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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Reaction Quotient

평형 상수 식은 평형 상태에서 반응물 A와 B의 몰농도에 대한 생성물 C와 D의 몰 농도의 비로 되며 여기서 각각은 개개의 화학양론 계수로 제곱됩니다. 식을 풀면 표현식은 평형 상수 Kc와 같습니다. 또한 농도가 어떻든 간에 반응물과 화합물에 대해 동일한 형식의 식을 작성할 수 있으며, 이 때 계산된 값을 반응 계수 Qc라고 합니다.

Qc와 마찬가지로, 부분 압력을 사용한 기체 반응에 대해 Qp 식을 작성할 수 있습니다. K는 농도에 관계없이 특정 온도에서 일정하게 유지되지만 생성물 또는 반응물에 대한 반응이 진행됨에 따라 Q의 값이 변경됩니다. 반응 지수를 사용하여 평형에 도달하기 위해 반응이 진행되는 방향을 결정할 수 있습니다.

반응이 시작될 때 생성물의 농도가 0이면 반응 지수는 0이 됩니다. 분모 내 반응물 농도가 높으면 Q가 K보다 작아지며 시스템이 평형에 도달할 때까지 더 많은 반응물을 생성하는 오른쪽으로 반응이 진행됩니다. 반응물의 농도가 0이면 반응 지수는 무한한 값입니다.

분자의 생성물 농도가 높으면 Q가 K보다 크게 되며 반응이 왼쪽으로 이동하여 반응물을 더 많이 생성합니다. Q가 K와 같으면 시스템이 평형 상태이고 양반응 및 역반응은 속도가 같습니다. 평형 상수가 50인 반응을 고려해봅시다 반응 혼합물이 수소 0.20 몰 요오드 0.20 몰 요오드화 수소 1.7 몰을 함유하고 있다면 Q를 계산하여 표시된 반응 방향을 결정할 수 있습니다.

주어진 농도 값을 식에 대입하면 Q는 K보다 큰 72입니다. 따라서 반응이 왼쪽으로 이동합니다.

<ol>
	<li>레더러, 로버트. "반응 지수는 평형 문제를 해결하는 데 필요하지 않습니다 : Q에 문제가 없습니다." 화학 교육 저널 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>

반응지수

가역적 반응의 상태는 반응 지수(Q)를 평가하여 편리하게 평가됩니다. m A + n B ⇌ x C + y D로 설명되는 가역 반응의 경우, 반응 지수는 균형 방정식의 화학량론에서 다음과 같이 직접 파생됩니다.

<img alt="Eq1" src="/files/ftp_upload/11389/11389_Equation_1.png" style="text-align: center; height: 90px;" />

여기서 아래 첨자 c는 표현에서 몰 농도의 사용을 나타냅니다. 반응물과 생성물이 기체인 경우, 부분압을 사용하여 반응 지수를 유사하게 유도할 수 있습니다.

<img alt="Eq2" src="/files/ftp_upload/11389/11389_Equation_2.png" style="text-align: center; height: 90px;" />

위의 반응지수 방정식은 절대값이 아닌 농도 및 압력에 대한 상대값을 사용하는 보다 엄격한 표현을 단순화한 것입니다. 이러한 상대 농도 및 압력 값은 차원이 없습니다(단위가 없음). 결과적으로 반응 지수도 마찬가지입니다.

Q의 수치는 반응이 평형을 향해 진행됨에 따라 달라집니다. 따라서 반응 상태를 나타내는 유용한 지표 역할을 할 수 있습니다. 이 점을 설명하기 위해 이산화황의 산화를 생각해 보세요.

<img alt="Eq3" src="/files/ftp_upload/11389/11389_Equation_3.png" style="text-align: center; height: 50px;" />

여기서는 두 가지 다른 실험 시나리오가 가능합니다. 하나는 이 반응이 반응물인 SO_2와 O_2의 혼합물로만 시작되는 것이고, 다른 하나는 생성물인 SO_3으로 시작되는 것입니다. 반응물의 혼합물만으로 시작되는 반응의 경우 Q는 초기에 0과 같습니다.

<img alt="Eq4" src="/files/ftp_upload/11389/11389_Equation_4.png" style="text-align: center; height: 90px;" />

반응이 순방향으로 평형을 향해 진행됨에 따라 반응물 농도는 감소하고(Q_c의 분모와 마찬가지로) 생성물 농도는 증가하며(Q_c의 분자와 마찬가지로) 결과적으로 반응 지수는 증가합니다. 평형이 달성되면 반응물과 생성물의 농도는 Q_c 값과 마찬가지로 일정하게 유지됩니다.

생성물만 존재하는 상태에서 반응이 시작되면 Q_c 값은 초기에 정의되지 않습니다(측정할 수 없을 정도로 크거나 무한함).

<img alt="Eq5" src="/files/ftp_upload/11389/11389_Equation_5.png" style="text-align: center; height: 90px;" />

이 경우 반응은 반대 방향으로 평형을 향해 진행된니다. 생성물 농도와 Q_c의 분자는 시간이 지남에 따라 감소하고, 반응물 농도와 Q_c의 분모는 증가하며, 결과적으로 반응 지수는 평형에서 일정해질 때까지 감소합니다. 평형 상태에서 시스템이 나타내는 Q의 상수 값을 평형 상수 K라고 합니다.

<img alt="Eq6" src="/files/ftp_upload/11389/11389_Equation_6.png" style="text-align: center; height: 50px;" />

반응 지수 평가

기체 이산화질소는 다음 방정식에 따라 사산화이질소를 형성합니다.

<img alt="Eq7" src="/files/ftp_upload/11389/11389_Equation_7.png" style="text-align: center; height: 50px;" />

25 °C에서 1.0 L 플라스크에 0.10 몰 NO_2를 첨가하면 평형 상태에서 [NO_2] = 0.016 M 및 [N_2O_4] = 0.042 M이 되도록 농도가 변합니다. 생성물이 형성되기 전에는 [NO_2] = 0.10입니다. M 및 [N_2O_4] = 0 M입니다. 따라서,

<img alt="Eq8" src="/files/ftp_upload/11389/11389_Equation_8.png" style="text-align: center; height: 90px;" />

평형 상태에서,

<img alt="Eq9" src="/files/ftp_upload/11389/11389_Equation_9.png" style="text-align: center; height: 90px;" />

입니다.

이 문서는 에서 발췌되었습니다 <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>

가역적 반응의 상태는 반응 지수(Q)를 평가하여 편리하게 평가할 수 있습니다. m A + n B ...

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Chemical Equilibrium

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KEY TERMS AND CONCEPTS

주요 용어 및 개념

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

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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

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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)

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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...

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

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Using Q to Track Equilibrium Changes

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