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

K12 Standards

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North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.5.2

Reaction Quotient

L'expression de la constante d'équilibre est exprimée par les concentrations molaires des produits C et D, par rapport aux réactifs, A et B, à l'équilibre, chacune étant élevée à son coefficient stœchiométrique respectif. Une fois résolue, l'expression est égale à la constante de l'équilibre, Kc.Une expression sous la même forme peut également être écrite pour les réactifs et les produits à toute concentration, et la quantité calculée est connue comme quotient de réaction, Qc.Comme Qc, l'expression Qp peut être écrit pour des réactions gazeuses utilisant des pressions partielles. Alors que K reste constant à une température spécifique quelle que soit la concentration, la valeur de Q change au fur et à mesure que la réaction se déroule avec les produits ou les réactifs.Le quotient de réaction peut être utilisé pour déterminer la direction dans laquelle une réaction se déroulera dans l'ordre pour atteindre l'équilibre. Au début d'une réaction donnée, si la concentration des produits est nulle, le quotient de réaction est nul. Chaque fois que la concentration des réactifs dans le dénominateur est élevée, de sorte que Q est plus petit que K, la réaction se déplacera vers la droite pour synthétiser plus de produits jusqu'à ce que le système atteigne l'équilibre.Si la concentration des réactifs est nulle, le quotient de réaction est infini. Chaque fois que la concentration des produits dans le numérateur est élevée, tel que Q est plus grand que K, la réaction se déplacera vers la gauche pour produire plus de réactifs. Si Q est égal à K, le système est à l'équilibre, et le taux des réactions directes et inverses est égal.Considérons la réaction donnée avec une constante d'équilibre 50. Si le mélange réactionnel contient 0, 20 molaire d'hydrogène, 0, 20 molaire d'iode et 1, 7 molaire d'iodure d'hydrogène, la direction de la réaction indiquée peut être déterminée en calculant Q.Substituer les concentrations données dans l'expression, Q égal à 72, qui est supérieur à K.Par conséquent, la réaction se déplacera vers la gauche.

<ol>
	<li>Lederer, Robert. « Le quotient de réaction n’est pas nécessaire pour résoudre les problèmes d’équilibre : aucun problème avec Q. » Journal of Chemical Education 82, n° 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>

quotient de réaction

L'état d'une réaction réversible est estimé en évaluant son quotient réactionnel (Q). Pour une réaction réversible décrite par m A + n B ⇌ x C + y D, le quotient de la réaction est déduit directement à partir de la stœchiométrie de l'équation équilibrée comme 
 <img alt="Eq1" src="/files/ftp_upload/11389/11389_Equation_1.png" style="text-align: center; height: 90px;" />
 où l'indice c indique l'utilisation de concentrations molaires dans l'expression. Si les réactifs et les produits sont gazeux, un quotient réactionnel peut être déduit de la même façon en utilisant des pressions partielles : 
 <img alt="Eq2" src="/files/ftp_upload/11389/11389_Equation_2.png" style="text-align: center; height: 90px;" />
 Notez que les équations du quotient réactionnel ci-dessus sont une simplification des expressions plus rigoureuses qui utilisent des valeurs relatives pour les concentrations et les pressions plutôt que des valeurs absolues. Ces valeurs relatives de la concentration et de la pression sont sans dimension (elles n'ont pas d'unités) ; par conséquent, les quotients réactionnels le sont aussi. 
 La valeur numérique de Q varie au fur et à mesure qu’une réaction se poursuit vers l’équilibre ; elle peut donc servir d’indicateur utile de l’état de la réaction. Pour illustrer ce point, considérez l'oxydation du dioxyde de soufre : 
 <img alt="Eq3" src="/files/ftp_upload/11389/11389_Equation_3.png" style="text-align: center; height: 50px;" />
 Deux scénarios expérimentaux différents sont possibles ici, l'un dans lequel cette réaction est initiée avec un mélange de réactifs seulement, SO2 et O2, et un autre qui commence avec seulement du produit, SO3. Pour la réaction qui commence par un mélange de réactifs uniquement, Q est initialement égal à zéro : 
 <img alt="Eq4" src="/files/ftp_upload/11389/11389_Equation_4.png" style="text-align: center; height: 90px;" />
 À mesure que la réaction se dirige vers l'équilibre dans le sens direct, les concentrations de réactifs diminuent (tout comme le dénominateur de Qc), la concentration du produit augmente (tout comme le numérateur de Qc) et le quotient de la réaction, par conséquent, augmente. Lorsque l'équilibre est atteint, les concentrations des réactifs et du produit restent constantes, tout comme la valeur de Qc. 
 Si la réaction commence avec seulement du produit présent, la valeur de Qc est initialement indéfinie (infiniment grande ou infinie) : 
 <img alt="Eq5" src="/files/ftp_upload/11389/11389_Equation_5.png" style="text-align: center; height: 90px;" />
 Dans ce cas, la réaction se poursuit vers l'équilibre dans la direction inverse. La concentration du produit et le numérateur de Qc diminuent avec le temps, les concentrations des réactifs et le dénominateur de Qc augmentent, et le quotient de la réaction diminue par conséquent jusqu'à ce qu'il devienne constant à l'équilibre. La valeur constante de Q qu'affiche un système à l’équilibre est appelée la constante d'équilibre, K : 
 <img alt="Eq6" src="/files/ftp_upload/11389/11389_Equation_6.png" style="text-align: center; height: 50px;" />
 Évaluation d'un quotient réactionnel 
 Le dioxyde d'azote gazeux forme du tétraoxyde de diazote selon cette équation : 
 <img alt="Eq7" src="/files/ftp_upload/11389/11389_Equation_7.png" style="text-align: center; height: 50px;" />
 Lorsque 0,10 mol de NO2 est ajouté à une fiole de 1,0 L à 25 °C, la concentration change de sorte qu'à l'équilibre, [NO2] = 0,016 M et [N2O4] = 0,042 M. Avant la formation d'un produit, [NO2] = 0,10 M et [N2O4] = 0 M. Ainsi, 
 <img alt="Eq8" src="/files/ftp_upload/11389/11389_Equation_8.png" style="text-align: center; height: 90px;" />
 À l'équilibre, 
 <img alt="Eq9" src="/files/ftp_upload/11389/11389_Equation_9.png" style="text-align: center; height: 90px;" />
 Ce texte a été adapté de<a href="https://openstax.org/books/chemistry-2e/pages/13-2-equilibrium-constants"> Openstax, Chimie 2e, Section 13.2 Constantes d'équilibre</a><a href="https://openstax.org/books/chemistry-2e/pages/13-2-equilibrium-constants">.</a>

Quotient de réaction

L’état d’une réaction réversible est commodément évalué en évaluant son quotient de réaction (Q). Pour une réaction réversible décrite par m A + n B ...

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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 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)

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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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Physical Science

Chemical Reactions

3.2.9-12.F

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Matter And Its Interactions

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Using Q to Track Equilibrium Changes

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