Name: using q to track equilibrium changes
Uploaded: 2020-09-24T15:06:50.000Z
Duration: 2 min 35 s
Channel: JoVE

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North Carolina K-12 Science Standard Course of Study

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Chemistry

Matter and its Interactions

PS.Chm.5.2

Reaction Quotient

平衡定数の発現は、平衡状態における反応物AとBに対する生成物CとDのモル濃度として記述され、それぞれがそれぞれの化学量論的係数に上昇します。解くと、式は平衡定数 Kc に等しくなります。
同じ形式の式は、任意の濃度の反応物と生成物についても書くことができ、計算された量は反応商Q cとして知られています。
Qc と同様に、Qp 式は分圧を使用した気体反応について記述できます。
Kは濃度に関係なく特定の温度で一定に保たれますが、反応が生成物または反応物に向かって進行するにつれて、Qの値は変化します。
反応商は、反応が平衡に達するために進行する方向を決定するために使用できます。
特定の反応の開始時に、生成物の濃度がゼロの場合、反応商はゼロです。
分母内の反応物の濃度が高い場合、つまりQがKよりも小さい場合、反応は右に移動してより多くの生成物を合成し、システムが平衡に達するまで続きます。
反応物の濃度がゼロの場合、反応商は無限大です。
分子内の生成物の濃度が高く、QがKよりも大きい場合は常に、反応は左に移動してより多くの反応物を生成します。
Q が K に等しい場合、システムは平衡状態にあり、順反応と逆反応の速度は等しくなります。
平衡定数50で与えられた反応を考えます。反応混合物に0.20モルの水素、0.20モルのヨウ素、および1.7モルのヨウ化水素が含まれている場合、示されている反応の方向はQを計算することで決定できます。
与えられた濃度を式に代入すると、Q は 72 に等しく、K よりも大きくなります。したがって、反応は左にシフトします。

<ol>
	<li>「反応商は平衡問題を解決するために不要です:Qに問題はありません。」 Journal of Chemical Education 82、no. 8(2005):1149。<a href="https://pubs.acs.org/doi/pdf/10.1021/ed082p1149.2">https://pubs.acs.org/doi/pdf/10.1021/ed082p1149.2</a></li>
</ol>

反応商

可逆反応の状態は、その反応商(Q)を評価することで簡単に知ることができます。m A + n B ⇌ x C + y D で記述される可逆反応の場合、反応商は平衡式の化学量論から次のように直接導かれます。
 <img alt="Eq1" src="/files/ftp_upload/11389/11389_Equation_1.png" style="text-align: center; height: 90px;" />
 ここで、添え字のcは、式の中でモル濃度を使用していることを示します。反応物と生成物が気体の場合は、分圧を用いて同様に反応商を求めることができます。
 <img alt="Eq2" src="/files/ftp_upload/11389/11389_Equation_2.png" style="text-align: center; height: 90px;" />
 上の反応商の式は、濃度や圧力に絶対値ではなく相対値を用いる、より厳密な式を単純化したものであることに注意しましょう。これらの相対的な濃度と圧力の値は無次元（無単位）であり、その結果、反応商も無次元となります。
 Qの値は、反応が平衡に近づくにつれて変化するため、反応の状態を示す指標として有効です。ここでは、二酸化硫黄の酸化反応について考えます。
 <img alt="Eq3" src="/files/ftp_upload/11389/11389_Equation_3.png" style="text-align: center; height: 50px;" />
 ここでは、この反応を反応物であるSO2とO2のみの混合物で開始する場合と、生成物であるSO3のみで開始する場合の2つの異なる実験が考えられます。反応物のみの混合物で始まる反応では、Qは最初ゼロに等しいです。
 <img alt="Eq4" src="/files/ftp_upload/11389/11389_Equation_4.png" style="text-align: center; height: 90px;" />
 反応が順方向に平衡に向かって進むと、反応物の濃度は減少する（Qcの分母も同様である）。そして生成物の濃度は増加し（Qcの分子も同様である）、結果的に反応商は増加します。平衡が達成されると、反応物と生成物の濃度は一定になり、Qcの値も一定になります。
 生成物のみが存在する状態で反応が始まると、Qcの値は最初は無限大となります。
 <img alt="Eq5" src="/files/ftp_upload/11389/11389_Equation_5.png" style="text-align: center; height: 90px;" />
 この場合、反応は逆方向に平衡へと向かって進む。生成物の濃度とQcの分子は時間とともに減少し、反応物の濃度とQcの分母は増加します。その結果、反応商は平衡状態で一定になるまで減少します。このように平衡状態にある系が示すQの一定値を平衡定数Kといいます。
 <img alt="Eq6" src="/files/ftp_upload/11389/11389_Equation_6.png" style="text-align: center; height: 50px;" />
 反応商の評価
 気体状の二酸化窒素は、次式によって四酸化二窒素を形成します。
 <img alt="Eq7" src="/files/ftp_upload/11389/11389_Equation_7.png" style="text-align: center; height: 50px;" />
 25°Cの1.0 Lのフラスコに0.10 molのNO2を加えると、その濃度が変化します。平衡状態では[NO2] = 0.016 M、[N2O4] = 0.042 Mとなります。 生成物が生成する前の状態では、[NO2] = 0.10 M、[N2O4] = 0 Mです。
 <img alt="Eq8" src="/files/ftp_upload/11389/11389_Equation_8.png" style="text-align: center; height: 90px;" />
 平衡状態において、
 <img alt="Eq9" src="/files/ftp_upload/11389/11389_Equation_9.png" style="text-align: center; height: 90px;" />
 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/13-2-equilibrium-constants">Openstax, Chemistry 2e, Section 13.2 Equilibrium Constants</a><a href="https://openstax.org/books/chemistry-2e/pages/13-2-equilibrium-constants">.</a>

可逆反応の状態は、その反応商(Q)を評価することで簡単に評価できます。m A + n B ...

Education

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化学

Chemical Equilibrium

化学平衡

KEY TERMS AND CONCEPTS

主要用語と概念

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 moves toward equilibrium, the value of Q changes, so it gives a snapshot of the system’s status.

The oxidation of sulfur dioxide shows how Q changes over time. If a reaction starts with only reactants, SO2 and O2, then Q is initially zero. As the reaction moves forward, reactant concentrations drop, the denominator of Qc gets smaller, product concentration rises, and Qc increases.

If the reaction starts with only product, SO3, then Qc is initially undefined and can be treated as immeasurably large or infinite. In that case, the reaction moves in reverse toward equilibrium. Product concentration and the numerator of Qc decrease, while reactant concentrations and the denominator increase. Qc keeps dropping until it becomes constant at equilibrium.

The constant value of Q for a system at equilibrium is the equilibrium constant, K. A nitrogen dioxide example shows how this works in practice. When 0.10 mol NO2 is placed in a 1.0-L flask at 25 °C, the initial concentrations are [NO2] = 0.10 M and [N2O4] = 0 M, so Qc can be found from the starting amounts. At equilibrium, [NO2] = 0.016 M and [N2O4] = 0.042 M, and Qc is calculated again from those concentrations.

Using Q to Track Equilibrium Changes

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

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

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 the reaction quotient Q shows whether a reversible reaction is moving forward, reversing, or reaching equilibrium using concentration and pressure examples.

Using Q to Track Equilibrium Changes | 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。

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Matter And Its Interactions

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Chemical reactions

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Matter and Its Interactions

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New York State Science Learning Standards

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Using Q to Track Equilibrium Changes

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