Name: shifting equilibrium by changing concentration
Uploaded: 2020-09-24T15:06:50.000Z
Duration: 2 min 27 s
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Le Chatelier's Principle: Changing Concentration

順方向反応の速度が逆反応の速度と等しい場合、システムは動的平衡状態にあります。反応はまだ両方向に進行していますが、反応物と生成物の正味濃度は一定のままです。
この時点で、反応物や生成物の濃度が変化すると、例えば分子Aを追加したり、分子Cを除去したりすると、平衡が乱れます。このような変更は、システムに対するストレスです。
ル・シャトリエの原理によれば、濃度の変化、体積や圧力の変化、温度の変化などにより、平衡状態にある系に応力がかかると、平衡位置は応力を最小化し、平衡を回復する方向に移動します。
濃度の変化に対するシステムの応答を理解するために、接続された 2 つの水タンク A と B について考えてみます。最初は、2つのタンクの水位は等しく、システムは平衡状態にあります。次に、タンクBに水を追加します。
このストレスにより、タンクBの水位が上昇し、平衡が乱れます。このストレスを相殺するために、水の一部がタンクAに流れ込み、平衡状態を回復します。
逆に、タンクBから水を取り出すと、水位が下がることでストレスも引き起こされます。
その結果、タンクAの水がタンクBに流れ込み、平衡状態に戻ります。
平衡状態のシステムの場合、反応商は平衡定数に等しくなります。平衡混合物に反応物を追加すると、Qが減少します。ル・シャトリエの原理によれば、システムは平衡位置を生成物にシフトして追加された反応物を消費し、平衡を再確立することにより、この応力を最小限に抑えます。 
同様に、平衡混合物から製品を取り除くと、平衡を回復するために同様の反応が得られます。
これに対して、製品を追加するとQが増加します。したがって、平衡位置は反応物に向かってシフトし、追加された生成物を消費して平衡を回復します。
濃度が変化すると、Kの値は変化せずに平衡位置がシフトします。

ルシャトリエの原理：濃度の変更

動的平衡状態にある系は、順方向と逆方向の反応が等しい速度で起こっています。平衡状態にある系が、これらの反応速度に異なる影響を与えるような条件の変化（外乱）を受けた場合、速度はもはや等しくなく、系は平衡状態ではありません。反応系は再び進行し（平衡シフト）、再び平衡状態になります。この現象は、ルシャトリエの原理として知られます。平衡状態にある系に外乱が起こると、その系は乱れに反応して平衡シフトし、再び平衡状態に達します。
 反応速度は、主に濃度（反応速度則）と温度（アレニウス式）の影響を受けます。したがって、濃度と温度の変化は、平衡を変化させる2つの外乱です。
 濃度変化の影響
 平衡系において、反応物または生成物の濃度が変化すると、順方向または逆方向の反応の速度が変化します。例として、次の平衡反応を考えます。
 <img alt="Eq1" src="/files/ftp_upload/11391/11391_Equation 1v2.png" style="text-align: center; height: 48px;" />
 この系が平衡状態にあるとき、順方向の反応速度と逆方向の反応速度は等しくなります。
 <img alt="Eq2" src="/files/ftp_upload/11391/11391_Equation 2.png" style="text-align: center; height: 50px;" />
 N2やO2などの反応物を加えて系に乱れを与えると、濃度の上昇によって順方向の反応の速度が逆方向の反応の速度を上回ります。
 <img alt="Eq3" src="/files/ftp_upload/11391/11391_Equation 3.png" style="text-align: center; height: 50px;" />
 系は再び平衡に向かおうとして、一時的に順方向の反応が優位に起こる（平衡が右にシフトする）。生成物であるNOが系から取り除かれて逆反応の速度が低下すると、これと同じようなシフトが起こり、再び速度の不均衡が生じます。
 同じ理屈で、平衡系から反応物を取り除いたり、生成物を加えたりしたときに生じる左側のシフトも説明できます。これらの外乱は、どちらも逆反応の速度を増加させます。
 <img alt="Eq4" src="/files/ftp_upload/11391/11391_Equation 4.png" style="text-align: center; height: 50px;" />
 そして、一時的に逆方向が優位に起こって平衡を回復させます。
 この動力学的な解釈に代わるものとして、濃度の変化が平衡に及ぼす影響を、反応速度の観点から説明することができます。系が平衡状態にあるときは、次が成立します。
 <img alt="Eq5" src="/files/ftp_upload/11391/11391_Equation 5.png" style="text-align: center; height: 70px;" />
 反応物が加えられたり（反応商の分母が大きくなる）、生成物が取り除かれたり（分子が小さくなる）すると、Qc < Kcとなり、平衡は右にシフトします。この外乱を誘発する3つの異なる方法は、平衡混合物の組成に3つの異なる変化をもたらします。N2を加えた場合、平衡が右にシフトすることで、平衡が再構築される際にO2が消費されてNOが生成されます。そのため、以前よりもN2とNOの濃度が高くなり、O2の濃度が低くなるような混合物が得られます。もしO2が加えられれば、新しい平衡混合物はO2とNOの濃度がより高く、N2の濃度がより低くなります。最後に、NOが除去された場合、新しい平衡混合物はN2とO2の濃度が高くなり、NOの濃度は低くなります。このような組成の違いにもかかわらず、平衡定数の値は、外乱の後でも以前と同じになる（質量作用の法則）。同じ理屈が、反応物の除去や生成物の追加を伴う外乱にも適用でき、その場合、Qc > Kcとなることから、平衡は左にシフトします。
 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/13-3-shifting-equilibria-le-chateliers-principle">Openstax, Chemistry 2e, Section 13.3 Shifting Equilibria: LeChatelier’s Principle</a>.

平衡状態にある系は、順方向反応と逆反応が同じ速度で起こる動的バランスの状態にあります。平衡システムが...

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Chemical Equilibrium

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KEY TERMS AND CONCEPTS

主要用語と概念

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 reactant or product can move the equilibrium in one direction or the other.

Adding a reactant, such as N2 or O2 in the example reaction, increases the forward reaction rate. The forward rate then becomes greater than the reverse rate, so the equilibrium shifts right. Removing the product NO has the same effect because it lowers the reverse reaction rate and creates the same imbalance.

The opposite changes cause a left shift. Removing reactant or adding product increases the reverse reaction rate compared with the forward rate. The system then moves in the reverse direction until equilibrium is reached again.

This shift can also be explained with the reaction quotient, Qc, and the equilibrium constant, Kc. When reactant is added, Qc becomes less than Kc. When product is removed, Qc also becomes less than Kc, so the equilibrium shifts right. If reactant is removed or product is added, then Qc is greater than Kc and the equilibrium shifts left.

Although the final composition changes, the equilibrium constant stays the same after the stress is removed and the system rebalances. If N2 is added, the new mixture has more N2 and NO and less O2. If O2 is added, the mixture has more O2 and NO and less N2. If NO is removed, the mixture ends with more N2 and O2 and less NO. The same idea applies to shifts caused by removing reactant or adding product.

Shifting Equilibrium by Changing Concentration

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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...

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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

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...

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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...

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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...

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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)

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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

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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...

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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 changing concentration shifts chemical equilibrium, including why adding or removing reactants or products moves the reaction left or right.

Shifting Equilibrium by Changing Concentration | JoVE K12

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Shifting Equilibrium by Changing Concentration

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