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

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 loi de l'action de masse introduit le constant d'équilibre pour relier les concentrations d'équilibre des réactifs et des produits. Pour une équation chimique équilibrée où A et B sont des réactifs, C et D sont des produits, et a, b, c et d minuscules, sont leurs coefficients stœchiométriques respectifs, l'expression constante d'équilibre est donnée par les concentrations molaires des produits divisés par les concentrations molaires des réactifs, tous élevés à leurs coefficients stœchiométriques. La constante d'équilibre est souvent exprimée par le symbole K ou Kc, où l'indice c indique la constante d'équilibre par rapport à la concentration molaire.Étant un rapport de concentrations molaires, la constante d'équilibre est sans unitée. Une grande constante d'équilibre bien supérieure à 1 signifie que le numérateur, c'est-à-dire la concentration des produits, est supérieur au dénominateur, la concentration des réactifs. Cela indique que l'équilibre se situe vers les produits et favorise la réaction directe.A l'inverse, une petite constante d'équilibre bien inférieure à 1 signifie que le dénominateur est supérieur au numérateur. Cela signifie que l'équilibre se situe vers les réactifs et favorise la réaction inverse. Pour une constante d'équilibre de grandeur égale ou proche à 1, les concentrations relatives des réactifs et des produits sont à peu près les mêmes.Cela indique qu'aucune des deux réactions, directe ou inverse, n'est favorisée. Les modifications de l'équation chimique modifie également la constante d'équilibre. Si une réaction chimique avec une constante d'équilibre x est inversée, l'expression constante d'équilibre car la réaction inverse est la réciproque de la réaction directe.Ainsi, la nouvelle constante d'équilibre serait 1 sur x. Si les coefficients sont multipliés par un autre facteur, n, la constante d'équilibre est élevé au même facteur. La nouvelle constante d'équilibre serait maintenant x élevé à la puissance n.Lorsque deux ou plusieurs réactions individuelles sont additionnées, la constante d'équilibre de la réaction globale est le produit des constantes d'équilibre individuelles. Ainsi, la nouvelle constante d'équilibre serait y fois z. Pour un équilibre hétérogène, l'expression constante d'équilibre exclut les entités solides pures ou liquides pures.Par exemple, l'expression constante d'équilibre pour la décomposition du nitrate d'ammonium solide en protoxyde d'azote gazeux et vapeur d'eau, à l'exclusion du nitrate d'ammonium un solide pur. C'est parce que la concentration molaire relative de solides ou liquides pures restent constantes pendant la réaction.

la constante d'équilibre

La loi d'action de masse stipule que le rapport entre la concentration des produits et la concentration des réactifs à l'équilibre, élevées à leurs coefficients stœchiométriques respectifs, est égal à une constante, appelée la constante d'équilibre, K ou Kc. 
 Ainsi, l'expression de la constante d'équilibre pour la réaction ci-dessus est écrite comme suit : 
 <img alt="Eq2" src="/files/ftp_upload/11386/11386_Equation_2.png" style="text-align: center; height: 100px;" />
 où, l'indice ‘ c ’ indique que la constante d'équilibre tient compte de la concentration molaire des réactifs et des produits. 
 La taille de la constante d'équilibre reflète explicitement la composition d'un mélange réactionnel à l'équilibre. Une réaction présentant un K élevé atteint l'équilibre lorsque la majeure partie du réactif a été convertie en produit, alors qu'un petit K indique que la réaction atteint l'équilibre après la conversion de très peu de réactif. Il est important de garder à l’esprit que la taille de K n'indique pas à quel point l’équilibre sera atteint rapidement ou lentement. Certains équilibres sont établis si rapidement qu'ils sont presque instantanés, et d'autres si lentement qu'aucun changement perceptible n'est observé pendant des jours, des années, voire plus. La constante d'équilibre d'une réaction peut être utilisée pour prédire le comportement de mélanges contenant ses réactifs et/ou ses produits. Comme l'a démontré le processus d'oxydation du dioxyde de soufre décrit ci-dessus, une réaction chimique se fera dans la direction nécessaire pour atteindre l'équilibre. 
 Équilibres couplés 
 De nombreux systèmes d'équilibre impliquent deux ou plusieurs réactions d'équilibre couplées , celles qui ont en commun une ou plusieurs espèces de réactifs ou de produits. La valeur de K d'un système impliquant des équilibres couplés peut être liée aux valeurs de K des réactions individuelles. Trois manipulations de base sont impliquées dans cette approche, comme décrites ci-dessous : 
 
 • Changer le sens d’une équation chimique permute essentiellement les identités des “ réactifs ” et des “ produits ”, de sorte que la constante d’équilibre de l’équation inverse est simplement l'inverse de celle de l’équation directe ; 
 
 • La modification des coefficients stœchiométriques dans une équation par un facteur  x entraîne une variation exponentielle de la constante d'équilibre par ce même facteur ; 
 
 • Additioner ensemble deux équations d’équilibre ou plus donne une équation globale dont la constante d’équilibre est le produit mathématique des valeurs de K des réactions individuelles. 
 Expression de la constante d'équilibre pour les équilibres hétérogènes 
 Pour les équilibres hétérogènes impliquant des réactifs et des produits dans deux ou plusieurs phases différentes, les concentrations de solides purs ou de liquides purs ne sont pas incluses dans l'expression de la constante d'équilibre, comme illustré dans l'exemple suivant : 
 <img alt="Eq3" src="/files/ftp_upload/11386/11386_Equation_3.png" style="text-align: center; height: 150px;" />
 En effet, les concentrations relatives des liquides purs et des solides purs restent constantes pendant la réaction. 
 Ce texte a été adapté de<a href="https://openstax.org/books/chemistry-2e/pages/13-2-equilibrium-constants"> Openstax, Chimie 2e, Section 13.2 Constantes d'équilibre</a>.

La constante d'équilibre

Considérez l’oxydation du dioxyde de soufre :

Pour une réaction qui commence avec un mélange de réactifs uniquement, la concentration du produit est initialement égale à ...

Education

Enseignement

JoVE Core

Chimie

Chemical Equilibrium

Équilibre chimique

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

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

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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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