Name: gas equilibrium under volume changes
Uploaded: 2020-09-24T15:06:51.000Z
Duration: 2 min 32 s
Channel: JoVE

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Le Chatelier's Principle: Changing Volume (Pressure)

Zasada Le Châteliera może być wykorzystana do przewidywania, w jaki sposób układ w stanie równowagi zareaguje na naprężenia związane ze zmianą objętości lub ciśnienia.
Objętość gazu jest odwrotnie proporcjonalna do jego ciśnienia, dlatego też dla układu w stanie równowagi zmniejszenie objętości zwiększa ciśnienie i zakłóca równowagę. W odpowiedzi pozycja równowagi przesunie się w kierunku, aby zminimalizować naprężenia.
Prawo gazu doskonałego mówi, że ciśnienie gazu jest wprost proporcjonalne do liczby moli. Zatem kierunek przesunięcia potrzebnego do przywrócenia równowagi zależy od liczby moli cząstek gazu po obu stronach reakcji.
Ponieważ więcej moli gazu powoduje wyższe ciśnienie, wzrost ciśnienia przesuwa pozycję równowagi na bok z mniejszą liczbą moli w celu obniżenia ciśnienia. Podobnie, spadek ciśnienia przesuwa pozycję równowagi na stronę, w której znajduje się więcej moli gazu.
Rozważmy równowagę chemiczną, w której jeden mol gazowego pięciochlorku fosforu rozkłada się na jeden mol trichlorku fosforu i jeden mol chloru gazowego - dwa całkowite mole produktu.
Jeśli tłok zostanie wciśnięty w dół, objętość układu równowagi zmniejsza się, zwiększając ciśnienie. Zaburza to równowagę i powoduje, że Q jest większe niż K. W ten sposób pozycja równowagi przesuwa się w kierunku reagentów, z mniejszą liczbą moli cząstek gazu, w celu obniżenia ciśnienia i przywrócenia równowagi.
I odwrotnie, pociągnięcie tłoka do góry zwiększa objętość i zmniejsza ciśnienie. W tym przypadku Q staje się mniejsze niż K. W celu podniesienia ciśnienia pozycja równowagi przesuwa się w kierunku produktów, strony z największą liczbą moli gazu i przywraca równowagę.
Zwiększenie ciśnienia przez dodanie gazu obojętnego do mieszaniny równowagi o stałej objętości nie wpływa na równowagę, ponieważ ciśnienia cząstkowe gazowych reagentów i produktów pozostają niezmienione.
W przypadku układów równowagi z równą liczbą moli gazowych reagentów i produktów, takich jak reakcja między jodem gazowym a chlorem gazowym w celu wytworzenia monochlorku jodu, zmiana objętości układu nie będzie miała wpływu na równowagę.

zasada le chateliera: zmiana objętości (ciśnienia)

W przypadku równowagi w fazie gazowej zmiany stężeń reagentów i produktów mogą wystąpić przy zmianie objętości i ciśnienia. Ciśnienie cząstkowe P gazu doskonałego jest proporcjonalne do jego stężenia molowego M.

<img alt="Eq1" src="/files/ftp_upload/11392/11392_Equation 1.png" style="text-align: center; height: 70px;" />

Zmiany ciśnień cząstkowych dowolnego reagenta lub produktu są zatem zasadniczo zmianami stężeń. Dlatego też zmiany te mają taki sam wpływ na równowagę. Oprócz dodawania lub usuwania reagentów lub produktów, ciśnienia (stężenia) substancji w równowadze w fazie gazowej można również zmieniać poprzez zmianę objętości zajmowanej przez system. Ponieważ wszystkie gatunki równowagi w fazie gazowej zajmują tę samą objętość, dana zmiana objętości spowoduje taką samą zmianę stężenia zarówno reagentów, jak i produktów. Aby określić, jakie przesunięcie (jeśli w ogóle) wywoła ten typ naprężenia, należy wziąć pod uwagę stechiometrię reakcji.

W równowadze reakcję N2 (g) + O2 (g) ⇌ 2 NO (g) opisuje się ilorazem reakcji

<img alt="Eq2" src="/files/ftp_upload/11392/11392_Equation 2.png" style="text-align: center; height: 70px;" />

Jeżeli objętość zajmowana przez mieszaninę równowagową tych substancji zmniejszy się 3-krotnie, ciśnienia cząstkowe wszystkich trzech substancji wzrosną 3-krotnie:

<img alt="Eq3" src="/files/ftp_upload/11392/11392_Equation 3.png" style="text-align: center; height: 70px;" />

Zatem zmiana objętości tej mieszaniny równowagowej w fazie gazowej nie powoduje przesunięcia równowagi.

Podobne traktowanie innego układu, 2 SO2 (g) + O2 (g) ⇌ 2 SO3 (g), daje inny wynik:

<img alt="Eq4" src="/files/ftp_upload/11392/11392_Equation 4.png" style="text-align: center; height: 200px;" />

W tym przypadku zmiana objętości skutkuje ilorazem reakcji mniejszym niż stała równowagi, a zatem równowaga przesunie się w prawo.

Wyniki te ilustrują związek pomiędzy stechiometrią równowagi fazy gazowej a wpływem zmiany ciśnienia (stężenia) wywołanej objętością. Jeżeli całkowite ilości molowe reagentów i produktów są równe, jak w pierwszym przykładzie, zmiana objętości nie powoduje przesunięcia równowagi. Jeżeli ilości molowe reagentów i produktów są różne, zmiana objętości przesunie równowagę w kierunku, który lepiej znosi zmianę objętości. W drugim przykładzie trzy mole reagenta (SO2 i O2) dają dwa mole produktu (SO3), zatem zmniejszenie objętości układu powoduje przesunięcie równowagi w prawo, ponieważ reakcja postępująca wytwarza mniej gazu (2 mole) niż reakcja odwrotna (3 mole). I odwrotnie, zwiększenie objętości tego układu równowagi spowodowałoby przesunięcie w stronę reagentów.

Zasada Le Chateliera: zmiana objętości (ciśnienia)

W przypadku równowagi fazy gazowej zmiany stężeń reagentów i produktów mogą wystąpić przy zmienionej objętości i ciśnieniu. Ciśnienie parcjalne, P, wynoszące ...

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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

the equilibrium constant

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...

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...

homogeneous equilibria for gaseous reactions

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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...

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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...

the small x assumption

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 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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