Name: chemical equilibrium and kc rules
Uploaded: 2025-08-11T09:37:07.000Z
Duration: 3 min 10 s
Channel: JoVE

K12 Standards

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NC K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.5.2

The Equilibrium Constant

Prawo działania masy wprowadza stałą równowagi w celu powiązania stężeń równowagowych reagentów i produktów.
W przypadku zrównoważonego równania chemicznego, w którym A i B są reagentami, C i D są produktami, a małe litery a, b, c i d są ich odpowiednimi współczynnikami stechiometrycznymi, wyrażenie stałej równowagi jest określone przez stężenia molowe produktów podzielone przez stężenia molowe reagentów, wszystkie podniesione do ich współczynników stechiometrycznych.
Stała równowagi jest często wyrażana symbolem K lub Kc, gdzie indeks dolny c wskazuje stałą równowagi w odniesieniu do stężenia molowego. Będąc stosunkiem stężeń molowych, stała równowagi jest bezjednostkowa.
Duża stała równowagi znacznie większa od 1 oznacza, że licznik, czyli stężenie produktów, jest większy niż mianownik, czyli stężenie reagentów. Wskazuje to, że równowaga leży w kierunku produktów i sprzyja reakcji naprzód.
I odwrotnie, mała stała równowagi znacznie mniejsza niż 1 oznacza, że mianownik jest większy niż licznik. Oznacza to, że równowaga leży w kierunku reagentów i sprzyja reakcji odwrotnej.
Dla stałej równowagi o wielkości równej lub bliskiej 1 względne stężenia reagentów i produktów są prawie takie same. Oznacza to, że ani reakcja, ani do przodu, ani do tyłu, nie jest faworyzowana.
Modyfikacje równania chemicznego zmieniają również stałą równowagi.
Jeżeli reakcja chemiczna ze stałą równowagi x jest odwrócona, wyrażenie stałej równowagi dla reakcji odwrotnej jest odwrotnością reakcji naprzód. Tak więc nowa stała równowagi wynosiłaby 1 przez x.
Jeśli współczynniki pomnoży się przez inny współczynnik, n, stała równowagi zostanie podniesiona do tego samego współczynnika. Nowa stała równowagi wynosiłaby teraz xn.
Gdy dwie lub więcej indywidualnych reakcji jest sumowanych, stała równowagi dla całej reakcji jest iloczynem indywidualnych stałych równowagi. Zatem nowa stała równowagi wynosiłaby y × z.
W przypadku równowagi heterogenicznej wyrażenie stałej równowagi wyklucza jednostki czyste stałe lub czyste ciecze.
Na przykład wyrażenie stałej równowagi dla rozkładu stałego azotanu amonu na gazowy podtlenek azotu i parę wodną nie obejmuje azotanu amonu - czystego ciała stałego.
Dzieje się tak, ponieważ względne stężenie molowe czystego ciała stałego lub cieczy pozostaje stałe podczas reakcji.

stała równowagi

Rozważmy utlenianie dwutlenku siarki:

<img alt="Eq1" src="/files/ftp_upload/11386/11386_Equation_1.png" style="text-align: center; height: 50px;" />

W przypadku reakcji, która rozpoczyna się tylko od mieszaniny reagentów, stężenie produktu jest początkowo równe zeru. W miarę jak reakcja postępuje w kierunku równowagi w kierunku do przodu, stężenia reagentów maleją, a stężenie produktu wzrasta. Po osiągnięciu równowagi stężenia reagentów i produktów pozostają stałe.

Jeśli reakcja rozpoczyna się z obecnymi produktami, reakcja przebiega w kierunku równowagi w odwrotnym kierunku. Stężenie produktu maleje z czasem, a stężenia reagentów rosną, aż będą stałe.

Prawo działania mas stwierdza, że stosunek stężenia produktów do stężenia reagentów w stanie równowagi, podniesiony do odpowiednich współczynników stechiometrycznych, jest równy stałej, zwanej stałą równowagi, K lub Kc.

Zatem wyrażenie stałej równowagi dla powyższej reakcji zapisuje się jako:

<img alt="Eq2" src="/files/ftp_upload/11386/11386_Equation_2.png" style="text-align: center; height: 100px;" />

gdzie indeks dolny „c” wskazuje, że stała równowagi uwzględnia stężenie molowe reagentów i produktów.

Wielkość stałej równowagi wyraźnie odzwierciedla skład mieszaniny reakcyjnej w stanie równowagi. Reakcja wykazująca duże K osiągnie równowagę, gdy większość reagenta zostanie przekształcona w produkt, podczas gdy małe K wskazuje, że reakcja osiąga równowagę po przekształceniu bardzo małej ilości reagenta. Należy pamiętać, że wielkość K nie wskazuje, jak szybko lub wolno zostanie osiągnięta równowaga. Niektóre równowagi ustalają się tak szybko, że są prawie natychmiastowe, a inne tak wolno, że nie obserwuje się żadnych zauważalnych zmian w ciągu dni, lat lub dłużej. Stałą równowagi reakcji można wykorzystać do przewidywania zachowania mieszanin zawierających jej reagenty i/lub produkty. Jak wykazano w opisanym powyżej procesie utleniania dwutlenku siarki, reakcja chemiczna będzie przebiegać w dowolnym kierunku niezbędnym do osiągnięcia równowagi.

Sprzężona równowaga

Wiele układów równowagi obejmuje dwie lub więcej sprzężonych reakcji równowagowych, które mają wspólny jeden lub więcej rodzajów reagentów lub produktów. Wartość K dla układu obejmującego sprzężone równowagi można powiązać z wartościami K poszczególnych reakcji. Podejście to obejmuje trzy podstawowe manipulacje, które opisano poniżej:

<ol style="list-style-type: disc;">
	<li>Zmiana kierunku równania chemicznego zasadniczo powoduje zamianę tożsamości „reagentów” i „produktów”, zatem stała równowagi dla odwróconego równania jest po prostu odwrotnością stałej równowagi dla równania forward.</li>
	<li>Zmiana współczynników stechiometrycznych w równaniu o pewien współczynnik x powoduje wykładniczą zmianę stałej równowagi o ten sam współczynnik.</li>
	<li>Dodanie dwóch lub więcej równań równowagi daje równanie ogólne, którego stała równowagi jest matematycznym iloczynem wartości K poszczególnych reakcji.</li>
</ol>

Wyrażenie stałej równowagi dla równowagi heterogenicznej

W przypadku równowag heterogenicznych, obejmujących reagenty i produkty w dwóch lub większej liczbie różnych faz, stężenia czystych substancji stałych lub czystych cieczy nie są uwzględniane w wyrażeniu stałej równowagi, co ilustruje następujący przykład:

<img alt="Eq3" src="/files/ftp_upload/11386/11386_Equation_3.png" style="text-align: center; height: 150px;" />

Dzieje się tak, ponieważ względne stężenia czystych cieczy i czystych substancji stałych pozostają stałe podczas reakcji.

Stała równowagi

Rozważ utlenianie dwutlenku siarki:

W przypadku reakcji, która rozpoczyna się tylko od mieszaniny reagentów, stężenie produktu jest początkowo równe ...

Education

JoVE Core

Chemia

Jeden

Chemical Equilibrium

Równowaga chemiczna

KEY TERMS AND CONCEPTS

Kluczowe terminy i pojęcia

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 in reverse toward equilibrium. The product concentration falls with time, and the reactant concentrations rise until both become constant. This shows that a chemical reaction will move in whatever direction is needed to reach equilibrium.

The law of mass action links equilibrium to a constant value. It states that the ratio of product concentrations to reactant concentrations, each raised to their stoichiometric coefficients, equals the equilibrium constant, K or Kc. The subscript c means the expression uses molar concentration. The size of K shows the mixture makeup at equilibrium. A large K means most reactant has changed to product before equilibrium is reached. A small K means only a little reactant changes to product. The value of K does not tell how fast equilibrium is reached.

Some equilibria form very quickly. Others take so long that no visible change may be seen for days, years, or longer. The equilibrium constant can still be used to predict how a mixture with reactants and/or products will behave. For coupled equilibria, two or more equilibrium reactions share one or more species. The K value for the overall system can be found from the K values of the separate reactions. Reversing a chemical equation changes K to its reciprocal. Multiplying the stoichiometric coefficients by a factor x changes K by that same factor as an exponent. Adding equilibrium equations gives an overall K equal to the product of the individual K values.

For heterogeneous equilibria, the reactants and products are in different phases. Pure solids and pure liquids are not included in the equilibrium constant expression. Their concentrations stay constant during the reaction, so they do not change the value of K. This rule is an important part of writing equilibrium expressions correctly.

Chemical Equilibrium and Kc Rules

the equilibrium constant

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...

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...

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...

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...

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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

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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

Explore chemical equilibrium, Kc, and the law of mass action. Learn how concentration changes, reaction direction, and mixed-phase equilibria shape equilibrium expressions.

Chemical Equilibrium and Kc Rules | JoVE K12

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Physical Science

Chemical Reactions

3.2.9-12.F

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Ohio’s Learning Standards and Model Curriculum

Interactions Of Matter

Chemical reactions

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HS+C.P1U1.6

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Matter and Its Interactions

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South Carolina College and Career Ready Science Standards

C-PS1-6

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New York State Science Learning Standards

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Chemical Equilibrium and Kc Rules

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