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

The Equilibrium Constant

De wet der massawerking introduceert de evenwichtsconstante om de evenwichtsconcentraties van de reactanten en producten te relateren. Voor een uitgebalanceerde chemische vergelijking waarbij A en B reactanten zijn, C en D producten zijn, en kleine letters a, b, c en d hun respectievelijke stoichiometrische coëfficiënten zijn, wordt de uitdrukking van de evenwichtsconstante gegeven door de molaire concentraties van de producten gedeeld door de molaire concentraties van de reactanten, allemaal verhoogd tot hun stoichiometrische coëfficiënten. De evenwichtsconstante wordt vaak uitgedrukt met het symbool K of Kc, waarbij het subscript c de evenwichtsconstante aangeeft met betrekking tot de molaire concentratie.Omdat het een verhouding is van molaire concentraties, is de evenwichtsconstante eenheidloos. Een grote evenwichtsconstante veel groter dan 1 betekent dat de teller, dat wil zeggen de concentratie van de producten, groter is dan de noemer, de concentratie van de reactanten. Dit geeft aan dat het evenwicht naar de producten toe ligt en bevordert de voorwaartse reactie.Omgekeerd betekent een kleine evenwichtsconstante van veel minder dan 1 dat de noemer groter is dan de teller. Dit betekent dat het evenwicht in de richting van de reactanten ligt en de omgekeerde reactie bevordert. Voor een evenwichtsconstante van grootte gelijk aan of dichtbij 1, zijn de relatieve concentraties van de reactanten en producten bijna hetzelfde.Dit geeft aan dat noch de reactie, vooruit noch achteruit, de voorkeur heeft. Wijzigingen in de chemische vergelijking veranderen ook de evenwichtsconstante. Als een chemische reactie met een evenwichtsconstante x wordt omgekeerd, is de uitdrukking van de evenwichtsconstante voor de omgekeerde reactie het omgekeerde van de voorwaartse reactie.Dus de nieuwe evenwichtsconstante zou 1 gedeeld door x zijn. Als de coëfficiënten worden vermenigvuldigd met een andere factor, n, wordt de evenwichtsconstante tot dezelfde factor verhoogd. De nieuwe evenwichtsconstante zou nu x worden verhoogd tot de macht n.Wanneer twee of meer individuele reacties bij elkaar worden opgeteld, is de evenwichtsconstante voor de algehele reactie het product van de individuele evenwichtsconstanten. Dus de nieuwe evenwichtsconstante zou y keer z zijn. Voor een heterogeen evenwicht sluit de uitdrukking van de evenwichtsconstante zuivere vaste of zuivere vloeibare entiteiten uit.De uitdrukking voor de evenwichtsconstante voor de ontleding van vast ammoniumnitraat in gasvormig lachgas en waterdamp sluit bijvoorbeeld ammoniumnitraat uit een zuivere vaste stof. Dit komt doordat de relatieve molaire concentratie van een zuivere vaste stof of vloeistof constant blijft tijdens de reactie.

de evenwichtsconstante

Beschouw de oxidatie van zwaveldioxide: <img alt="Eq1" src="/files/ftp_upload/11386/11386_Equation_1.png" style="text-align: center; height: 50px;"> Voor een reactie die begint met alleen een mengsel van reactanten, is de productconcentratie aanvankelijk gelijk aan nul. Naarmate de reactie in voorwaartse richting richting evenwicht gaat, nemen de concentraties van de reactanten af en neemt de productconcentratie toe. Wanneer het evenwicht is bereikt, blijven de concentraties van reactanten en producten constant. Als de reactie begint met alleen de aanwezige producten, verloopt de reactie in de omgekeerde richting naar evenwicht. De productconcentratie neemt met de tijd af en de concentraties van de reactanten nemen toe totdat de concentraties constant worden bij evenwicht. De wet van massaactie stelt dat de verhouding van de concentratie van producten tot de concentratie van reactanten bij evenwicht, verhoogd tot hun respectieve stoichiometrische coëfficiënten, gelijk is aan een constante, de evenwichtsconstante genaamd, K of Kc. De evenwichtsconstante voor de bovenstaande reactie wordt dus geschreven als: <img alt="Eq2" src="/files/ftp_upload/11386/11386_Equation_2.png" style="text-align: center; height: 100px;"> waarbij het subscript ‘c’ aangeeft dat de evenwichtsconstante rekening houdt met de molaire concentratie van reactanten en producten. De grootte van de evenwichtsconstante weerspiegelt expliciet de samenstelling van een reactiemengsel bij evenwicht. Een reactie die een grote K vertoont, zal een evenwicht bereiken wanneer het grootste deel van de reactant in product is omgezet, terwijl een kleine K aangeeft dat de reactie een evenwicht bereikt nadat heel weinig reactant is omgezet. Het is belangrijk om in gedachten te houden dat de grootte van K niet aangeeft hoe snel of langzaam het evenwicht zal worden bereikt. Sommige evenwichten worden zo snel tot stand gebracht dat ze vrijwel ogenblikkelijk tot stand komen, en andere zo langzaam dat er geen waarneembare verandering wordt waargenomen in de loop van dagen, jaren of langer. De evenwichtsconstante voor een reactie kan worden gebruikt om het gedrag te voorspellen van mengsels die de reactanten en/of producten ervan bevatten. Zoals aangetoond door het hierboven beschreven zwaveldioxide-oxidatieproces zal een chemische reactie verlopen in welke richting dan ook nodig is om evenwicht te bereiken. Gekoppelde evenwichten Bij veel evenwichtssystemen zijn twee of meer gekoppelde evenwichtsreacties betrokken, waarbij één of meer reactanten of productsoorten gemeenschappelijk zijn. De K-waarde voor een systeem met gekoppelde evenwichten kan worden gerelateerd aan de K-waarden van de individuele reacties. Bij deze aanpak zijn drie basismanipulaties betrokken, zoals hieronder beschreven: • Door de richting van een chemische vergelijking te veranderen, worden feitelijk de identiteiten van 'reactanten' en 'producten' verwisseld, en dus is de evenwichtsconstante voor de omgekeerde vergelijking eenvoudigweg het omgekeerde van die voor de voorwaartse vergelijking. • Het veranderen van de stoichiometrische coëfficiënten in een vergelijking met een factor x resulteert in een exponentiële verandering in de evenwichtsconstante met diezelfde factor. • Het optellen van twee of meer evenwichtsvergelijkingen levert een algemene vergelijking op waarvan de evenwichtsconstante het wiskundige product is van de K-waarden van de individuele reactie. Evenwicht Constante uitdrukking voor heterogene evenwichten Voor heterogene evenwichten, waarbij reactanten en producten in twee of meer verschillende fasen betrokken zijn, worden de concentraties van zuivere vaste stoffen of zuivere vloeistoffen niet opgenomen in de evenwichtsconstante-uitdrukking, zoals geïllustreerd door het volgende voorbeeld: <img alt="Eq3" src="/files/ftp_upload/11386/11386_Equation_3.png" style="text-align: center; height: 150px;"> Dit komt omdat de relatieve concentraties voor zuivere vloeistoffen en zuivere vaste stoffen tijdens de reactie constant blijven.

De evenwichtsconstante

Consider the oxidation of sulfur dioxide:

For a reaction that begins with a mixture of reactants only, the product concentration is initially equal to ...

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Chemical Equilibrium

Chemisch evenwicht

KEY TERMS AND CONCEPTS

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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...

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...

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...

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...

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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Interactions Of Matter

Chemical reactions

Arizona Science Standards

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HS+C.P1

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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