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

Принцип Ле Шателье можно использовать для предсказания того, как система в состоянии равновесия отреагирует на стресс, возникающее при изменении объема или давления. Объем газа обратно пропорционален его давлению;поэтому для системы, находящейся в состоянии равновесия, уменьшение объема увеличивает давление и нарушает равновесие. В ответ равновесие сместится в направлении минимизации стресса.Закон идеального газа гласит, что давление газа прямо пропорционально количеству молей. Таким образом, направление смещения, необходимого для восстановления равновесия, зависит от количества молей частиц газа по обе стороны реакции. Поскольку большее количество молей газа приводит к более высокому давлению, увеличение давления смещает равновесие в сторону с меньшим количеством молей, чтобы снизить давление.Точно так же снижение давления смещает равновесие в сторону с большим количеством молей газа. Рассмотрим химическое равновесие, при котором один моль газообразного пентахлорида фосфора разлагается на один моль треххлористого фосфора и один моль газообразного хлора всего два моля продукта. Если поршень опускается, объем равновесной системы уменьшается, увеличивая давление.Это нарушает равновесие и приводит к тому, что Q становится больше, чем K.Таким образом, равновесие смещается в сторону исходных веществ с меньшим количеством молей частиц газа, чтобы снизить давление и восстановить равновесие. И наоборот, поднятие поршня вверх увеличивает объем и снижает давление. В этом случае Q становится меньше, чем K.Для повышения давления равновесие смещается в сторону продуктов, стороны с наибольшим количеством молей газа, и восстанавливает равновесие.Повышение давления путем добавления инертного газа к равновесной смеси при постоянном объеме не влияет на равновесие, поскольку парциальные давления газообразных исходных веществ и продуктов реакции остаются неизменными. Для равновесных систем с равным числом молей газообразных исходных веществ и продуктов реакции, таких как реакция между газообразным йодом и газообразным хлором с образованием монохлорида йода, изменение объема системы не влияет на равновесие.

Принцип Ле Шателье: изменение объема (давления)

При равновесии газовой фазы изменения концентрации реагенты и продуктов могут происходить при изменении объема и давления. Парциальное давление, P, идеального газа пропорционально его молярной концентрации, М. 
 <img alt="Eq1" src="/files/ftp_upload/11392/11392_Equation 1.png" style="text-align: center; height: 70px;" />
 Таким образом, изменения парциального давления любого реагент или продукта по существу изменятся в концентрациях, поэтому эти изменения оказывают такое же влияние на равновесие. Помимо добавления или удаления реагенты или продуктов, давление (концентрации) видов в равновесии газовой фазы также можно изменить, изменив объем, занимаемый системой. Поскольку все виды газового равновесия занимают один и тот же объем, то изменение объема приведет к одинакового изменения концентрации как для реагенты, так и для продуктов. Для того чтобы определить, какой сдвиг, если он есть, этот тип напряжения вызовет, необходимо рассмотреть стехиометрию реакции. 
 При равновесии реакция N2 (g) + O2 (g) ⇌ 2 НЕТ (g) описывается реактивным соотношением 
 <img alt="Eq2" src="/files/ftp_upload/11392/11392_Equation 2.png" style="text-align: center; height: 70px;" />
 Если объем, занимаемый равновесной смесью этих видов, уменьшается в 3 раз, парциальное давление всех трех видов будет увеличено в 3 раз: 
 <img alt="Eq3" src="/files/ftp_upload/11392/11392_Equation 3.png" style="text-align: center; height: 70px;" />
 Таким образом, изменение объема этой газовой смеси не приводит к смещению равновесия. 
 Аналогичное лечение другой системы, 2 SO2 (g) + O2 (g) ⇌ 2 SO3 (g), однако, дает другой результат: 
 <img alt="Eq4" src="/files/ftp_upload/11392/11392_Equation 4.png" style="text-align: center; height: 200px;" />
 В этом случае изменение объема приводит к тому, что коэффициент реакции меньше, чем константа равновесия, и поэтому равновесие будет смещаться вправо. 
 Эти результаты иллюстрируют взаимосвязь между стехиометрией газового фазового равновесия и эффектом изменения давления (концентрации), вызванного объемом. Если общее молярное количество реагенты и продуктов равно, как и в первом примере, изменение объема не смеет равновесие. Если молярное количество реагенты и продуктов отличается, изменение объема смещает равновесие в направлении, которое лучше «приспосабливает» изменение объема. Во втором примере три моль реагент (SO2 и O2) дают две моль продукта (SO3), и поэтому уменьшение объема системы вызывает смещение равновесия вправо, так как при прямой реакции образуется меньше газа (2 моль), чем при обратной реакции (3 моль). И наоборот, увеличение объема этой равновесной системы приведет к смещению в сторону реагента. 
 Этот текст был адаптирован к Openstax<a href="https://openstax.org/books/chemistry-2e/pages/13-3-shifting-equilibria-le-chateliers-principle">, Химия 2е изд., раздел 13.3 Сдвиг равновесия: Принцип Ле-Шателье</a>.

При газофазном равновесии изменения концентраций реагентов и продуктов могут происходить при изменении объема и давления. Парциальное давление, P, ...

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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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