g), decreasing the volume by a factor of 3 increases the partial pressures of all three gases by a factor of 3. In this case, the reaction quotient still matches the equilibrium position, so the equilibrium does not shift. The total moles of reactants and products are equal, and that is why the volume change has no effect.

A different result appears for 2 SO2(g) + O2(g) ⇌ 2 SO3(g). Here, decreasing the volume makes the reaction quotient smaller than the equilibrium constant, so the equilibrium shifts right. Three moles of reactant form two moles of product, so the forward reaction makes less gas. Increasing the volume would shift this system toward the reactants.

Gas Equilibrium Under Volume Changes

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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

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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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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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Equilibrium Concentrations from an ICE Table

Equilibrium concentrations can be found from initial concentrations and an equilibrium constant. This skill matters in many areas of science and technology, including pharmaceutical product formulation and dosing. After a drug is ingested or injected, it may take part in several chemical equilibria that change its final concentration in the body system of interest. Scientists use these quantitative equilibrium relationships to help calculate a dosage that will produce the desired therapeutic...

le chatelier's principle: changing volume (pressure)

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.

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Learn how changing volume changes pressure and concentration in gas-phase equilibrium, and why stoichiometry decides whether the system shifts left or right.

Gas Equilibrium Under Volume Changes | JoVE K12

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Gas Equilibrium Under Volume Changes

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