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

Eine reversible chemische Reaktion ist eine Reaktion, bei der die Umwandlung von Reaktanten in Produkte und die Umwandlung von Produkten zurück in Reaktanten gleichzeitig erfolgt. Ein Doppelpfeil zwischen den Reaktanten und Produkten zeigt seine reversible Natur an.
Wenn die Geschwindigkeit der Bildung von Produkten oder die Geschwindigkeit der Vorwärtsreaktion gleich der Geschwindigkeit der Bildung von Reaktanten oder der Geschwindigkeit der Umkehrreaktion ist, wird das chemische Gleichgewicht erreicht.
Das chemische Gleichgewicht ist in der Tat ein dynamischer Prozess. Von außen betrachtet erscheint das gesamte System statisch – auf molekularer Ebene ist es jedoch hochaktiv.
Denken Sie an die thermische Zersetzung von Phosphorpentachlorid.
Beim Erhitzen in einem geschlossenen Behälter zerfällt gasförmiges Phosphorpentachlorid in einer reversiblen Reaktion in ein Gemisch aus Phosphortrichlorid und Chlorgas. Im Laufe der Reaktion nimmt die Konzentration von Phosphorpentachlorid ab, während Phosphortrichlorid und Chlorgas zunehmen.
Infolgedessen nimmt die Geschwindigkeit der Vorwärtsreaktion ab und die der Rückreaktion zu.
Schließlich werden die Raten der Vorwärts- und Rückwärtsreaktion gleich, und das System erreicht ein chemisches Gleichgewicht. In diesem Zustand ändern sich die relativen Konzentrationen von Phosphorpentachlorid, Phosphortrichlorid und Chlor nicht mehr.
Ein solcher Gleichgewichtszustand wird auch als dynamisches Gleichgewicht bezeichnet, da sowohl die Vorwärts- als auch die Rückwärtsreaktion weiterhin ablaufen, jedoch mit der gleichen Geschwindigkeit. Da sich die Reaktanten und Produkte mit der gleichen Geschwindigkeit bilden, mit der sie konsumiert werden, bleiben ihre Konzentrationen konstant, wenn auch nicht unbedingt gleich.
Ein chemisches Gleichgewicht ist homogen, wenn sich alle Reaktanten und Produkte einer gegebenen reversiblen Reaktion in derselben Phase befinden – entweder gasförmig oder wässrig. In einem heterogenen Gleichgewicht existieren Reaktanten und Produkte in unterschiedlichen Phasen.

dynamisches gleichgewicht

Eine reversible chemische Reaktion stellt einen chemischen Prozess dar, der sowohl in Vorwärtsrichtung (von links nach rechts) als auch in Rückwärtsrichtung (von rechts nach links) abläuft. Wenn die Geschwindigkeiten der Hin- und Rückreaktion gleich sind, bleiben die Konzentrationen der Reaktanten- und Produktspezies über die Zeit konstant und das System befindet sich im Gleichgewicht. Ein spezieller Doppelpfeil wird verwendet, um die Reversibilität der Reaktion hervorzuheben. Die relativen Konzentrationen von Reaktanten und Produkten in Gleichgewichtssystemen variieren stark; Einige Systeme enthalten hauptsächlich Produkte im Gleichgewicht, andere enthalten hauptsächlich Reaktanten und einige enthalten nennenswerte Mengen von beidem. Dynamisches Gleichgewicht in chemischen Reaktionen Betrachten Sie die reversible Zersetzung von farblosem Distickstofftetroxid zu braunem Stickstoffdioxid, beschrieben durch die Gleichung: <img alt="Eq1" src="/files/ftp_upload/11385/11385_Equation_1.png" style="text-align: center; height: 50px;"> Zu Beginn der Reaktion (Zeit = 0) ist die Konzentration des N2O4-Reaktanten endlich und die des NO2-Produkts null, sodass die Vorwärtsreaktion mit einer endlichen Geschwindigkeit abläuft, während die Rückreaktionsgeschwindigkeit null ist. Mit der Zeit wird N2O4 verbraucht und seine Konzentration sinkt, während NO2 entsteht und seine Konzentration zunimmt. Die abnehmende Konzentration des Reaktanten verlangsamt die Vorwärtsreaktionsgeschwindigkeit und die zunehmende Produktkonzentration beschleunigt die Rückreaktionsgeschwindigkeit. Dieser Prozess wird fortgesetzt, bis die Vorwärts- und Rückwärtsreaktionsgeschwindigkeiten gleich sind. Zu diesem Zeitpunkt hat die Reaktion das Gleichgewicht erreicht. Es ist wichtig zu betonen, dass chemische Gleichgewichte dynamisch sind; Eine Reaktion im Gleichgewicht ist nicht „gestoppt“, sondern läuft mit der gleichen Geschwindigkeit in Vorwärts- und Rückwärtsrichtung ab. Somit ändern sich im Gleichgewicht die Konzentrationen von N2O4 und NO2 nicht mehr, da die Geschwindigkeit der Bildung von NO2 genau der Geschwindigkeit des Verbrauchs von NO2 entspricht und die Geschwindigkeit der Bildung von N2O4 genau gleich der Geschwindigkeit des Verbrauchs von N2O4 ist. Homogene und heterogene Gleichgewichte Ein homogenes Gleichgewicht liegt vor, wenn alle Reaktanten und Produkte (und ggf. Katalysatoren) in derselben Phase vorliegen, entweder in wässriger oder gasförmiger Phase, wie die folgenden Beispiele verdeutlichen: <img alt="Eq2" src="/files/ftp_upload/11385/11385_Equation_2.png" style="text-align: center; height: 90px;"> Ein heterogenes Gleichgewicht umfasst Reaktanten und Produkte in zwei oder mehr verschiedenen Phasen, wie das folgende Beispiel zeigt: <img alt="Eq3" src="/files/ftp_upload/11385/11385_Equation_3.png" style="text-align: center; height: 130px;"> Dieser Text ist adaptiert von <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>

Dynamisches Gleichgewicht

Eine reversible chemische Reaktion stellt einen chemischen Prozess dar, der sowohl in Vorwärtsrichtung (von links nach rechts) als auch in umgekehrter Richtung (von rechts nach links) abläuft. Wann...

Education

Lehre

JoVE Core

Chemie

Eins

Chemical Equilibrium

Chemisches Gleichgewicht

KEY TERMS AND CONCEPTS

KERNBEGRIFFE UND KONZEPTE

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...

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...

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...

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

chemistry

JoVE Core Chemie erweckt allgemeine Konzepte der Chemie durch kompakte und leicht verständliche animierte Videolektionen zum Leben. Außerdem zeigen wir in den Videos Wissenschaftler in Aktion bei klassischen und neuartigen Forschungsexperimenten, wie sie heutzutage in Laboren weltweit durchgeführt wurden.

Pennsylvania State Academic Standards

Physical Science

Chemical Reactions

3.2.9-12.F

Louisiana Student Standards for Science

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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New York State Science Learning Standards

HS. Chemical Reactions

HS-PS1-6.

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Chemical Equilibrium in Reversible Reactions

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