Name: relating kc and kp in gas equilibria
Uploaded: 2020-09-24T15:06:50.000Z
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NC K-12 Science Standard Course of Study

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Chemistry

Matter and its Interactions

PS.Chm.5.2

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Homogeneous Equilibria for Gaseous Reactions

בתגובות כימיות, בהן המגיבים והתוצרים כולם גזים, את קבוע שיווי המשקל ניתן לחשב באמצעות הלחצים החלקיים של כל חומר ואין צורך בריכוזים מולריים. וכך, כאשר גזים A ו-B הופכים לגזים C ו-D בתגובה הפיכה, משוואת שיווי המשקל יכולה להיכתב במקום כלחץ החלקי של כל גז, בחזקת המקדם הסטויכומטרי שלו. קבוע שיווי המשקל מסומן כ-Kp כשהאות p מסמנת את הלחץ.בכל תגובה כימית נתונה, Kp לא שווה בהכרח ל-Kc, משום שהלחצים החלקיים של גז והריכוזים המולריים שלו הם ערכים נפרדים. יחד עם זאת, ניתן למצוא את היחס בין שני הקבועים באמצעות משוואת הגזים האידיאלית וההגדרה של מולריות. כדי לחלץ את היחס, נבחן את נוסחאות שיווי המשקל של Kc ו-Kp בתגובה כימית נתונה.משוואת הגז האידיאלי מנסחת יחס בין לחץ של גז למספר המולים שלו ונפחו בטמפרטורה נתונה. הצבת ריכוז מולרי ביחס בין מולים לנפח במשוואת הגז האידיאלי מאפשרת ביטוי של הלחץ של גז אידיאלי באמצעות הריכוז המולרי שלו. כך ניתן להציב את הלחצים החלקיים הספציפיים בנוסחה של Kp הערך השווה של הריכוז בכל גז.המקדמים הסטויכומטריים נותרים ללא שינוי. בביטוי המותאם והחדש עבור Kp, היחס בין ריכוז התוצרים לריכוז המגיבים יכול להיות מוחלף ב-Kc. משוואה זו נותנת לנו את היחס בין שני הקבועים Kp שווה ל-Kc כפול RT בחזקת סכום המקדמים של התוצרים פחות סכום המקדמים של המגיבים.ההבדל בין שני המקדמים של המגיבים והתוצרים הגזיים יכול להיות מיוצג כדלתא n.

שיווי משקל הומוגני לתגובות גזים

<div style="direction:rtl">שיווי משקל הומוגני לתגובות גזים עבור תגובות גז, קבוע שיווי המשקל עשוי להתבטא במונחים של הריכוזים המולריים (Kc) או הלחצים החלקיים (Kp) של המגיבים והתוצרים. קשר בין שני ערכי K אלה יכול להיגזר בפשטות ממשוואת הגז האידיאלית ומהגדרת המולריות. לפי משוואת הגז האידיאלית: <img alt="Eq1" src="/files/ftp_upload/11387/11387_Equation_1.png" style="text-align: center; height: 50px;"> ריכוז מולרי או מולריות ניתנים לפי מספר מולות חלקי הנפח: <img alt="Eq2" src="/files/ftp_upload/11387/11387_Equation_2.png" style="text-align: center; height: 100px;"> לכן, <img alt="Eq3" src="/files/ftp_upload/11387/11387_Equation_3.png" style="text-align: center; height: 90px;"> כאשר P הוא לחץ חלקי, V הוא נפח, n הוא מספר מולות, R הוא קבוע הגז, T הוא טמפרטורה ו-M הוא ריכוז מולרי. לתגובת שלב הגז: m A + n B ⇌ x C + y D <img alt="Eq4" src="/files/ftp_upload/11387/11387_Equation_4.png" style="text-align: center; height: 130px;"> וכך, היחס בין Kc ל-KP הוא <img alt="Eq5" src="/files/ftp_upload/11387/11387_Equation_5.png" style="text-align: center; height: 50px;"> כאשר Δn הוא ההבדל בכמויות המולריות של גזי תוצר וגזים מגיבים, במקרה זה: <img alt="Eq6" src="/files/ftp_upload/11387/11387_Equation_6.png" style="text-align: center; height: 50px;"> טקסט זה מותאם מ<a href="https://openstax.org/books/chemistry-2e/pages/13-2-equilibrium-constants">Openstax, Chemistry 2e, Section 13.2 Equilibrium Constants</a>.</div>

שיווי משקל הומוגני לתגובות גזיות
עבור תגובות פאזת גז, קבוע שיווי המשקל יכול לבוא לידי ביטוי במונחים של הטוחנת ...

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Chemical Equilibrium

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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 temperature. Molar concentration is written as M.

For a gas reaction written as mA + nB ⇌ xC + yD, the two equilibrium constants are connected by a simple relation. That relation depends on Δn, which is the difference between the molar amounts of gaseous products and gaseous reactants. This connection helps show how Kc and Kp describe the same equilibrium in different ways.

Relating Kc and Kp in Gas Equilibria

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

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

le-chatelier-s-principle-changing-concentration

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...

calculating the equilibrium constant

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

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...

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)

le-chatelier-s-principle-changing-volume-pressure

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-changing-temperature

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 gas-phase equilibrium constants are connected by the ideal gas law. Compare Kc and Kp using molarity, partial pressure, and Δn.

Relating Kc and Kp in Gas Equilibria | JoVE K12

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Relating Kc and Kp in Gas Equilibria

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