Name: shifting equilibrium by changing concentration
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Le Chatelier's Principle: Changing Concentration

מערכת נמצאת בשיווי משקל דינמי כשקצב התגובה הישרה שווה לקצב התגובה ההפוכה. למרות שהתגובה ממשיכה לשני הכיוונים, ריכוזי הנטו של המגיבים ותוצרים נשארים קבועים. בשלב זה, אם ריכוז של מגיב או תוצר משתנה, לדוגמה על ידי הוספת מולקולה A או הורדת מולקולה C, שיווי המשקל מופר.שינויים אלו מפעילים הפרעה על המערכת. על פי עיקרון לה שטלייה, כשמפעילים הפרעה על מערכת בשיווי משקל בעקבות שינוי בריכוז, בנפח, בלחץ או בטמפרטורה, שיווי המשקל נע בכיוון מסוים כדי להפחית את ההפרעה ולהשיב את שיווי המשקל. כדי להבין תגובה של מערכת לשינויים בריכוז, חשבו על שני מכלי מים מחוברים, A ו-B.בתחילה, גובה המים בשני המכלים שווה, והמערכת בשיווי משקל. לאחר מכן מוסיפים מים למכל B.ההפרעה הזו מעלה את פני המים במכל B ומפרה את שיווי המשקל. כדי לאזן את ההפרעה, מים עוברים למכל A כדי להשיב את שיווי המשקל.לעומת זאת, אם מים מוצאים ממכל B, הנמכת גובה המים גורמת אף היא להפרעה. כתוצאה מכך, מים ממכל A יזרמו למכל B כדי להשיב את שיווי המשקל. במערכת הנמצאת בשיווי משקל מנת התגובה שווה לקבוע שיווי המשקל.הוספת עוד מגיבים לתערובות שיווי המשקל מפחיתה את Q.על פי עקרון לה שטלייה, המערכת מצמצת את ההפרעה על ידי שינוי כיוון שיווי המשקל לכיוון התוצרים כדי לבזבז את המגיב הנוסף ולחזור לשיווי משקל. בדומה לכך, הסרת תוצר מתערובות שיווי המשקל תוביל לתגובה דומה כדי להשיב את שיווי המשקל. לעומת זאת, הוספת תוצרים מגדילה את Q.לכן, כיוון שיווי המשקל נע לכיוון המגיבים כדי לבזבז את התוצר הנוסף ולהשיב את שיווי המשקל.שינוי בריכוז מזיז את כיוון שיווי המשקל בלי לשנות את ערך K.

עקרון לה שטלייה: שינוי ריכוז

<div style="direction:rtl">מערכת בשיווי משקל נמצאת במצב של איזון דינמי, כאשר תגובות קדמיות והפוכות מתרחשות בקצבים שווים. אם מערכת שיווי משקל נתונה לשינוי בתנאים המשפיעים על קצבי תגובה אלו בצורה שונה (לחץ), אז הקצבים כבר אינם שווים והמערכת אינה בשיווי משקל. המערכת תחווה לאחר מכן תגובה נטו בכיוון של קצב גדול יותר (הסטה) שתקים מחדש את שיווי המשקל. תופעה זו מסוכמת על ידי עקרון לה שטלייה: אם מערכת שיווי משקל נלחצת, המערכת תחווה שינוי בתגובה ללחץ שמשקם מחדש את שיווי המשקל. קצבי התגובה מושפעים בעיקר מריכוזים, כפי שמתואר על ידי חוק קצב התגובה, ומטמפרטורה, כפי שמתואר במשוואת ארניוס. כתוצאה מכך, שינויים בריכוז ובטמפרטורה הם שני הלחצים שיכולים לשנות שיווי משקל. השפעת שינוי בריכוז אם מערכת שיווי משקל נתונה לשינוי בריכוז של מגיב או מין תוצר, קצב התגובה הקדמית או ההפוכה ישתנה. כדוגמה, שקול את תגובת שיווי המשקל: <img alt="Eq1" src="/files/ftp_upload/11391/11391_Equation 1v2.png" style="text-align: center; height: 48px;"> כאשר מערכת זו נמצאת בשיווי משקל, קצב התגובה הקדמית וההפוכה שווים. <img alt="Eq2" src="/files/ftp_upload/11391/11391_Equation 2.png" style="text-align: center; height: 50px;"> אם המערכת נלחצת על ידי הוספת מגיב, או N2 או O2, העלייה המתקבלת בריכוז גורמת לעלייה בקצב התגובה הקדמית, העולה על זה של התגובה ההפוכה: <img alt="Eq3" src="/files/ftp_upload/11391/11391_Equation 3.png" style="text-align: center; height: 50px;"> המערכת תחווה תגובה נטו זמנית בכיוון קדימה כדי ליצור מחדש שיווי משקל (שיווי המשקל יעבור ימינה). אותה תזוזה תגרום אם מוצר NO יוסר מהמערכת, מה שמקטין את קצב התגובה ההפוכה, הגורמת שוב לאותו חוסר איזון בקצבים. ניתן להשתמש באותו היגיון כדי להסביר את ההזזה השמאלית הנובעת מהסרת מגיב או הוספת תוצר למערכת שיווי משקל. שני הלחצים הללו מביאים לקצב מוגבר של התגובה ההפוכה <img alt="Eq4" src="/files/ftp_upload/11391/11391_Equation 4.png" style="text-align: center; height: 50px;"> ותגובה נטו זמנית בכיוון ההפוך לשיקום שיווי משקל. כחלופה לפרשנות קינטית זו, ניתן לתרץ את ההשפעה של שינויים בריכוז על שיווי המשקל במונחים של מנות תגובה. כאשר המערכת נמצאת בשיווי משקל, <img alt="Eq5" src="/files/ftp_upload/11391/11391_Equation 5.png" style="text-align: center; height: 70px;"> אם מוסיפים מגיב (הגדלת המכנה של מנת התגובה) או מסירים תוצר (הקטנת המונה), אז Qc < Kcושיווי המשקל יזוז ימינה. שימו לב ששלושת הדרכים השונות לגרימת מתח זה מביאות לשלושה שינויים שונים בהרכב תערובת שיווי המשקל. אם יתווסף N2, ההסטה הימנית תצרוך O2 ותיצור NO כאשר שיווי המשקל יתבסס מחדש, ותניב תערובת עם ריכוז גדול יותר של N2 ו-NO וריכוז נמוך יותר של O2 ממה שהיה קיים קודם לכן. אם מוסיפים O2, לתערובת שיווי המשקל החדשה יהיו ריכוזים גדולים יותר של O2 ו-NO וריכוז נמוך יותר של N2. לבסוף, אם מסירים את NO, לתערובת שיווי המשקל החדשה יהיו ריכוזים גדולים יותר של N2 ו-O2 וריכוז נמוך יותר של NO. למרות הבדלים אלה בהרכב, ערכו של קבוע שיווי המשקל יהיה זהה לאחר הלחץ כפי שהיה קודם (לפי חוק פעולת המסות). אותו היגיון עשוי להיות מיושם עבור מתחים הכוללים הסרת מגיבים או הוספת תוצר, ובמקרה זה Qc > Kc ושיווי המשקל יעבור שמאלה. 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: LeChatelier’s Principle</a>.</div>

מערכת בשיווי משקל נמצאת במצב של איזון דינמי, כאשר תגובות קדימה ואחורה מתרחשות בשיעורים שווים. אם מערכת שיווי משקל היא ...

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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 reactant or product can move the equilibrium in one direction or the other.

Adding a reactant, such as N2 or O2 in the example reaction, increases the forward reaction rate. The forward rate then becomes greater than the reverse rate, so the equilibrium shifts right. Removing the product NO has the same effect because it lowers the reverse reaction rate and creates the same imbalance.

The opposite changes cause a left shift. Removing reactant or adding product increases the reverse reaction rate compared with the forward rate. The system then moves in the reverse direction until equilibrium is reached again.

This shift can also be explained with the reaction quotient, Qc, and the equilibrium constant, Kc. When reactant is added, Qc becomes less than Kc. When product is removed, Qc also becomes less than Kc, so the equilibrium shifts right. If reactant is removed or product is added, then Qc is greater than Kc and the equilibrium shifts left.

Although the final composition changes, the equilibrium constant stays the same after the stress is removed and the system rebalances. If N2 is added, the new mixture has more N2 and NO and less O2. If O2 is added, the mixture has more O2 and NO and less N2. If NO is removed, the mixture ends with more N2 and O2 and less NO. The same idea applies to shifts caused by removing reactant or adding product.

Shifting Equilibrium by Changing Concentration

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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...

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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

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

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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.

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Learn how changing concentration shifts chemical equilibrium, including why adding or removing reactants or products moves the reaction left or right.

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PS.Chm.3.2

Analyze the structure of atoms and isotopes.

Shifting Equilibrium by Changing Concentration

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