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

Uma reação química reversível é a que a conversão de reagentes em produtos e a conversão de produtos novamente reagentes ocorrem simultaneamente. Uma seta dupla entre os reagentes e produtos significa a sua natureza reversível. Quando a taxa de formação de produtos, ou a taxa da reação de antecipação, é igual à taxa de formação de reagentes, ou a taxa da reação inversa, é alcançado o equilíbrio químico.O equilíbrio químico é de facto, um processo dinâmico. Do exterior, o todo parece estático seja como for, a nível molecular, é altamente ativo. Considere-se a decomposição térmica do pentacloreto de fósforo.Quando aquecido num recipiente fechado, O pentacloreto fósforo gasoso decompõe-se numa mistura de tricloreto de fósforo e gás de cloro numa reação reversível. À medida que a reação prossegue, a concentração de pentacloreto de fósforo diminui enquanto o tricloreto de fósforo e o gás de cloro aumenta. Como resultado, a taxa da reação de antecipação decresce e a da reação inversa aumenta.Eventualmente, as taxas das reações avançadas e inversas tornan-se igual, e o sistema alcança o equilíbrio químico. Neste estado, as concentrações relativas de pentacloreto de fósforo, tricloreto de fósforo, e o cloro já não mudam. Este estado de equilíbrio é também chamado de equilíbrio dinâmico porque tanto as reações avançadas como as inversas continuam a ocorrer, mas com as mesmas taxas.Uma vez que os reagentes e os produtos formam-se às mesmas taxas à medida que são consumidos, as suas concentrações permanecem constantes, embora não necessariamente iguais. Um equilíbrio químico é homogéneo se todos os reagentes e produtos numa dada reação reversível são na mesma fase quer gasosa ou aquosa. Num equilíbrio heterogéneo, os reagentes e produtos existem em diferentes fases.

equilíbrio dinâmico

Uma reação química reversível representa um processo químico que prossegue tanto na direção direta (da esquerda para a direita) como inversa (da direita para a esquerda). Quando as velocidades das reações direta e inversa são iguais, as concentrações das espécies de reagentes e produtos permanecem constantes ao longo do tempo e o sistema está em equilíbrio. Uma seta dupla especial é utilizada para enfatizar a natureza reversível da reação. As concentrações relativas de reagentes e produtos em sistemas de equilíbrio variam muito; alguns sistemas contêm principalmente produtos em equilíbrio, alguns contêm principalmente reagentes, e alguns contêm quantidades apreciáveis de ambos.
Equilíbrio Dinâmico em Reações Químicas
Considere a decomposição reversível de tetróxido de dinitrogénio incolor para produzir dióxido de nitrogénio castanho, descrita pela equação:
<img alt="Eq1" src="/files/ftp_upload/11385/11385_Equation_1.png" style="text-align: center; height: 50px;" />
Quando a reação começa (tempo = 0), a concentração do reagente N2O4 é finita e a do produto NO2 é zero, pelo que a reação direta prossegue a uma velocidade finita enquanto que a velocidade da reação inversa é zero. À medida que o tempo passa, o N2O4 é consumido e a sua concentração diminui, enquanto que o NO2 é produzido, e a sua concentração aumenta. A diminuição da concentração do reagente diminui a velocidade da reação direta e a concentração crescente do produto acelera a velocidade da reação inversa. Este processo continua até que as velocidades de reação direta e inversa se tornem iguais, altura em que a reação atingiu o equilíbrio. É importante enfatizar que os equilíbrios químicos são dinâmicos; uma reação em equilíbrio não “parou”", mas está a decorrer nas direções direta e inversa à mesma velocidade. Assim, em equilíbrio, as concentrações de N2O4 e NO2 já não mudam porque a velocidade de formação de NO2 é exactamente igual à velocidade de consumo de NO2, e a velocidade de formação de N2O4 é exactamente igual à velocidade de consumo de N2O4.
Equilíbrios Homogéneos e Heterogéneos
Um equilíbrio homogéneo é aquele em que todos os reagentes e produtos (e quaisquer catalisadores, se aplicável) estão presentes na mesma fase, quer na fase aquosa quer na fase gasosa, como ilustrado nos seguintes exemplos:
<img alt="Eq2" src="/files/ftp_upload/11385/11385_Equation_2.png" style="text-align: center; height: 90px;" />
Um equilíbrio heterogéneo envolve reagentes e produtos em duas ou mais fases diferentes, como ilustrado pelo exemplo a seguir:
<img alt="Eq3" src="/files/ftp_upload/11385/11385_Equation_3.png" style="text-align: center; height: 130px;" />
Este texto foi adaptado de <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>.

Equilíbrio Dinâmico

Uma reação química reversível representa um processo químico que prossegue nas direções direta (da esquerda para a direita) e reversa (da direita para a esquerda). Quando...

Education

JoVE Core

Química

Chemical Equilibrium

Equilíbrio Químico

KEY TERMS AND CONCEPTS

TERMOS-CHAVE E CONCEITOS

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 dá vida a conceitos gerais de química usando vídeos de animação fáceis de entender. Além disso, nos vídeos Cientistas em ação, demonstramos experimentos de pesquisa clássicos e originais realizados atualmente por cientistas em laboratórios em todo o 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

ohios-learning-standards-and-model-curriculum

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

south-carolina-college-and-career-ready-science-standards

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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