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'espressione della costante di equilibrio è scritta come le concentrazioni molari dei prodotti, C e D, sui reagenti, A e B, in equilibrio, ciascuno elevato ai rispettivi coefficienti stechiometrici. Quando è risolta, l'espressione è uguale alla costante di equilibrio, Kc.Un'espressione nella stessa forma può anche essere scritta per i reagenti ed i prodotti a qualsiasi concentrazione, e la quantità calcolata è nota come quoziente di reazione, Qc.Come Qc, l'espressione Qp può essere scritta per le reazioni gassose, utilizzando le pressioni parziali. Mentre K rimane costante ad una temperatura specifica indipendentemente dalla concentrazione, il valore di Q cambia mentre la reazione procede verso i prodotti o i reagenti.

Il quoziente di reazione può essere utilizzato per determinare la direzione in cui procederà una reazione per raggiungere l'equilibrio. All'inizio di una data reazione, se la concentrazione dei prodotti è zero, il quoziente di reazione sarà zero. Ogni volta che la concentrazione dei reagenti nel denominatore è elevata, laddove Q sia minore di K, la reazione si sposterà a destra per sintetizzare più prodotti, fino a quando il sistema non raggiunge l'equilibrio.

Se la concentrazione dei reagenti è zero, il quoziente di reazione è infinito. Ogni volta che la concentrazione dei prodotti nel numeratore è elevata, laddove Q è maggiore di K, la reazione si sposterà a sinistra per produrre più reagenti. Se Q è uguale a K, il sistema è in equilibrio e la velocità delle reazioni diretta e inversa sono uguali.

Considerate la reazione data con una costante di equilibrio 50. Se la miscela di reazione contiene 0, 20 idrogeno molare, 0, 20 iodio molare e 1, 7 ioduro di idrogeno molare, la direzione della reazione mostrata può essere determinata calcolando Q.Sostituendo le concentrazioni date nell'espressione, Q è uguale a 72, che è maggiore di K.Pertanto, la reazione si sposterà verso sinistra.

<ol>
	<li>Lederer, Robert. "Il quoziente di reazione non è necessario per risolvere problemi di equilibrio: nessun problema con 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>

quoziente di reazione

Lo stato di una reazione reversibile viene convenientemente valutato valutando il suo quoziente di reazione (Q). Per una reazione reversibile descritta da m A + n B ⇌ x C + y D, il quoziente di reazione si deriva direttamente dalla stechiometria dell'equazione bilanciata come

<img alt="Eq1" src="/files/ftp_upload/11389/11389_Equation_1.png" style="text-align: center; height: 90px;" />

dove il pedice c denota l'uso delle concentrazioni molari nell'espressione. Se i reagenti e i prodotti sono gassosi, un quoziente di reazione può essere derivato in modo simile utilizzando pressioni parziali:

<img alt="Eq2" src="/files/ftp_upload/11389/11389_Equation_2.png" style="text-align: center; height: 90px;" />

Si noti che le equazioni del quoziente di reazione di cui sopra sono una semplificazione di espressioni più rigorose che utilizzano valori relativi per concentrazioni e pressioni anziché valori assoluti. Questi valori relativi di concentrazione e pressione sono adimensionali (non hanno unità); di conseguenza, lo sono anche i quozienti di reazione.

Il valore numerico di Q varia man mano che una reazione procede verso l'equilibrio; pertanto, può servire come utile indicatore dello stato della reazione. Per illustrare questo punto, consideriamo l’ossidazione del biossido di zolfo:

<img alt="Eq3" src="/files/ftp_upload/11389/11389_Equation_3.png" style="text-align: center; height: 50px;" />

Qui sono possibili due diversi scenari sperimentali, uno in cui questa reazione viene avviata solo con una miscela di reagenti, SO_2 e O_2, e un altro che inizia solo con il prodotto, SO_3. Per la reazione che inizia con una sola miscela di reagenti, Q è inizialmente uguale a zero:

<img alt="Eq4" src="/files/ftp_upload/11389/11389_Equation_4.png" style="text-align: center; height: 90px;" />

Man mano che la reazione procede verso l'equilibrio nella direzione in avanti, le concentrazioni dei reagenti diminuiscono (come fa il denominatore di Q_c), la concentrazione del prodotto aumenta (come fa il numeratore di Q_c) e di conseguenza il quoziente di reazione aumenta. Quando viene raggiunto l'equilibrio, le concentrazioni dei reagenti e del prodotto rimangono costanti, così come il valore di Q_c.

Se la reazione inizia con la presenza del solo prodotto, il valore di Q_c è inizialmente indefinito (incommensurabilmente grande o infinito):

<img alt="Eq5" src="/files/ftp_upload/11389/11389_Equation_5.png" style="text-align: center; height: 90px;" />

In questo caso la reazione procede verso l'equilibrio nella direzione opposta. La concentrazione del prodotto e il numeratore di Q_c diminuiscono con il tempo, le concentrazioni dei reagenti e il denominatore di Q_c aumentano e il quoziente di reazione di conseguenza diminuisce fino a diventare costante nell'equilibrio. Il valore costante di Q esibito da un sistema in equilibrio è chiamato costante di equilibrio, K:

<img alt="Eq6" src="/files/ftp_upload/11389/11389_Equation_6.png" style="text-align: center; height: 50px;" />

Valutazione di un quoziente di reazione

Il biossido di azoto gassoso forma tetrossido di diazoto secondo questa equazione:

<img alt="Eq7" src="/files/ftp_upload/11389/11389_Equation_7.png" style="text-align: center; height: 50px;" />

Quando si aggiungono 0,10 mol di NO_2 in un pallone da 1,0 L a 25 °C, la concentrazione cambia in modo che all'equilibrio, [NO_2] = 0,016 M e [N_2O_4] = 0,042 M. Prima che si formi qualsiasi prodotto, [NO_2] = 0,10 M e [N_2O_4] = 0 M. Pertanto,

<img alt="Eq8" src="/files/ftp_upload/11389/11389_Equation_8.png" style="text-align: center; height: 90px;" />

In equilibrio,

<img alt="Eq9" src="/files/ftp_upload/11389/11389_Equation_9.png" style="text-align: center; height: 90px;" />

Questo testo è adattato da <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>

Quoziente di reazione

Lo stato di una reazione reversibile viene opportunamente valutato valutando il suo quoziente di reazione (Q). Per una reazione reversibile descritta da m A + n B ...

Education

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Chemical Equilibrium

Equilibrio chimico

KEY TERMS AND CONCEPTS

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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

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

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Pennsylvania State Academic Standards

Physical Science

Chemical Reactions

3.2.9-12.F

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High School

Matter And Its Interactions

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Interactions Of Matter

Chemical reactions

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Matter and Its Interactions

Florida Department of Education

SC.912.P.12

SC.912.P.12.13

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New York State Science Learning Standards

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Using Q to Track Equilibrium Changes

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