Name: le chatelier's principle in heating and cooling
Uploaded: 2020-09-24T15:06:51.000Z
Duration: 2 min 19 s
Channel: JoVE

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Le Chatelier's Principle: Changing Temperature

درجة الحرارة تؤثر في معدل سرعة التفاعل الكيميائي،وعليه فإن التغيير في درجة الحرارة لتفاعل في حالة توازن يكون بمثابة إخلال في النظام. يتنبأ مبدأ لو شاتليه بكيفية استجابة النظام للحد من مثل هذه الاخلالات. التغيير في درجة الحرارة يغير قيمة ثابت التوازن،خلافًا للتغيير في التركيز أو الحجم،الذي يحرك التوازن دون تغيير قيمة K.لنتأمل تفكك غاز خماسي كلوريد الفوسفور إلى ثلاثي كلوريد الفوسفور وغاز الكلور،بالنسبة لهذا التفاعل الامتصاصي،يمكن التفكير في الحرارة الممتصة على أنها مادة متفاعلة.زيادة درجة الحرارة تضيف حرارة للنظام وهذا يشبه إضافة المزيد من مادة متفاعلة. وعليه،تتحرك وضعية التوازن باتجاه النواتج،وتنتج المزيد من ثلاثي كلوريد الفوسفور والكلور،لاستهلاك الحرارة الزائدة لأن قيمة ثابت التوازن K قد ازدادت. وفي المقابل،فإن نقصان درجة الحرارة يزيل حرارة من النظام،وهو ما يشبه إزالة مادة متفاعلة.تتحرك وضعية التوازن باتجاه المواد المتفاعلة،وتنتج المزيد من خماسي كلوريد الفوسفور،للتخلص من الحرارة لأن قيمة K قد انخفضت. بالنسبة لتفاعل طارد للحرارة،كالتفاعل الغازي بين ثاني أكسيد الكبريت والأكسجين،لإنتاج ثالث أكسيد الكبريت،يمكن التفكير بالحرارة الصادرة على أنها أحد النواتج. الزيادة في درجة الحرارة تشبه إضافة المزيد من أحد النواتج.وهذا يجعل وضعية التوازن تتحرك باتجاه المواد المتفاعلة،فتنتج المزيد من ثاني أكسيد الكبريت والأكسجين،لاستيعاب بعض الحرارة المضافة لأن قيمة K قد انخفضت. وخلافًا لذلك،فإن تخفيض درجة الحرارة لهذا التفاعل الطارد للحرارة يزيل الحرارة،كإزالة أحد النواتج. تتحرك وضعية التوازن باتجاه النواتج لإنتاج المزيد من ثالث أكسيد الكبريت،لطرد الحرارة لأن قيمة K قد ارتفعت.وعليه،فإن زيادة درجة الحرارة يميل إلى النواتج في تفاعل امتصاصي،بينما تخفيض درجة الحرارة يميل إلى النواتج في تفاعل طارد للحرارة.

مبدأ لوشاتيليير: تغيير درجة الحرارة

بما يتوافق مع قانون الحركة الجماعية ، فإن التوازن الذي يؤكده تغيير التركيز سوف يتحول إلى إعادة التوازن دون أي تغيير في قيمة ثابت التوازن ، K. عند حدوث توازن التحولات استجابة لتغير في درجة الحرارة ، ومع ذلك ، يتم إعادة إنشائها بتكوين نسبي مختلف يعرض قيمة مختلفة لثابت التوازن. لفهم هذه الظاهرة ، ضع في اعتبارك التفاعل الأولي:  <img alt="Eq1" src="/files/ftp_upload/11393/11393_Equation_1.png" style="height: 50px;" /> نظرًا لأن هذا رد فعل أولي ، يمكن اشتقاق قوانين المعدلات للأمام والعكس مباشرة من القياس المتكافئ للمعادلة المتوازنة:  <img alt="Eq2" src="/files/ftp_upload/11393/11393_Equation_2.png" style="height: 65px;" /> عندما يكون النظام في حالة توازن ،  <img alt="Eq3" src="/files/ftp_upload/11393/11393_Equation_3.png" style="height: 50px;" /> استبدال قوانين المعدلات في هذه المساواة وإعادة الترتيب يعطي  <img alt="Eq4" src="/files/ftp_upload/11393/11393_Equation_4.png" style="height: 100px;" /> يمكن التعبير عن ثابت التوازن كدالة رياضية لثوابت المعدل للتفاعلات الأمامية والعكسية. نظرًا لأن ثوابت المعدل تختلف باختلاف درجة الحرارة كما هو موضح في معادلة أرهينيو ، فمن المنطقي أن ثابت التوازن سيتغير بالمثل مع درجة الحرارة (بافتراض أن ثوابت المعدل تتأثر بدرجات مختلفة بتغير درجة الحرارة). بالنسبة للتفاعلات الأكثر تعقيدًا التي تتضمن آليات تفاعل متعددة الخطوات، توجد علاقة رياضية مماثلة ولكنها أكثر تعقيدًا بين ثابت التوازن وثوابت معدل الخطوات في الآلية. بغض النظر عن مدى تعقيد التفاعل، يستمر الاعتماد على درجة الحرارة في ثابت التوازن الخاص به. إن توقع التحول الذي سيشهده التوازن استجابةً لتغير في درجة الحرارة هو الأكثر ملاءمةً من خلال النظر في تغير المحتوى الحراري للتفاعل. على سبيل المثال، يعتبر تكوين الأمونيا بواسطة عملية هابر’ عملية طاردة للحرارة (منتجة للحرارة):  <img alt="Eq5" src="/files/ftp_upload/11393/11393_Equation_5.png" style="height: 50px;" /> لأغراض تطبيق مبدأ âلو شاتيلير#8217; يمكن اعتبار الحرارة ، q ، على أنها منتج:  <img alt="Eq6" src="/files/ftp_upload/11393/11393_Equation_6.png" style="height: 50px;" /> يشبه رفع درجة حرارة النظام زيادة كمية المنتج، وبالتالي سيتحول التوازن إلى اليسار. وبالمثل، سيؤدي خفض درجة حرارة النظام إلى تحول التوازن إلى اليمين. بالنسبة للعمليات الماصة للحرارة، يُنظر إلى الحرارة على أنها مادة متفاعلة للتفاعل ولذلك يتم ملاحظة الاعتماد المعاكس لدرجة الحرارة. 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: Le </a><a href="https://openstax.org/books/chemistry-2e/pages/13-3-shifting-equilibria-le-chateliers-principle">Ch</a>â<a href="https://openstax.org/books/chemistry-2e/pages/13-3-shifting-equilibria-le-chateliers-principle">t</a><a href="https://openstax.org/books/chemistry-2e/pages/13-3-shifting-equilibria-le-chateliers-principle">elier’s Principle</a>.

تماشيا مع قانون العمل الجماعي ، فإن التوازن الذي يتم التأكيد عليه من خلال التغيير في التركيز سيتحول إلى إعادة التوازن دون أي تغيير في ...

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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 forward and reverse rate laws based on the balanced equation. At equilibrium, the forward and reverse reaction rates are equal. When those rate laws are combined and rearranged, the equilibrium constant can be expressed as a function of the forward and reverse rate constants.

Because rate constants change with temperature, as described by the Arrhenius equation, the equilibrium constant can also change with temperature. This idea applies to both elementary reactions and more complex multistep mechanisms. In multistep reactions, the math is more complicated, but the temperature dependence of the equilibrium constant still remains.

A practical way to predict the direction of the shift is to use the reaction’s enthalpy change. In the Haber process, ammonia formation is exothermic, which means it produces heat. For Le Chatelier's principle, heat can be treated like a product. Raising the temperature adds heat, so the equilibrium shifts left. Lowering the temperature removes heat, so the equilibrium shifts right. For endothermic processes, heat acts like a reactant, so the temperature effect is opposite.

Le Chatelier's Principle in Heating and Cooling

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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...

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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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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 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 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)

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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

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 heating and cooling change chemical equilibrium, why K changes with temperature, and how exothermic and endothermic reactions shift.

Le Chatelier's Principle in Heating and Cooling | JoVE K12

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Le Chatelier's Principle in Heating and Cooling

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