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

K12 Standards

north-carolina-k-12-science-standard-course-of-study

NC K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.5.2

Dynamic Equilibrium

可逆的化学反应是 反应物转化为产物 和产物再转化回反应物 同时发生的反应。反应物和产物之间的双箭头 表示其可逆的性质。当产物的生成速率 或正向反应速率，等于反应物的生成速率 或逆向反应速率时，则达到化学平衡。化学平衡实际上是一个动态过程。从外面看，整个系统似乎是静态的—然而，在分子水平上，它是高度活跃的。考虑一下五氯化磷的 热分解。当在一个封闭容器中加热时，气态的五氯化磷在一个可逆反应中 分解成三氯化磷和氯气 气体的混合物。随着反应的进行，五氯化磷的浓度降低，而三氯化磷和氯气的浓度增加。结果，正向反应速率 减小，而逆向反应速率增大。最终，正向反应和逆向反应的速率 变得相等，系统就达到化学平衡。在这种状态下，五氯化磷、三氯化磷和氯气的 相对浓度不再变化。这样的一种平衡状态也被称为动态平衡，因为正向反应和逆向反应 继续发生，但速率相同。由于反应物和产物的形成速率与 消耗速率相同，因此它们的浓度 保持不变，尽管不一定相等。如果在一个给定的可逆反应中，所有的 反应物和产物都处于同一相 不论是气态还是水相，则化学平衡是均相的。在非均相平衡中，反应物和产物 以不同的相存在。

动态平衡

可逆化学反应是指正向 (从左到右) 和反向 (从右到左) 方向进行的化学过程。 当正向和反向反应的速率相等时，反应物和生成物物种的浓度随着时间的推移保持不变，系统处于平衡状态。 使用特殊的双箭头来强调反应的可反转性。 平衡系统中的反应物和生成物的相对浓度差异很大；有些系统中大部分是平衡状态下的生成物，有些系统中大部分是反应物，有些系统中两者的浓度都相当可观。 
 化学反应中的动态平衡 
 考虑无色二氧化三丁二烯二氧化氮可逆分解为产率棕色二氧化氮，如方程所述： 
 <img alt="Eq1" src="/files/ftp_upload/11385/11385_Equation_1.png" style="text-align: center; height: 50px;" />
 当反应开始时 (时间 = 0) ， N2O4 反应物的浓度是有限的， NO2 生成物的率值是零，因此正向反应在有限的率值下进行，而反向反应 是零。 随着时间的推移， N2O4 被消耗，其浓度下降， NO2 被生产，其浓度增加。 反应物的浓度越低，正向反应率值就越慢，而生成物浓度越高，反向反应率值就越快。 这一过程一直持续到正向和反向反应速度相等，此时反应已达到平衡。 必须强调的是，化学平衡是动态的；平衡的反应并没有“停止”，但它在同一个率值上正向和反向进行。 因此，在平衡状态下， N2O4 和 NO2 的浓度不再发生变化，因为 NO2 形成的率值与 NO2 消费的率值完全相同，而N2O4形成的率值与N2O4消费的率值完全相同。 
 均质和异质的平衡 
 均质平衡是指所有反应物和生成物 (以及任何催化剂，如果适用) 均存在于相同相位 (水相或气相) 中，如下例所示： 
 <img alt="Eq2" src="/files/ftp_upload/11385/11385_Equation_2.png" style="text-align: center; height: 90px;" />
 异构平衡涉及两个或多个不同阶段的反应物和生成物，如下例所示： 
 <img alt="Eq3" src="/files/ftp_upload/11385/11385_Equation_3.png" style="text-align: center; height: 130px;" />
 本文改编自<a href="https://openstax.org/books/chemistry-2e/pages/13-1-chemical-equilibria"> Openstax, 化学 2e, </a><a href="https://openstax.org/books/chemistry-2e/pages/13-1-chemical-equilibria"> 第13.1节：化学平衡</a>。

可逆化学反应指一种能够同时向正向（从左到右）和逆向（从右到左）进行的化学过程。当……

Education

教育

JoVE Core

化学

Chemical Equilibrium

化学平衡

KEY TERMS AND CONCEPTS

关键术语与概念

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 化学通过简单明了且通俗易懂的动画视频课程，将普通化学概念带入到现实生活当中。此外，在科学家行动视频中，我们演示了在实验室中进行的经典和原始研究实验。

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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