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

Une réaction chimique réversible est celui dans laquelle la conversion des réactifs et des produits en réactifs se déroulent simultanément. Une double flèche entre les réactifs et les produits signifie sa nature réversible. Lorsque la vitesse de formation des produits, ou la vitesse de la réaction directe, est égal à la vitesse de formation des réactifs, ou à la vitesse de la réaction inverse, et ainsi, l'équilibre chimique est atteint.L'équilibre chimique est en fait un processus dynamique. De l'extérieur, l'ensemble du système semble statique cependant, au niveau moléculaire, il est très actif. Considérez la décomposition thermique de pentachlorure de phosphore.Lorsqu'il est chauffé dans un contenant fermé, le pentachlorure de phosphore gazeux se décompose dans un mélange de trichlorure de phosphore et de gaz de chlore dans une réaction réversible. Au fur et à mesure que la réaction progresse, la concentration de pentachlorure de phosphore diminue tandis que le trichlorure de phosphore et le chlore gazeux augmentent. En conséquence, la vitesse de la réaction directe diminue et celle de la réaction inverse augmente.Finalement, les taux des réactions directes et inverses deviennent égaux, et le système atteint l'équilibre chimique. Dans cet état, les concentrations relatives de pentachlorure de phosphore, de trichlorure de phosphore, et du chlore ne change plus. Un tel état d'équilibre est également appelé équilibre dynamique parce que les réactions avant et arrière continuent à se produire, mais aux mêmes vitesses.Étant donné que les réactifs et les produits se forment aux mêmes vitesses et qu'ils sont consommés, leurs concentrations restent constants, mais pas nécessairement égaux. Un équilibre chimique est homogène si tout les réactifs et les produits d'une réaction réversible donnée sont dans la même phase gazeuse ou aqueuse. Dans un équilibre hétérogène, réactifs et produits existent en différentes phases.

Équilibre dynamique

Une réaction chimique réversible représente un processus chimique qui se déroule à la fois dans les sens direct (de gauche à droite) et inverse (de droite à gauche). Lorsque les vitesses des réactions directes et inverses sont égales, les concentrations des espèces de réactifs et des espèces de produits restent constantes au fil du temps et le système est à l'équilibre. Une double flèche spéciale est utilisée pour souligner la nature réversible de la réaction. Les concentrations relatives des réactifs et des produits dans les systèmes en équilibre varient considérablement ; certains systèmes contiennent principalement des produits à l'équilibre, d'autres contiennent principalement des réactifs et d'autres contiennent des quantités notables des deux. 
 Équilibre dynamique dans les réactions chimiques 
 Envisagez la décomposition réversible du tétraoxyde de diazote incolore pour produire du dioxyde d'azote brun, décrite par l'équation : 
 <img alt="Eq1" src="/files/ftp_upload/11385/11385_Equation_1.png" style="text-align: center; height: 50px;" />
 Lorsque la réaction commence (temps = 0), la concentration du réactif N2O4 est finie et celle du produit NO2 est nulle, de sorte que la réaction directe se produit à une vitesse finie alors que la vitesse de la réaction inverse est nulle. À mesure que le temps passe, N2O4 est consommé et sa concentration diminue, tandis que NO2 est produit, et sa concentration augmente. La diminution de la concentration du réactif ralentit la vitesse de la réaction directe et l'augmentation de la concentration du produit accélère la vitesse de la réaction inverse. Ce processus se poursuit jusqu'à ce que les vitesses des réactions directe et inverse deviennent égales, moment auquel la réaction a atteint l'équilibre. Il est important de souligner que les équilibres chimiques sont dynamiques ; une réaction à l'équilibre ne s'est pas “ arrêtée ”, mais elle se poursuit dans les directions directe et inverse à la même vitesse. Ainsi, à l'équilibre, les concentrations de N2O4 et de NO2 ne changent plus parce que la vitesse de formation de NO2 est exactement égale à la vitesse de consommation de NO2, et la vitesse de formation de N2O4 est exactement égale à la vitesse de consommation de N2O4. 
 Équilibres homogènes et hétérogènes 
 Un équilibre homogène est un équilibre dans lequel tous les réactifs et produits (et tous les catalyseurs, le cas échéant) sont présents dans la même phase, soit aqueuse, soit gazeuse, comme illustré par les exemples suivants : 
 <img alt="Eq2" src="/files/ftp_upload/11385/11385_Equation_2.png" style="text-align: center; height: 90px;" />
 Un équilibre hétérogène implique des réactifs et des produits dans deux ou plusieurs phases différentes, comme l'illustre l'exemple suivant : 
 <img alt="Eq3" src="/files/ftp_upload/11385/11385_Equation_3.png" style="text-align: center; height: 130px;" />
 Ce texte a été adapté d'<a href="https://openstax.org/books/chemistry-2e/pages/13-1-chemical-equilibria">Openstax, Chimie 2e, Section</a><a href="https://openstax.org/books/chemistry-2e/pages/13-1-chemical-equilibria"> 13.1 Équilibre chimique</a>.

Une réaction chimique réversible représente un processus chimique qui se déroule à la fois dans les directions avant (de gauche à droite) et inverse (de droite à gauche). Quand...

Education

Enseignement

JoVE Core

Chimie

Chemical Equilibrium

Équilibre chimique

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

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

chemistry

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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

new-york-state-science-learning-standards

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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