Name: gas equilibrium under volume changes
Uploaded: 2020-09-24T15:06:51.000Z
Duration: 2 min 32 s
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Chemistry

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Le Chatelier's Principle: Changing Volume (Pressure)

עיקרון לה שטלייה משמש לחיזוי האופן בו מערכת בשיווי משקל תגיב להפרעת שינוי בנפח או לחץ. נפח הגז נמצא ביחס הפוך ללחץ שלו;לכן, עבור מערכת בשיווי משקל, הפחתה בנפח מגדילה את הלחץ ומפרה את שיווי המשקל. כתגובה לכך, כיוון שיווי המשקל ינוע בכיוון שיצמצם את ההפרעה.חוק הגזים האידאלים מציין שלחץ הגז נמצא ביחס ישר למספר המולים. לכן, כיוון השינוי הנדרש כדי להשיב את שיווי המשקל תלוי במספר המולים של חלקיקי הגז בכל צד של התגובה. מכיוון שמספר מולים רב יותר גורם ללחץ גבוה יותר, עלייה בלחץ מניעה את כיוון שיווי המשקל לצד שבו יש פחות מולים, להפחתת הלחץ.בדומה לכך, הפחתה בלחץ מניעה את מיקום שיווי המשקל לצד עם יותר מולים של גז. דמיינו שיווי משקל כימי, שבו מול אחד של גז זרחן פנטכלוריד מתפרק למול של זרחן טריכלוריד ולמול של גז כלורי. שני מולים סך הכול כתוצר.אם הבוכנה נלחצת מטה, הנפח של מערכת שיווי המשקל קטן, ובכך הלחץ גדל. זה מפזר את שיווי המשקל וגורם ל-Q גדול מ-K. לכן, מיקום שיווי המשקל ינוע לכיוון המגיבים, עם פחות מולים של חלקיקי גז, כדי להפחית את הלחץ ולהשיב את שיווי המשקל.לעומת זאת, משיכת הבוכנה למעלה מגדילה את הנפח ומורידה את הלחץ. במקרה זה, Q הופך לקטן מ-K. כדי להגדיל את הלחץ, כיוון שיווי המשקל נע לכיוון התוצר, הצד עם יותר מולי הגז, ומשיב את שיווי המשקל.הגדלת הלחץ על ידי הוספת גז לא פעיל לתערובת שיווי המשקל בנפח קבוע, לא תשפיע על שיווי המשקל בגלל שהלחצים החלקיים של המגיבים הגזיים, והתוצרים יישארו ללא שינוי. במערכות בשיווי משקל, עם מספר שווה של מולי גז מגיבים ותוצרים, כמו התגובה בין גז יוד לגז כלור ליצירת יוד מונוכלוריד, שינוי בנפח המערכת לא יגרום לשינוי בשיווי המשקל.

עקרון לה שטלייה: שינוי הנפח (לחץ)

<div style="direction:rtl">

עבור שיווי משקל בשלב גז, שינויים בריכוזים של מגיבים ותוצרים יכולים להתרחש עם נפח ולחץ משתנים. הלחץ החלקי,P, של גז אידיאלי הוא פרופורציונלי לריכוז המולרי שלו,M. 
<img alt="Eq1" src="/files/ftp_upload/11392/11392_Equation 1.png" style="text-align: center; height: 70px;"> 

אז שינויים בלחצים החלקיים של כל מגיב או תוצר הם בעצם שינויים בריכוזים; לכן, שינויים אלה מניבים את אותן השפעות על שיווי המשקל. מלבד הוספה או הסרה של מגיבים או תוצרים, ניתן לשנות את הלחצים (ריכוזים) של מינים בשיווי משקל בשלבי גז על ידי שינוי הנפח שתופסת המערכת. מכיוון שכל המינים של שיווי משקל בשלבי גז תופסים את אותו נפח, שינוי נתון בנפח יגרום לאותו שינוי בריכוז הן עבור המגיבים והן עבור התוצרים. כדי להבחין איזה שינוי, אם בכלל, סוג זה של מתח יגרום, יש לשקול את הסטוכיומטריה של התגובה. 

בשיווי משקל, התגובה N2 (g) + O2 (g)⇌ 2 NO (g) מתוארת על ידי מנת התגובה
<img alt="Eq2" src="/files/ftp_upload/11392/11392_Equation 2.png" style="text-align: center; height: 70px;"> 

אם הנפח התפוס על ידי תערובת שיווי משקל של מינים אלה יקטן בגורם של 3, הלחצים החלקיים של כל שלושת המינים יוגדלו בגורם של 3: 
<img alt="Eq3" src="/files/ftp_upload/11392/11392_Equation 3.png" style="text-align: center; height: 70px;"> 

ולכן, שינוי נפח של תערובת שיווי משקל זו בשלבי גז אינו גורם לשינוי בשיווי המשקל. 

טיפול דומה במערכת שונה, 2 SO2 (g)+ O2 (g) ⇌ 2 SO3 (g), לעומת זאת, מניב תוצאה שונה: 
<img alt="Eq4" src="/files/ftp_upload/11392/11392_Equation 4.png" style="text-align: center; height: 200px;"> 

במקרה זה, השינוי בנפח מביא למנת תגובה קטנה מקבוע שיווי המשקל, ולכן שיווי המשקל יזוז ימינה. 

תוצאות אלו ממחישות את הקשר בין הסטוכיומטריה של שיווי משקל של שלבי גז לבין ההשפעה של שינוי לחץ (ריכוז) המושרה על ידי נפח. אם כל הכמויות המולריות של המגיבים והתוצרים שוות, כמו בדוגמה הראשונה, שינוי בנפח אינו מעביר את שיווי המשקל. אם הכמויות המולריות של המגיבים והתוצרים שונות, שינוי בנפח יעביר את שיווי המשקל לכיוון ש"מתאים" טוב יותר לשינוי הנפח. בדוגמה השנייה, שלוש מולים של מגיב (SO2 ו-O2) מניבים שני מולים של תוצר (SO3), ולכן הקטנת נפח המערכת גורמת לשיווי המשקל לעבור ימינה מכיוון שהתגובה הקדמית מייצרת פחות גז (2 מול) מהתגובה ההפוכה (3 מול). לעומת זאת, הגדלת הנפח של מערכת שיווי משקל זו תגרום למעבר לעבר המגיבים. 

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>

עבור שיווי משקל פאזות גז, שינויים בריכוזים של מגיבים ותוצרים יכולים להתרחש עם שינוי נפח ולחץ. הלחץ החלקי, P, של ...

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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...

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...

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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...

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 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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