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

Wyrażenie stałej równowagi jest zapisywane jako stężenia molowe produktów C i D nad reagentami A i B w stanie równowagi, z których każdy jest podniesiony do odpowiednich współczynników stechiometrycznych. Po rozwiązaniu wyrażenie jest równe stałej równowagi, Kc.
Wyrażenie w tej samej postaci można również zapisać dla reagentów i produktów o dowolnym stężeniu, a obliczona ilość jest znana jako iloraz reakcji, Qc.
Podobnie jak Qc, wyrażenie Qp można zapisać dla reakcji gazowych przy użyciu ciśnień cząstkowych.
Podczas gdy K pozostaje stały w określonej temperaturze niezależnie od stężenia, wartość Q zmienia się w miarę postępu reakcji w kierunku produktów lub reagentów.
Iloraz reakcji można wykorzystać do określenia kierunku, w którym będzie przebiegać reakcja, aby osiągnąć równowagę.
Na początku danej reakcji, jeśli stężenie produktów wynosi zero, iloraz reakcji wynosi zero.
Zawsze, gdy stężenie reagentów w mianowniku jest wysokie, tak że Q jest mniejsze niż K, reakcja przesunie się w prawo, aby zsyntetyzować więcej produktów, aż układ osiągnie równowagę.
Jeśli stężenie reagentów wynosi zero, iloraz reakcji jest nieskończony.
Zawsze, gdy stężenie produktów w liczniku jest wysokie, tak że Q jest większe niż K, reakcja przesunie się w lewo, aby wytworzyć więcej reagentów.
Jeśli Q jest równe K, układ jest w równowadze, a szybkość reakcji do przodu i do tyłu jest równa.
Rozważmy daną reakcję ze stałą równowagi 50. Jeśli mieszanina reakcyjna zawiera 0,20 molowego wodoru, 0,20 molowego jodu i 1,7 molowego jodku wodoru, pokazany kierunek reakcji można określić, obliczając Q.
Podstawiając podane stężenia do wyrażenia, Q jest równe 72, które jest większe niż K. W związku z tym reakcja przesunie się w lewo.

<ol>
	<li>Lederer, Robert. "Iloraz reakcji jest niepotrzebny do rozwiązania problemów równowagi: nie ma problemów z Q." Journal of Chemical Education 82, nr 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>

iloraz reakcji

Stan odwracalnej reakcji dogodnie ocenia się poprzez ocenę jej współczynnika reakcji (Q). Dla odwracalnej reakcji opisanej przez m A + n B ⇌ x C + y D, iloraz reakcji wyprowadza się bezpośrednio ze stechiometrii zrównoważonego równania jako

<img alt="Eq1" src="/files/ftp_upload/11389/11389_Equation_1.png" style="text-align: center; height: 90px;" />

gdzie indeks dolny c oznacza użycie stężeń molowych w wyrażeniu. Jeśli reagenty i produkty są gazowe, współczynnik reakcji można w podobny sposób wyprowadzić, stosując ciśnienia cząstkowe:

<img alt="Eq2" src="/files/ftp_upload/11389/11389_Equation_2.png" style="text-align: center; height: 90px;" />

Należy zauważyć, że powyższe równania ilorazu reakcji są uproszczeniem bardziej szczegółowych wyrażeń, które wykorzystują względne wartości stężeń i ciśnień, a nie wartości bezwzględne. Te względne wartości stężeń i ciśnienia są bezwymiarowe (nie mają jednostek); w konsekwencji takie same są ilorazy reakcji.

Wartość liczbowa Q zmienia się w miarę jak reakcja zmierza do równowagi, dlatego może służyć jako użyteczny wskaźnik stanu reakcji. Aby zilustrować ten punkt, rozważmy utlenianie dwutlenku siarki:

<img alt="Eq3" src="/files/ftp_upload/11389/11389_Equation_3.png" style="text-align: center; height: 50px;" />

Możliwe są tutaj dwa różne scenariusze eksperymentalne: jeden, w którym reakcję inicjuje się wyłącznie mieszaniną reagentów, SO2 i O2, oraz drugi, w którym reakcja rozpoczyna się wyłącznie produktem, SO3. W przypadku reakcji rozpoczynającej się tylko od mieszaniny reagentów Q jest początkowo równe zeru:

<img alt="Eq4" src="/files/ftp_upload/11389/11389_Equation_4.png" style="text-align: center; height: 90px;" />

W miarę jak reakcja zmierza do równowagi w kierunku do przodu, stężenia reagentów maleją (podobnie jak mianownik Qc), wzrasta stężenie produktu (podobnie jak licznik Qc), a w konsekwencji wzrasta iloraz reakcji. Po osiągnięciu równowagi stężenia reagentów i produktu pozostają stałe, podobnie jak wartość Qc.

Jeśli reakcja rozpoczyna się w obecności samego produktu, wartość Qc jest początkowo nieokreślona (niezmierzalnie duża lub nieskończona):

<img alt="Eq5" src="/files/ftp_upload/11389/11389_Equation_5.png" style="text-align: center; height: 90px;" />

W tym przypadku reakcja przebiega w kierunku równowagi w odwrotnym kierunku. Stężenie produktu i licznik Qc zmniejszają się z czasem, stężenia reagentów i mianownik Qc rosną, w wyniku czego iloraz reakcji maleje, aż do osiągnięcia stałego stanu w równowadze. Stała wartość Q wykazywana przez układ w równowadze nazywana jest stałą równowagi K:

<img alt="Eq6" src="/files/ftp_upload/11389/11389_Equation_6.png" style="text-align: center; height: 50px;" />

Obliczanie ilorazu reakcji

Gazowy dwutlenek azotu tworzy czterotlenek diazotu zgodnie z następującym równaniem:

<img alt="Eq7" src="/files/ftp_upload/11389/11389_Equation_7.png" style="text-align: center; height: 50px;" />

Po dodaniu 0,10 mola NO2 do 1,0-litrowej kolby w temperaturze 25°C stężenie zmienia się tak, że w stanie równowagi [NO2] = 0,016 M i [N2O4] = 0,042 M. Przed utworzeniem się jakiegokolwiek produktu [NO2] = 0,10 M i [N2O4] = 0 M. Zatem

<img alt="Eq8" src="/files/ftp_upload/11389/11389_Equation_8.png" style="text-align: center; height: 90px;" />

W stanie równowagi

<img alt="Eq9" src="/files/ftp_upload/11389/11389_Equation_9.png" style="text-align: center; height: 90px;" />

Iloraz reakcji

Stan reakcji odwracalnej jest wygodnie oceniany poprzez ocenę jej ilorazu reakcji (Q). Dla reakcji odwracalnej opisanej przez 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...

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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...

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

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Using Q to Track Equilibrium Changes

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