Name: chemical equilibrium and kc rules
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K12 Standards

north-carolina-k-12-science-standard-course-of-study

NC K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.5.2

The Equilibrium Constant

La ley de acción de masas introduce la constante de equilibrio para relacionar las concentraciones de equilibrio de los reactivos y los productos. Para una ecuación química balanceada donde A y B son reactivos, C y D son productos y las minúsculas a, b, c y d son sus respectivos coeficientes estequiométricos, la expresión constante de equilibrio está dada por las concentraciones molares de los productos divididos por las concentraciones molares de los reactivos, todas elevadas a sus coeficientes estequiométricos. La constante de equilibrio a menudo se expresa con el símbolo K o Kc, donde el subíndice c indica la constante de equilibrio con respecto a la concentración molar.Al ser una relación de concentraciones molares, la constante de equilibrio no tiene unidades. Una constante de equilibrio grande mucho mayor que 1 indica que el numerador, que es la concentración de los productos, es mayor que el denominador, la concentración de los reactivos. Esto indica que el equilibrio se inclina hacia los productos y favorece la reacción hacia adelante.Por el contrario, una pequeña constante de equilibrio mucho menor que 1 indica que el denominador es mayor que el numerador. Esto significa que el equilibrio se inclina hacia los reactivos y favorece la reacción inversa. Para una constante de equilibrio de magnitud igual o cercana a 1, las concentraciones relativas de los reactivos y productos son casi iguales.Esto indica que no se favorece la reacción, ni hacia adelante ni hacia atrás. Las modificaciones de la ecuación química también cambian la constante de equilibrio. Si se invierte una reacción química con una constante de equilibrio x, la expresión de la constante de equilibrio para la reacción inversa es la recíproca de la reacción directa.Entonces, la nueva constante de equilibrio sería 1 sobre x. Si los coeficientes se multiplican por otro factor, n, la constante de equilibrio se eleva al mismo factor. La nueva constante de equilibrio ahora sería x elevada a la potencia n.Cuando se suman dos o más reacciones individuales, la constante de equilibrio para la reacción general es el producto de las constantes de equilibrio individuales. Entonces, la nueva constante de equilibrio sería y por z Para un equilibrio heterogéneo, la expresión constante de equilibrio excluye las entidades sólidas o líquida puras. Por ejemplo, la expresión constante de equilibrio para la descomposición de nitrato de amonio sólido en óxido nitroso gaseoso y vapor de agua excluye el nitrato de amonio:un sólido puro.Esto se debe a que la concentración molar relativa de un sólido o líquido puro permanece constante durante la reacción.

la constante de equilibrio

Considere la oxidación del dióxido de azufre:
 <img alt="Eq1" src="/files/ftp_upload/11386/11386_Equation_1.png" style="text-align: center; height: 50px;" />
 Para una reacción que comienza sólo con una mezcla de reactivos, la concentración del producto es inicialmente igual a cero. A medida que la reacción avanza hacia el equilibrio en la dirección de avance, las concentraciones de los reactivos disminuyen y la concentración del producto aumenta. Cuando se alcanza el equilibrio, las concentraciones de los reactivos y productos permanecen constantes.
 Si la reacción comienza sólo con los productos presentes, la reacción avanza hacia el equilibrio en la dirección inversa. La concentración del producto disminuye con el tiempo y las concentraciones de los reactivos aumentan hasta que las concentraciones se mantienen constantes en equilibrio.
 La ley de acción de masas establece que la relación entre la concentración de los productos y la concentración de los reactivos en equilibrio, elevada a sus respectivos coeficientes estequiométricos, es igual a una constante, denominada constante de equilibrio, K o Kc.
 Por lo tanto, la expresión de la constante de equilibrio para la reacción anterior se escribe como:
 <img alt="Eq2" src="/files/ftp_upload/11386/11386_Equation_2.png" style="text-align: center; height: 100px;" />
 donde, el subíndice ‘c’ indica que la constante de equilibrio considera la concentración molar de reactivos y productos.
 La magnitud de la constante de equilibrio refleja explícitamente la composición de una mezcla de reacción en equilibrio. Una reacción que muestre una K grande alcanzará el equilibrio cuando la mayor parte del reactivo se haya convertido en producto, mientras que una K pequeña indica que la reacción alcanza el equilibrio después de que se haya convertido muy poco reactivo. Es importante tener en cuenta que la magnitud de K no indica cuán rápida o lentamente se alcanzará el equilibrio. Algunos equilibrios se establecen tan rápidamente como para ser casi instantáneos, y otros tan lentamente que ningún cambio perceptible se observa en el curso de días, años, o más tiempo. La constante de equilibrio de una reacción puede utilizarse para predecir el comportamiento de las mezclas que contienen sus reactivos y/o productos. Como lo demuestra el proceso de oxidación del dióxido de azufre descrito anteriormente, una reacción química procederá en cualquier dirección que sea necesaria para lograr el equilibrio.
 Equilibrios acoplados
 Muchos sistemas en equilibrio implican dos o más reacciones de equilibrio acopladas&#1, aquellas que tienen en común una o más especies de productos o reactantes. El valor de  K  de un sistema que implica equilibrios acoplados puede estar relacionado con los valores de  K  de las reacciones individuales. En este enfoque intervienen tres premisas básicas, como se describe a continuación: 
 
 • cambiar la dirección de una ecuación química esencialmente intercambia las identidades de los “reactantes” y “productos”, y por lo tanto la constante de equilibrio para la ecuación invertida es simplemente la recíproca de la ecuación de avance. 
 
 • El cambio de los coeficientes estequiométricos en una ecuación por algún factor x resulta en un cambio exponencial en la constante de equilibrio por ese mismo factor. 
 
 • agregar dos o más ecuaciones en equilibrio juntas produce una ecuación general cuya constante de equilibrio es el producto matemático de los valores de  K de las reacciones individuales.
 Expresión de la constante de equilibrio para equilibrios heterogéneos
 Para equilibrios heterogéneos, que implican reactivos y productos en dos o más fases diferentes, las concentraciones de sólidos puros o líquidos puros no se incluyen en la expresión de la constante de equilibrio, como se ilustra en el siguiente ejemplo:
 <img alt="Eq3" src="/files/ftp_upload/11386/11386_Equation_3.png" style="text-align: center; height: 150px;" />
 Esto se debe a que las concentraciones relativas de líquidos puros y sólidos puros permanecen constantes durante la reacción.</em
 Este texto ha sido adaptado de<a href="https://openstax.org/books/chemistry-2e/pages/13-2-equilibrium-constants"> las constantes de equilibrio Openstax, Química 2e, Sección 13.2</a>.

La Constante de Equilibrio

Considere la oxidación del dióxido de azufre:

Para una reacción que comienza con una mezcla de reactivos solamente, la concentración del producto es inicialmente igual a ...

Education

Educación

JoVE Core

Química

Chemical Equilibrium

Equilibrio químico

KEY TERMS AND CONCEPTS

TÉRMINOS Y CONCEPTOS CLAVE

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...

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

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...

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)

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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Pennsylvania State Academic Standards

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

Arizona Science Standards

High School

HS+C.P1

HS+C.P1U1.6

Next Generation Science Standards

Matter and Its Interactions

HS-PS1-6

Florida Department of Education

SC.912.P.12

SC.912.P.12.13

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South Carolina College and Career Ready Science Standards

C-PS1-6

new-york-state-science-learning-standards

New York State Science Learning Standards

HS. Chemical Reactions

HS-PS1-6.

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Chemical Equilibrium and Kc Rules

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