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

Una reacción química reversible es aquella en la que la conversión de reactivos en productos y la conversión de productos nuevamente en reactivos ocurren simultáneamente. Una flecha doble entre los reactivos y los productos indica su naturaleza reversible. Cuando la velocidad de formación de los productos, o la velocidad de reacción directa, es igual a la velocidad de formación de los reactivos, o la velocidad de la reacción inversa, se logra el equilibrio químico.El equilibrio químico es de hecho un proceso dinámico. Desde el exterior, todo el sistema parece estático;sin embargo, a nivel molecular, es muy activo. Considere la descomposición térmica del pentacloruro de fósforo.Cuando se calienta en un recipiente cerrado, el pentacloruro de fósforo gaseoso se descompone en una mezcla de tricloruro de fósforo y cloro gaseoso en una reacción reversible. A medida que avanza la reacción, la concentración de pentacloruro de fósforo disminuye mientras que el tricloruro de fósforo y el gas cloro aumentan. Como resultado, la velocidad de la reacción directa disminuye y la de la reacción inversa aumenta.Finalmente, las velocidades de las reacciones directa e inversa se igualan y el sistema alcanza el equilibrio químico. En este estado, las concentraciones relativas de pentacloruro de fósforo, tricloruro de fósforo, y cloro ya no cambian. Este estado de equilibrio también se llama equilibrio dinámico porque las reacciones directa e inversa continúan ocurriendo, pero a la misma velocidad.Dado que los reactivos y productos se forman a la misma velocidad a la que se consumen, sus concentraciones permanecen constantes, aunque no necesariamente iguales. Un equilibrio químico es homogéneo si todos los reactivos y productos en una reacción reversible dada están en la misma fase, ya sea gaseosa o acuosa. En un equilibrio heterogéneo, los reactivos y productos existen en diferentes fases.

equilibrio dinámico

Una reacción química reversible representa un proceso químico que procede tanto en dirección de avance (de izquierda a derecha) como de retroceso (de derecha a izquierda). Cuando las velocidades de las reacciones hacia delante y hacia atrás son iguales, las concentraciones de las especies de reactivos y productos permanecen constantes con el tiempo y el sistema está en equilibrio. Se utiliza una flecha doble especial para enfatizar la naturaleza reversible de la reacción. Las concentraciones relativas de reactivos y productos en los sistemas en equilibrio varían enormemente; algunos sistemas contienen principalmente productos en equilibrio, algunos contienen principalmente reactivos y otros contienen cantidades apreciables de ambos.
 El equilibrio dinámico en las reacciones químicas
 Considere la descomposición reversible del tetroxido de dinitrógeno incoloro para producir dióxido de nitrógeno marrón, descrito por la ecuación:
 <img alt="Eq1" src="/files/ftp_upload/11385/11385_Equation_1.png" style="text-align: center; height: 50px;" />
 A medida que comienza la reacción (tiempo = 0), la concentración del reactivo N2O4 es finita y la del producto NO2 es cero, por lo que la reacción directa procede a una velocidad finita mientras que la velocidad de reacción inversa es cero. A medida que pasa el tiempo, se consume el N2O4 y su concentración disminuye, mientras que se produce NO2, y su concentración aumenta. La disminución de la concentración del reactivo ralentiza la velocidad de reacción hacia delante y la creciente concentración del producto acelera la velocidad de reacción hacia atrás. Este proceso continúa hasta que las velocidades de reacción hacia delante y hacia atrás se igualan, momento en el que la reacción ha alcanzado el equilibrio. Es importante enfatizar que los equilibrios químicos son dinámicos; una reacción en equilibrio no se “ha detenido”, pero está avanzando en las direcciones hacia adelante y hacia atrás al mismo ritmo. Así, en equilibrio, las concentraciones de N2O4 y de NO2 ya no cambian porque la velocidad de formación de NO2 es exactamente igual a la velocidad de consumo de NO2, y la velocidad de formación de N2O4 es exactamente igual a la tasa de consumo de N2O4.
 Equilibrios homogéneos y heterogéneos
 Un equilibrio homogéneo es aquel en el que todos los reactivos y productos (y cualquier catalizador, si procede) están presentes en la misma fase, ya sea acuosa o gaseosa, como se ilustra en los siguientes ejemplos:
 <img alt="Eq2" src="/files/ftp_upload/11385/11385_Equation_2.png" style="text-align: center; height: 90px;" />
 Un equilibrio heterogéneo implica reactivos y productos en dos o más fases diferentes, como se ilustra en el siguiente ejemplo:
 <img alt="Eq3" src="/files/ftp_upload/11385/11385_Equation_3.png" style="text-align: center; height: 130px;" />
 Este texto ha sido adaptado de<a href="https://openstax.org/books/chemistry-2e/pages/13-1-chemical-equilibria"> Openstax, Química 2e, Sección</a><a href="https://openstax.org/books/chemistry-2e/pages/13-1-chemical-equilibria"> 13.1 Equilibrio químico</a>.

Equilibrio Dinámico

Una reacción química reversible representa un proceso químico que procede tanto hacia adelante (de izquierda a derecha) como hacia atrás (de derecha a izquierda). Cuando...

Education

Educación

JoVE Core

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Chemical Equilibrium

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KEY TERMS AND CONCEPTS

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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

JoVE Core Química da vida a los conceptos de química general mediante lecciones de vídeo basadas en animaciones, concisas y fáciles de entender. Además, en los vídeos de científicos en acción, se muestran experimentos de investigación clásicos y originales realizados en laboratorios de todo el mundo.

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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