Name: gas equilibrium under volume changes
Uploaded: 2020-09-24T15:06:51.000Z
Duration: 2 min 32 s
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Le Chatelier's Principle: Changing Volume (Pressure)

يمكن استخدام مبدأ لو شاتليه للتنبؤ بردة فعل نظام في حالة توازن،تجاه الاختلال نتيجة تغيير في الضغط أو الحجم. يتناسب حجم الغاز عكسيًا مع ضغطه،وعليه فبالنسبة لنظام متوازن،فإن إنقاص الحجم يؤدي إلى زيادة الضغط ويسبب اختلالًا في التوازن. واستجابة لذلك،ستتحرك وضعية التوازن باتجاه يحد من الاختلال.ينص قانون الغاز النموذجي على أن ضغط الغاز يتناسب طرديًا مع عدد المولات. بالتالي،يعتمد اتجاه الحركة اللازم لاستعادة التوازن،على عدد مولات جسيمات الغاز في كل من طرفي التفاعل. بما أن ارتفاع عدد المولات يؤدي إلى زيادة الضغط،فإن زيادة الضغط تحرك وضعية التوازن باتجاه الطرف ذو العدد الأقل من المولات لتخفيض الضغط.وبالمثل،فإن انخفاض الضغط يحرك وضعية التوازن باتجاه الطرف ذو العدد الأكبر من مولات الغاز. لنتأمل توازنًكيميائيًا،حيث يتفكك مول واحد من خماسي كلوريد الفوسفور الغازي إلى مول واحد من ثلاثي كلوريد الفوسفور ومول واحد من غاز الكلور كلياًمولان اثنان من الناتج. إذا دُفعت الوضعية للأسفل،سينخفض حجم نظام التوازن،مما سيزيد الضغط.سيؤدي ذلك إلى اختلال التوازن وتصبح قيمة Q أكبر من K.وبالتالي،ستتحرك وضعية التوازن باتجاه المواد المتفاعلة ذات العدد الأقل من مولات جسيمات الغاز،من أجل تخفيض الضغط واستعادة التوازن. وفي المقابل،فإن سحب الوضعية للأعلى يزيد الحجم ويقلل الضغط. في هذه الحالة،تصبح قيمة Q أقل من K.لزيادة الضغط،تتحرك وضعية التوازن باتجاه النواتج،وهي الطرف الأعلى من حيث عدد مولات الغاز،فتستعيد التوازن.زيادة الضغط بإضافة غاز خامل إلى خليط متوازن عند حجم ثابت،لا يؤثر في التوازن،لأن الضغوط الجزئية للمواد المتفاعلة الغازية والنواتج لا تتغير. بالنسبة لأنظمة التوازن،التي تتساوى فيها أعداد مولات المواد المتفاعلة الغازية والنواتج،كالتفاعل بين غاز اليود وغاز الكلور لإنتاج أحادي كلوريد اليود،فإن التغيير في حجم النظام لن يكون له أي تأثير على التوازن.

مبدأ لوشاتيليير: تغيير الحجم (الضغط)

بالنسبة لتوازن الطور الغازي، يمكن أن تحدث تغيرات في تركيزات المواد المتفاعلة والمنتجات مع تغير الحجم والضغط. ويتناسب الضغط الجزئي، P, للغاز المثالي مع تركيزه المولي، M.<img alt="Eq1" src="/files/ftp_upload/11392/11392_Equation 1.png" style="text-align: center; height: 70px;" /> لذا فإن التغييرات في الضغوط الجزئية لأي مادة متفاعلة أو منتج هي في الأساس تغيرات في التركيزات؛ لذلك، فإن هذه التغييرات تنتج نفس التأثيرات على التوازن. بصرف النظر عن إضافة أو إزالة المواد المتفاعلة أو المنتجات، يمكن أيضًا تغيير ضغوط (تركيزات) الأنواع في توازن الطور الغازي عن طريق تغيير الحجم الذي يشغله النظام. نظرًا لأن جميع أنواع توازن الطور الغازي تشغل نفس الحجم، فإن تغييرًا معينًا في الحجم سيؤدي إلى نفس التغيير في التركيز لكل من المواد المتفاعلة والمنتجات. من أجل تمييز التحول، إن وجد، الذي سيحدثه هذا النوع من الإجهاد ، يجب مراعاة قياس العناصر المتفاعلة في التفاعل. عند التوازن ، التفاعل N2 (g) + O2 (g) ⇌ 2 NO (g) موصوفة في حاصل قسمة التفاعل <img alt="Eq2" src="/files/ftp_upload/11392/11392_Equation 2.png" style="text-align: center; height: 70px;" /> إذا انخفض الحجم الذي يشغله خليط التوازن لهذه الأنواع بمعامل 3 ، فإن الضغوط الجزئية لجميع الأنواع الثلاثة ستزداد بمعامل 3: <img alt="Eq3" src="/files/ftp_upload/11392/11392_Equation 3.png" style="text-align: center; height: 70px;" /> وهكذا، فإن تغيير حجم خليط توازن الطور الغازي لا يؤدي إلى حدوث تحول في التوازن. معاملة مماثلة لنظام مختلف، 2 SO2 (g) + O2 (g) ⇌ 2 SO3 (g) بكل الأحوال، ينتج عنها عن نتيجة مختلفة: <img alt="Eq4" src="/files/ftp_upload/11392/11392_Equation 4.png" style="text-align: center; height: 200px;" /> في هذه الحالة، ينتج عن التغيير في الحجم حاصل قسمة التفاعل أصغر من ثابت التوازن، وبالتالي فإن التوازن سوف يتحول إلى اليمين. توضح هذه النتائج العلاقة بين القياس المتكافئ لتوازن الطور الغازي وتأثير تغيير الضغط (التركيز) الناجم عن الحجم. إذا كانت الكميات المولية الإجمالية للمواد المتفاعلة والمنتجات متساوية ، كما في المثال الأول ، فإن التغيير في الحجم لا يغير التوازن. إذا كانت الكميات المولية للمواد المتفاعلة والنواتج مختلفة ، فإن التغيير في الحجم سيغير التوازن في اتجاه أفضل “يستوعب” تغيير الحجم. في المثال الثاني، ثلاثة مولات من المادة المتفاعلة (SO2 و O2) تنتج مولات من المنتج (SO3) ، وبالتالي تناقص يتسبب حجم النظام في تحول التوازن إلى اليمين لأن التفاعل الأمامي ينتج غازًا أقل (2 مول) من التفاعل العكسي (3 مول). على العكس من ذلك، فإن زيادة حجم نظام التوازن هذا سيؤدي إلى تحول نحو المواد المتفاعلة. تم اقتباس هذا النص من <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>.

بالنسبة لتوازنات الطور الغازي ، يمكن أن تحدث تغيرات في تركيزات المواد المتفاعلة والمنتجات مع تغيير الحجم والضغط. الضغط الجزئي ، P ، من ...

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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...

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...

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.

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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