Name: relating kc and kp in gas equilibria
Uploaded: 2020-09-24T15:06:50.000Z
Duration: 2 min 15 s
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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

homogeneous-equilibria-for-gaseous-reactions

Homogeneous Equilibria for Gaseous Reactions

Pour les réactions chimiques, où les réactifs et les produits sont tous des gaz, la constante d'équilibre peut également être calculée en utilisant les pressions partielles individuelles plutôt que leurs concentrations molaires. Ainsi, lorsque les gaz A et B se transforment en gaz C et D dans une réaction réversible, l'expression d'équilibre peut être écrite à la place comme la pression partielle de chaque gaz, élevé à leurs coefficients stœchiométriques. La constante d'équilibre est désignée comme Kp, où l'indice p indique la pression.Pour une réaction gazeuse donnée, Kp n'est pas nécessairement égal à Kc, car la pression partielle d'un gaz et sa concentration molaire sont des valeurs distinctes. Cependant, une relation peut être établie entre les deux constantes utilisant l'équation des gaz parfaits et la définition de molarité. Pour dériver cette relation, considérons les expressions d'équilibre de Kc et Kp pour la réaction chimique donnée.L'équation des gaz parfaits rapporte la pression d'un gaz à son nombre de moles et à son volume à une température donnée. Remplacer le rapport des moles au volume pour la molarité dans l'équation des gaz parfaits permet la pression d'un gaz parfait à exprimer en termes de concentration molaire. De cette manière, les pressions partielles individuelles dans l'expression pour Kp peuvent être substituées pour l'équivalent de concentration de chaque gaz.Les coefficients stoechiométriques restent inchangés. Dans l'expression modifiée de Kp, le rapport de la concentration des produits à la concentration des réactifs peuvent être substitués à Kc.Cette équation donne la relation entre les deux constantes Kp est égal à Kc fois RT élevé à la somme des coefficients des produits moins la somme des coefficients des réactifs. La différence entre les coefficients de réactifs et produits gazeux peut être représentée par delta n.

Équilibre homogène pour les réactions gazeuses

Équilibre homogène pour les réactions gazeuses 
 Pour les réactions en phase gazeuse, la constante d'équilibre peut être exprimée en termes de concentrations molaires (Kc) ou de pressions partielles (Kp) des réactifs et des produits. Une relation entre ces deux valeurs de K peut être simplement déduite de l'équation des gaz parfaits et de la définition de la molarité. Selon l'équation des gaz parfaits : 
 <img alt="Eq1" src="/files/ftp_upload/11387/11387_Equation_1.png" style="text-align: center; height: 50px;" />
 La concentration molaire ou la molarité est donnée par le nombre de moles divisé par le volume : 
 <img alt="Eq2" src="/files/ftp_upload/11387/11387_Equation_2.png" style="text-align: center; height: 100px;" />
 Ainsi, 
 <img alt="Eq3" src="/files/ftp_upload/11387/11387_Equation_3.png" style="text-align: center; height: 90px;" />
 où P est la pression partielle, V le volume, n le nombre de moles, R la constante des gaz, T la température et M la concentration molaire. 
 Pour la réaction en phase gazeuse : m A + n B ⇌ x C + y D 
 <img alt="Eq4" src="/files/ftp_upload/11387/11387_Equation_4.png" style="text-align: center; height: 130px;" />
 Ainsi, la relation entre Kc et Kp est 
 <img alt="Eq5" src="/files/ftp_upload/11387/11387_Equation_5.png" style="text-align: center; height: 50px;" />
 où Δn est la différence entre les quantités molaires des produits et réactifs gazeux, dans ce cas : 
 <img alt="Eq6" src="/files/ftp_upload/11387/11387_Equation_6.png" style="text-align: center; height: 50px;" />
 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>.

Équilibres homogènes pour les réactions gazeuses
Pour les réactions en phase gazeuse, la constante d’équilibre peut être exprimée en fonction de la molaire ...

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Chemical Equilibrium

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KEY TERMS AND CONCEPTS

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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 temperature. Molar concentration is written as M.

For a gas reaction written as mA + nB ⇌ xC + yD, the two equilibrium constants are connected by a simple relation. That relation depends on Δn, which is the difference between the molar amounts of gaseous products and gaseous reactants. This connection helps show how Kc and Kp describe the same equilibrium in different ways.

Relating Kc and Kp in Gas Equilibria

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...

homogeneous equilibria for gaseous reactions

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

Learn how gas-phase equilibrium constants are connected by the ideal gas law. Compare Kc and Kp using molarity, partial pressure, and Δn.

Relating Kc and Kp in Gas Equilibria | 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

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High School

HS+C.P1

HS+C.P1U1.6

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Matter and Its Interactions

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New York State Science Learning Standards

HS. Chemical Reactions

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Relating Kc and Kp in Gas Equilibria

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