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

可逆的な化学反応とは、反応物の生成物への変換と生成物の反応物への再変換が同時に起こる反応です。反応物と生成物の間の二重矢印は、その可逆的な性質を示しています。
生成物の形成速度、または順反応の速度が反応物の形成速度、または逆反応の速度と等しいとき、化学平衡が達成されます。
化学平衡は実際には動的なプロセスです。外から見ると、システム全体が静止しているように見えますが、分子レベルでは非常に活性があります。
五塩化リンの熱分解を考えてみましょう。
密閉容器内で加熱すると、ガス状の五塩化リンは可逆反応で三塩化リンと塩素ガスの混合物に分解します。反応が進行すると、五塩化リンの濃度は減少し、三塩化リンと塩素ガスは増加します。
その結果、順反応の速度は減少し、逆反応の速度は増加します。
やがて、順反応と逆反応の速度は等しくなり、系は化学平衡を達成します。この状態では、五塩化リン、三塩化リン、および塩素の相対濃度は変化しなくなります。
このような平衡状態は、順反応と逆反応の両方が同じ速度で発生し続けるため、動的平衡とも呼ばれます。反応物と生成物は消費されるのと同じ速度で形成されるため、それらの濃度は一定のままですが、必ずしも等しいとは限りません。
化学平衡は、特定の可逆反応のすべての反応物と生成物が同じ相(気体または水性)にある場合、均一です。不均一平衡では、反応物と生成物は異なる相で存在します。

動的平衡

可逆的な化学反応とは、順方向（左から右）と逆方向（右から左）の両方に進行する化学プロセスのことです。順方向と逆方向の反応の速度が等しい場合、反応種と生成種の濃度は時間の経過とともに一定になり、系は平衡状態に達します。特別な二重矢印は、このような反応の可逆性を強調するために使用されます。平衡系における反応物と生成物の相対的な濃度は大きく異なり、平衡状態でほとんどが生成物である系もあれば、ほとんどが反応物である系もあり、かなりの量の両方が含まれる系もあります。
 化学反応における動的平衡
 無色の四酸化二窒素が可逆的に分解されて茶色の二酸化窒素を生成する反応は、次の式で表されます。
 <img alt="Eq1" src="/files/ftp_upload/11385/11385_Equation_1.png" style="text-align: center; height: 50px;" />
 反応が始まると（time = 0）、反応物であるN2O4の濃度は有限であり、生成物であるNO2の濃度は0であるため、順方向の反応は有限の速度で進行し、逆方向の反応速度は0です。時間が経過すると、N2O4が消費されてその濃度が下がり、NO2が生成されてその濃度が上がります。反応物の濃度が下がると順方向の反応速度が遅くなり、生成物の濃度が上がると逆方向の反応速度が速くなります。このプロセスは、順方向の反応速度と逆方向の反応速度が等しくなるまで続き、その時点で反応は平衡状態に達します。ここで重要なのは、化学平衡は動的なものであるということです。平衡状態にある反応は止まっているわけではなく、順方向と逆方向に同じ速度で進行しています。したがって、平衡状態では、NO2の生成速度とNO2の消費速度が完全に等しく、N2の生成速度とN2O4の消費速度が完全に等しいため、N2とNO2の濃度は変化しません。
 均一系と不均一系における平衡状態
 均一系における平衡とは、以下の例に示すように、すべての反応物と生成物（および該当する場合は触媒）が液相または気相のいずれかの同一相に存在する状態をいいます。
 <img alt="Eq2" src="/files/ftp_upload/11385/11385_Equation_2.png" style="text-align: center; height: 90px;" />
 不均一系における平衡とは、以下の例で示すように、2つ以上の異なる相にある反応物と生成物を含むものです。
 <img alt="Eq3" src="/files/ftp_upload/11385/11385_Equation_3.png" style="text-align: center; height: 130px;" />
 上記の文章は以下から引用しました。 <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>.

可逆的な化学反応は、順方向(左から右)と逆方向(右から左)の両方に進む化学プロセスを表します。いつ。。。

Education

JoVE Core

化学

Chemical Equilibrium

化学平衡

KEY TERMS AND CONCEPTS

主要用語と概念

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 化学は、簡潔で理解しやすいアニメーションビデオレッスンを使用して、一般的な化学の概念を生き生きとしたものにします。さらに、活動する科学者のビデオでは、現在世界中の実験室で行われている古典的そして独創的な研究・実験をお見せします。

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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