Name: chemical equilibrium and kc rules
Uploaded: 2025-08-11T09:37:07.000Z
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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

A lei da ação de massas introduz o equilíbrio constante para relacionar as concentrações de equilíbrio dos reagentes e produtos. Para uma equação química equilibrada onde A e B são reagentes, C e D são produtos, e as minúsculas a, b, c, e d, são os seus respetivos coeficientes estequiométricos, a expressão de equilíbrio constante é dada pelas concentrações molares dos produtos divididas pelas concentrações molares dos reagentes, todos elevados aos seus coeficientes estequiométricos. O equilíbrio constante é frequentemente expresso com o símbolo K ou Kc, onde o subscrito c indica o equilíbrio constante relativo à concentração molar.Sendo uma proporção de concentrações molares, o equilíbrio constante não tem unidade. Um grande equilíbrio constante muito superior a 1 significa que o numerador, ou seja a concentração dos produtos, é maior do que o denominador, a concentração dos reagentes. Isto indica que o equilíbrio está voltado para os produtos e favorece a reação frontal.Inversamente, um pequeno equilíbrio constante muito menos de 1 significa que o denominador é maior do que o numerador. Isto significa que o equilíbrio fica direcionado para os reagentes e favorece a reação inversa. Para uma constante de equilíbrio de magnitude igual ou próxima a 1, as concentrações relativas dos reagentes e dos produtos são quase as mesmas.Isto indica que nenhuma das duas reações, avançadas ou reversas, é favorecida. Modificações na equação química também alteram a constante de equilíbrio. Se uma reação química com uma constante de equilíbrio x é invertida, a expressão de equilíbrio constante para a reação inversa é o recíproco da reação de antecipação.Assim, o novo equilíbrio constante seria 1 sobre x. Se os coeficientes são multiplicados por outro fator, n, a constante de equilíbrio é elevada ao mesmo fator. O novo equilíbrio constante seria agora x elevado à potência n.Quando são somadas duas ou mais reações individuais, a constante de equilíbrio para a reação global é o produto das constantes de equilíbrio individual. Assim, o novo equilíbrio constante seria y vezes z. Para um equilíbrio heterogéneo, a expressão de equilíbrio constante exclui as entidades puramente sólidas ou líquidas.Por exemplo, a expressão de equilíbrio constante para a decomposição do nitrato de amónio sólido em óxido nitroso gasoso e vapor de água, exclui o nitrato de amónio-um puro sólido. Isto porque a concentração molar relativa de um sólido puro ou líquido mantém-se constante durante a reação.

a constante de equilíbrio

Considere a oxidação do dióxido de enxofre:
<img alt="Eq1" src="/files/ftp_upload/11386/11386_Equation_1.png" style="text-align: center; height: 50px;" />
Para uma reação que começa apenas com uma mistura de reagentes, a concentração do produto é inicialmente igual a zero. À medida que a reação avança para o equilíbrio, as concentrações de reagentes diminuem e a concentração do produto aumenta. Quando o equilíbrio é alcançado, as concentrações de reagentes e produtos permanecem constantes.
Se a reação começar apenas com os produtos presentes, a reação avança para o equilíbrio na direção inversa. A concentração do produto diminui com o tempo e as concentrações de reagentes aumentam até que as concentrações se tornem constantes em equilíbrio.
A lei de ação das massas afirma que a relação entre a concentração de produtos e a concentração de reagentes em equilíbrio, elevada aos respectivos coeficientes estequiométricos, é igual a uma constante, chamada constante de equilíbrio, K ou Kc.
Assim, a expressão da constante de equilíbrio para a reação acima é escrita como:
<img alt="Eq2" src="/files/ftp_upload/11386/11386_Equation_2.png" style="text-align: center; height: 100px;" />
onde, o subscrito ‘c’ indica que a constante de equilíbrio considera a concentração molar de reagentes e produtos.
A magnitude da constante de equilíbrio reflete explicitamente a composição de uma mistura de reação em equilíbrio. Uma reação que exiba um K grande alcançará o equilíbrio quando a maioria dos regeantes tiver sido convertida em produto, enquanto que um K pequeno indica que a reação alcançará o equilíbrio depois de muito pouco reagente ter sido convertido. É importante ter em mente que a magnitude de K não indica o quão rapidamente ou lentamente o equilíbrio será alcançado. Alguns equilíbrios são estabelecidos tão rapidamente que são quase instantâneos, e outros tão lentamente que nenhuma mudança perceptível é observada ao longo dos dias, anos, ou mais. A constante de equilíbrio para uma reação pode ser utilizada para prever o comportamento de misturas que contenham os seus reagentes e/ou produtos. Como demonstrado pelo processo de oxidação do dióxido de enxofre descrito acima, uma reação química prosseguirá em qualquer direção que seja necessária para alcançar o equilíbrio.
Equilíbrio Acoplado
Muitos sistemas de equilíbrio envolvem duas ou mais reações de equilíbrio acoplado , aquelas que têm em comum uma ou mais espécies reagentes ou de produtos. O valor de K para um sistema que envolva equilíbrios acoplados pode estar relacionado com os valores de K das reações individuais. Três manipulações básicas estão envolvidas nesta abordagem, conforme descrito abaixo: 
 
• A alteração da direção de uma equação química basicamente troca as identidades de “reagentes” e “produtos” e, portanto, a constante de equilíbrio para a equação inversa é simplesmente a recíproca daquela para a equação direta. 
 
•  A alteração dos coeficientes estequiométricos em uma equação por algum factor x resulta em uma mudança exponencial da constante de equilíbrio por esse mesmo factor. 
 
• A soma de duas ou mais equações de equilíbrio gera uma equação geral cuja constante de equilíbrio é o produto matemático dos valores de K da reação individual.
Expressão da Constante de Equilíbrio para Equilíbrios Heterogéneos
Para equilíbrios heterogéneos, envolvendo reagentes e produtos em duas ou mais fases diferentes, as concentrações de sólidos puros ou líquidos puros não são incluídas na expressão da constante de equilíbrio, como ilustrado no exemplo seguinte:
<img alt="Eq3" src="/files/ftp_upload/11386/11386_Equation_3.png" style="text-align: center; height: 150px;" />
Isto deve-se ao facto de as concentrações relativas de líquidos puros e sólidos puros permanecerem constantes durante a reação.
Este texto foi adaptado de <a href="https://openstax.org/books/chemistry-2e/pages/13-2-equilibrium-constants">Openstax, Chemistry 2e, Section 13.2 Equilibrium Constants</a>.

A Constante de Equilíbrio

Considere a oxidação do dióxido de enxofre:

Para uma reação que começa apenas com uma mistura de reagentes, a concentração do produto é inicialmente igual a ...

Education

JoVE Core

Química

Chemical Equilibrium

Equilíbrio Químico

KEY TERMS AND CONCEPTS

TERMOS-CHAVE E CONCEITOS

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

chemistry

JoVE Core - Química dá vida a conceitos gerais de química usando vídeos de animação fáceis de entender. Além disso, nos vídeos Cientistas em ação, demonstramos experimentos de pesquisa clássicos e originais realizados atualmente por cientistas em laboratórios em todo o mundo.

Pennsylvania State Academic Standards

Physical Science

Chemical Reactions

3.2.9-12.F

ohios-learning-standards-and-model-curriculum

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

south-carolina-college-and-career-ready-science-standards

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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