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

הטמפרטורה משפיעה על קצב תגובה כימית;ולכן, שינוי בטמפרטורה בתגובה בשיווי משקל פועל כהפרעה על המערכת. עיקרון לה שטלייה מנבא איך מערכת תגיב כדי לצמצם הפרעות אלו. שינוי בטמפרטורה משנה את הנפח של קבוע שיווי המשקל, בשונה משינוי בריכוז או בנפח, מה שמזיז את שיווי המשקל בלי לשנות את ערך K.חשבו על פירוק הגז זרחן פנטכלוריד לזרחן טריכלוריד וגז כלור.בתגובה אנדותרמית זו, החום הנספג יכול להיחשב כמגיב. עלייה בטמפרטורה מוסיפה חום למערכת, בדומה להוספת עוד מגיב. לכן, כיוון שיווי המשקל נע לעבר התוצרים ויוצר עוד זרחן טריכלוריד וכלור כדי לצרוך את החום הנוסף, מפני שהערך של קבוע שיווי המשקל K גדל.מצד שני, הפחתה של הטמפרטורה מוציאה חום מהמערכת, בדומה להוצאת מגיב. כיוון שיווי המשקל נע לעבר המגיבים ויוצר עוד זרחן פנטכלוריד כדי לשחרר חום, משום שהערך של K קטן. בתגובה אקסותרמית, כמו בתגובה הגזית בין גופרית דו חמצנית וחמצן ליצירת גופרית תלת חמצנית, החום המשתחרר יכול להיחשב כתוצר.עלייה בטמפרטורה דומה להוספת עוד תוצר. זה גורם לכיוון שיווי המשקל לנוע לעבר המגיבים, ויוצר עוד גופרית דו חמצנית וחמצן לספיגת חלק מהחום שנוסף מכיוון שהערך של K פחת. לעומת זאת, הפחתת הטמפרטורה בתגובה אקסותרמית זו מפחיתה חום, בדומה להפחתת תוצר.כיוון שיווי המשקל נע לעבר התוצרים ויוצר עוד גופרית תלת חמצנית לשחרור חום מכיוון ש-K גדל. לכן, עליית טמפרטורה תורמת לתוצרים בתגובה אנדותרמית, בעוד שירידה בטמפרטורה תורמת לתוצרים בתגובה אקסותרמית.

עקרון לה שטלייה: שינוי טמפרטורה

<div style="direction:rtl">בהתאם לחוק פעולת המסות, שיווי משקל שנלחץ על ידי שינוי בריכוז יעבור לשיקום שיווי משקל ללא כל שינוי בערך קבוע שיווי המשקל, K. כאשר שיווי משקל משתנה בתגובה לשינוי בטמפרטורה, לעומת זאת, זה מתבסס מחדש עם הרכב יחסי שונה המציג ערך שונה עבור קבוע שיווי המשקל. כדי להבין את התופעה, שקול את התגובה האלמנטרית:  <img alt="Eq1" src="/files/ftp_upload/11393/11393_Equation_1.png" style="height: 50px;"> מכיוון שזו תגובה אלמנטרית, חוקי הקצבים עבור קדימה והפוך עשויים להיגזר ישירות מהסטוכיומטריה של המשוואה המאוזנת:  <img alt="Eq2" src="/files/ftp_upload/11393/11393_Equation_2.png" style="height: 65px;"> כאשר המערכת נמצאת בשיווי משקל,  <img alt="Eq3" src="/files/ftp_upload/11393/11393_Equation_3.png" style="height: 50px;"> החלפת חוקי הקצבים בשוויון זה וסידור מחדש נותן  <img alt="Eq4" src="/files/ftp_upload/11393/11393_Equation_4.png" style="height: 100px;"> ניתן לבטא את קבוע שיווי המשקל כפונקציה מתמטית של קבועי הקצב עבור התגובות קדמיות והפוכות. מכיוון שקבועי הקצב משתנים עם הטמפרטורה כפי שמתואר במשוואת ארניוס, ברור שקבוע שיווי המשקל ישתנה גם הוא עם הטמפרטורה (בהנחה שקבועי הקצב מושפעים במידה שונה משינוי הטמפרטורה). עבור תגובות מורכבות יותר המערבות מנגנוני תגובה מרובי-שלבים, קיים קשר מתמטי דומה אך מורכב יותר בין קבוע שיווי המשקל לבין קבועי הקצב של השלבים במנגנון. לא משנה כמה מורכבת יכולה להיות התגובה, תלות הטמפרטורה של קבוע שיווי המשקל שלה נמשכת. חיזוי השינוי שיחווה שיווי משקל בתגובה לשינוי בטמפרטורה מתבצע בצורה נוחה ביותר על ידי התחשבות בשינוי האנתלפיה של התגובה. לדוגמה, היווצרות אמוניה על ידי תהליך הבר-בוש שזה תהליך אקסותרמי (מייצר חום):  <img alt="Eq5" src="/files/ftp_upload/11393/11393_Equation_5.png" style="height: 50px;"> למטרות יישום עקרון לה שטלייה, ניתן לראות בחום, q, כתוצר:  <img alt="Eq6" src="/files/ftp_upload/11393/11393_Equation_6.png" style="height: 50px;"> העלאת הטמפרטורה של המערכת דומה להגדלת כמות התוצר, ולכן שיווי המשקל יעבור שמאלה. הורדת טמפרטורת המערכת תגרום גם לשיווי המשקל לעבור ימינה. עבור תהליכים אנדותרמיים, חום נתפס כמגיב של התגובה ולכן נצפית תלות בטמפרטורה הפוכה. טקסט זה מותאם מ<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>.</div>

בהתאם לחוק הפעולה ההמונית, שיווי משקל המודגש על ידי שינוי בריכוז ישתנה כדי ליצור מחדש שיווי משקל ללא כל שינוי ב ...

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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

le-chateliers-principle-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...

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)

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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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