Name: using q to track equilibrium changes
Uploaded: 2020-09-24T15:06:50.000Z
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Reaction Quotient

معادلة ثابت التوازن تُكتب على شكل تركيزات مولية للنواتج C وD،مقسومة على المواد المتفاعلة A وB،في حالة التوازن،وترفع كل منهما إلى قوة أسية تعادل معاملات التكافؤ لكل عنصر. عند حل المعادلة،تكون مساوية لثابت التوازن Kc.كما يمكن كتابة معادلة بالصيغة ذاتها للمواد المتفاعلة والنواتج عند أي تركيز،وتُعرف الكمية المحسوبة بحاصل التفاعل Qc.ومثل Qc،يمكن كتابة معادلة Qp للتفاعلات الغازية باستخدام الضغوط الجزئية. بينما تظل K ثابتة عند درجة حرارة محددة بغض النظر عن التركيز،تتغير قيمة Q مع استمرار التفاعل باتجاه النواتج أو المواد المتفاعلة.يمكن استخدام حاصل التفاعل،لتحديد الاتجاه الذي سيسير فيه تفاعل معين للوصول إلى توازن. في بداية تفاعل معين،إذا كان تركيز النواتج صفرًا،يكون حاصل قسمة التفاعل صفرًا. متى كان تركيز المواد المتفاعلة في المقام مرتفعًا،كأن تكون Q أصغر من K،سيتحرك التفاعل نحو اليمين ليصنع المزيد من النواتج،إلى أن يصل النظام إلى التوازن.إذا كان تركيز المواد المتفاعلة صفرًا،يكون حاصل قسمة التفاعل لانهائي. كلما كان تركيز النواتج في البسط مرتفعًا،كأن تكون Q أكبر من K،سيتحرك التفاعل إلى اليسار لإنتاج مزيد من المواد المتفاعلة. إذا كانت Q تساوي K،يكون النظام متوازنًا،وتكون معدلات التفاعلات الأمامية والعكسية متساوية.لنأخذ تفاعلًا معينًا ثابت توازنه 50. إذا كان خليط التفاعل يحتوي على 0.20 مولار من الهيدروجين،و0.20 مولار من اليود،و1.7 مولار من يوديد الهيدروجين،يمكن تحديد اتجاه التفاعل المبيّن بحساب قيمة Q.باستبدال التركيزات المعطاة في المعادلة،تكون قيمة Q تساوي 72،وهي أكبر من قيمة K.وبالتالي،سيتحول التفاعل نحو اليسار.

<ol>
	<li>ليدرر ، روبرت. "حاصل التفاعل غير ضروري لحل مشاكل التوازن: لا توجد مشاكل مع Q." مجلة التعليم الكيميائي 82 ، العدد 8 (2005): 1149. <a href="https://pubs.acs.org/doi/pdf/10.1021/ed082p1149.2">https://pubs.acs.org/doi/pdf/10.1021/ed082p1149.2</a></li>
</ol>

حاصل قسمة التفاعل

يتم تقييم حالة التفاعل القابل للعكس بشكل ملائم من خلال تقييم حاصل قسمة التفاعل (Q. بالنسبة إلى رد الفعل القابل للعكس الموضح بواسطة m A + n B ⇌ x C + y D، حاصل قسمة التفاعل مشتق مباشرة من قياس العناصر المتوازنة للمعادلة المتوازنة مثل <img alt="Eq1" src="/files/ftp_upload/11389/11389_Equation_1.png" style="text-align: center; height: 90px;" /> حيث يشير الرمز السفلي c إلى استخدام التركيزات المولية في التعبير. إذا كانت المواد المتفاعلة والمنتجات غازية ، فيمكن اشتقاق حاصل التفاعل بالمثل باستخدام الضغوط الجزئية: <img alt="Eq2" src="/files/ftp_upload/11389/11389_Equation_2.png" style="text-align: center; height: 90px;" /> لاحظ أن معادلات حاصل رد الفعل أعلاه هي تبسيط لتعبيرات أكثر صرامة تستخدم قيمًا نسبية للتركيزات والضغوط بدلاً من القيم المطلقة. قيم التركيز والضغط النسبية هذه بلا أبعاد (لا تحتوي على وحدات) ؛ وبالتالي ، كذلك حاصل قسمة التفاعل. تختلف القيمة العددية لـ Q مع تقدم التفاعل نحو التوازن ؛ لذلك ، يمكن أن يكون بمثابة مؤشر مفيد لحالة التفاعل’. لتوضيح هذه النقطة ، ضع في اعتبارك أكسدة ثاني أكسيد الكبريت: <img alt="Eq3" src="/files/ftp_upload/11389/11389_Equation_3.png" style="text-align: center; height: 50px;" /> هناك سيناريوهان تجريبيان مختلفان ممكنان هنا ، أحدهما يبدأ فيه هذا التفاعل بمزيج من المواد المتفاعلة فقط ، SO2 و O2، والآخر يبدأ بـ المنتج الوحيد ، SO3. بالنسبة للتفاعل الذي يبدأ بخليط من المواد المتفاعلة فقط ، فإن Q تساوي في البداية صفرًا: <img alt="Eq4" src="/files/ftp_upload/11389/11389_Equation_4.png" style="text-align: center; height: 90px;" /> مع تقدم التفاعل نحو التوازن في الاتجاه الأمامي ، تنخفض تركيزات المادة المتفاعلة (كما يفعل مقام Qc) ، ويزداد تركيز المنتج (كما يفعل بسط Qc) ، وبالتالي يزيد حاصل التفاعل. عند تحقيق التوازن، تظل تركيزات المواد المتفاعلة والمنتج ثابتة، كما هو الحال مع قيمة Q c . إذا بدأ التفاعل مع وجود المنتج فقط ، فإن قيمة Qc تكون في البداية غير محددة (كبيرة بشكل لا يقاس ، أو لانهائية): <img alt="Eq5" src="/files/ftp_upload/11389/11389_Equation_5.png" style="text-align: center; height: 90px;" /> في هذه الحالة ، يتقدم التفاعل نحو التوازن في الاتجاه العكسي. تركيز منتج وبسط Qc يتناقصان بمرور الوقت ، وتركيزات المادة المتفاعلة ومقام Qc يزداد، وبالتالي ينخفض ​​حاصل قسمة التفاعل حتى يصبح ثابتًا عند التوازن. تسمى القيمة الثابتة لـ Q التي يعرضها نظام عند التوازن بثابت التوازن ، K: <img alt="Eq6" src="/files/ftp_upload/11389/11389_Equation_6.png" style="text-align: center; height: 50px;" /> تقييم حاصل قسمة التفاعل يشكل ثاني أكسيد النيتروجين الغازي رباعي أكسيد ثنائي النيتروجين وفقًا لهذه المعادلة: <img alt="Eq7" src="/files/ftp_upload/11389/11389_Equation_7.png" style="text-align: center; height: 50px;" /> عند إضافة 0.10 مول NO2 إلى دورق سعة 1.0 لتر عند 25 °C، يتغير التركيز بحيث عند التوازن ، [NO2] = 0.016 M و [N2 O 4] = 0.042 M. قبل تكوين أي منتج ، [NO2] = 0.10 M و [ N 2O4] = 0 M. وهكذا ، <img alt="Eq8" src="/files/ftp_upload/11389/11389_Equation_8.png" style="text-align: center; height: 90px;" /> في حالة التوازن ، <img alt="Eq9" src="/files/ftp_upload/11389/11389_Equation_9.png" style="text-align: center; height: 90px;" />تم اقتباس هذا النص من <a href="https://openstax.org/books/chemistry-2e/pages/13-2-equilibrium-constants">Openstax, Chemistry 2e, Section 13.2 Equilibrium Constants</a><a href="https://openstax.org/books/chemistry-2e/pages/13-2-equilibrium-constants">.</a>

يتم تقييم حالة التفاعل القابل للانعكاس بشكل ملائم من خلال تقييم حاصل التفاعل (Q). بالنسبة للتفاعل القابل للانعكاس الموصوف بواسطة m A + n B ...

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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 moves toward equilibrium, the value of Q changes, so it gives a snapshot of the system’s status.

The oxidation of sulfur dioxide shows how Q changes over time. If a reaction starts with only reactants, SO2 and O2, then Q is initially zero. As the reaction moves forward, reactant concentrations drop, the denominator of Qc gets smaller, product concentration rises, and Qc increases.

If the reaction starts with only product, SO3, then Qc is initially undefined and can be treated as immeasurably large or infinite. In that case, the reaction moves in reverse toward equilibrium. Product concentration and the numerator of Qc decrease, while reactant concentrations and the denominator increase. Qc keeps dropping until it becomes constant at equilibrium.

The constant value of Q for a system at equilibrium is the equilibrium constant, K. A nitrogen dioxide example shows how this works in practice. When 0.10 mol NO2 is placed in a 1.0-L flask at 25 °C, the initial concentrations are [NO2] = 0.10 M and [N2O4] = 0 M, so Qc can be found from the starting amounts. At equilibrium, [NO2] = 0.016 M and [N2O4] = 0.042 M, and Qc is calculated again from those concentrations.

Using Q to Track Equilibrium 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...

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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...

reaction quotient

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...

dynamic equilibrium

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...

calculating equilibrium concentrations

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)

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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

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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 the reaction quotient Q shows whether a reversible reaction is moving forward, reversing, or reaching equilibrium using concentration and pressure examples.

Using Q to Track Equilibrium Changes | JoVE K12

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Using Q to Track Equilibrium Changes

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