Name: chemical equilibrium in reversible reactions
Uploaded: 2020-09-24T15:06:50.000Z
Duration: 2 min 20 s
Channel: JoVE

K12 Standards

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NC K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.5.2

Dynamic Equilibrium

תגובה כימית הפיכה מאופיינת בכך שהפיכת המגיבים לתוצרים וההפיכה חזרה של תוצרים למגיבים מתרחשות בו זמנית. חץ כפול בין המגיבים לתוצרים מסמל את אופי התגובה ההפיך. כשקצב יצירת התוצרים, או קצב התגובה הישירה, שווה לקצב היווצרות המגיבים או לקצב התגובה ההפוכה, מושג שיווי משקל כימי.שיווי משקל כימי הוא למעשה תהליך דינמי. ממבט מבחוץ, המערכת בכללותה נדמית כסטטית, אבל ברמה המולקולרית, היא פעילה ביותר. חשבו על הפירוק התרמי של זרחן פנטכלורידי.כשמחממים אותו במיכל סגור, הזרחן הפנטכלורידי הנמצא במצב גז מתפרק לתערובת של זרחן טריכלורידי וגז כלור בתגובה הפיכה. ככל שהתגובה מתקדמת, ריכוז הזרחן הפנטכלורידי פוחת ואילו כמות הזרחן הטריכלורידי וגז הכלור גדלה. כתוצאה מכך, קצב התגובה הישירה דועך וקצב התגובה ההפוכה עולה.בסופו של דבר, קצב התגובה הישירה וקצב התגובה ההפוכה משתווים, והמערכת מגיעה לשיווי משקל כימי. במצב זה, הריכוזים היחסיים של זרחן פנטכלורידי, זרחן טריכלורידי וכלור לא משתנים. מצב שיווי משקל כזה נקרא שיווי משקל דינמי משום שגם התגובה הישירה וגם התגובה ההפוכה ממשיכות להתרחש באותו הקצב.מאחר שהמגיבים והתוצרים נוצרים באותו הקצב בו הם נצרכים, הריכוזים שלהם נותרים קבועים, אבל לא בהכרח שווים. שיווי משקל כימי יהיה הומוגני אם כל המגיבים וכל התוצרים בתגובה הפיכה מסוימת נמצאים באותו מצב צבירה גז או נוזל. בשיווי משקל כימי הטרוגני, המגיבים והתוצרים מתקיימים במצבים צבירה שונים.

שיווי משקל דינמי

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תגובה כימית הפיכה מייצגת תהליך כימי שמתקדם הן בכיוון קדימה (משמאל לימין) והן בכיוון ההפוך (מימין לשמאל). כאשר קצב התגובה הקדמית וההפוכה שווים, הריכוזים של המגיבים והמינים של התוצר נשארים קבועים לאורך זמן והמערכת נמצאת בשיווי משקל. חץ כפול מיוחד משמש להדגשת האופי הפיך של התגובה. הריכוזים היחסיים של מגיבים ותוצרים במערכות שיווי משקל משתנים מאוד; חלק מהמערכות מכילות בעיקר תוצרים בשיווי משקל, חלקן מכילות בעיקר מגיבים, וחלקן מכילות כמויות ניכרות של שניהם. 

שיווי משקל דינמי בתגובות כימיות 

שקול את הפירוק ההפיך של דיניטרוגן טטרוקסיד חסר צבע כדי להניב חנקן דו חמצני חום, המתואר במשוואה: 
<img alt="Eq1" src="/files/ftp_upload/11385/11385_Equation_1.png" style="text-align: center; height: 50px;"> 

כשהתגובה מתחילה (זמן= 0), ריכוז המגיב N2O4 הוא סופי וזה של תוצר NO2 הוא אפס, כך שהתגובה הקדמית ממשיכה בקצב סופי בעוד שקצב התגובה ההפוכה הוא אפס. ככל שעובר הזמן,N2O4נצרך וריכוזו יורד, בעוד NO2 נוצר, וריכוזו עולה. הריכוז היורד של המגיב מאט את קצב התגובה הקדמית, וריכוז התוצר הגובר מאיץ את קצב התגובה ההפוכה. תהליך זה נמשך עד שקצב התגובה הקדמית וההפוכה הופכים להיות שווים, אז התגובה הגיעה לשיווי משקל. חשוב להדגיש ששיווי משקל כימיים הם דינמיים; תגובה בשיווי משקל לא "נפסקה", אבל היא מתקדמת לכיוון קדימה והפוך באותו קצב. כך, בשיווי משקל, הריכוזים של N2O4 ו-NO2 אינם משתנים עוד מכיוון שקצב היווצרותNO2שווה בדיוק לקצב הצריכה של NO2, וקצב היווצרות N2O4 שווה בדיוק לקצב הצריכה של N2O4.


שיווי משקל הומוגניים והטרוגניים 

שיווי משקל הומוגני הוא כזה שבו כל המגיבים והתוצרים (וכל הזרזים, אם רלוונטי) נמצאים באותו שלב, שלב מימי או גז, כפי שמודגם בדוגמאות הבאות: 
<img alt="Eq2" src="/files/ftp_upload/11385/11385_Equation_2.png" style="text-align: center; height: 90px;"> 

שיווי משקל הטרוגני כולל מגיבים ותוצרים בשני שלבים שונים או יותר, כפי שממחישה הדוגמה הבאה: 
<img alt="Eq3" src="/files/ftp_upload/11385/11385_Equation_3.png" style="text-align: center; height: 130px;"> 

טקסט זה מותאם מ<a href="https://openstax.org/books/chemistry-2e/pages/13-1-chemical-equilibria">Openstax, Chemistry 2e, Section </a><a href="https://openstax.org/books/chemistry-2e/pages/13-1-chemical-equilibria">13.1 Chemical Equilibria</a>.</div>

תגובה כימית הפיכה מייצגת תהליך כימי המתקדם הן בכיוון קדימה (משמאל לימין) והן בכיוון הפוך (מימין לשמאל). מתי...

Education

JoVE Core

כימיה

אחד

Chemical Equilibrium

שיווי משקל כימי

KEY TERMS AND CONCEPTS

מונחים ומושגים מרכזיים

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 N2O4 and no NO2, so the forward reaction begins while the reverse reaction is still zero. As N2O4 is used up, its concentration falls. As NO2 forms, its concentration rises.

These concentration changes affect the reaction rates. Less N2O4 slows the forward reaction, and more NO2 speeds the reverse reaction. The process continues until both rates match. At that point, the system has reached equilibrium.

Chemical equilibrium is dynamic, not stopped. The reaction still proceeds in both directions, but at the same rate. So the amount of NO2 does not change because it is formed and consumed at equal rates, and the amount of N2O4 stays constant for the same reason.

The video also distinguishes homogeneous and heterogeneous equilibria. A homogeneous equilibrium has all reactants and products in the same phase, such as all aqueous or all gaseous species. A heterogeneous equilibrium has reactants and products in two or more different phases.

Chemical Equilibrium in Reversible Reactions

chemical-equilibrium-in-reversible-reactions

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

homogeneous-equilibria-for-gaseous-reactions

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

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 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 reversible reactions reach chemical equilibrium, why it is dynamic, and how homogeneous and heterogeneous equilibria differ in high school chemistry.

Chemical Equilibrium in Reversible Reactions | JoVE K12

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Pennsylvania State Academic Standards

Physical Science

Chemical Reactions

3.2.9-12.F

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High School

Matter And Its Interactions

HS-PS1-2

HS-PS1-6

HS-PS1-7

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Ohio’s Learning Standards and Model Curriculum

Interactions Of Matter

Chemical reactions

Arizona Science Standards

HS+C.P1

HS+C.P1U1.6

Next Generation Science Standards

Matter and Its Interactions

Florida Department of Education

SC.912.P.12

SC.912.P.12.13

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South Carolina College and Career Ready Science Standards

C-PS1-2

C-PS1-6

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HS. Chemical Reactions

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Construct an explanation to infer the atomic size, reactivity, electronegativity, and ionization energy of an element based on its position in the Periodic Table.

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Chemical Equilibrium in Reversible Reactions

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