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

La legge dell'azione di massa introduce la costante di equilibrio per mettere in relazione le concentrazioni di equilibrio dei reagenti e dei prodotti. Per un'equazione chimica bilanciata in cui A e B sono reagenti, C e D sono prodotti e a, b, c e d minuscole sono i loro rispettivi coefficienti stechiometrici, l'espressione della costante di equilibrio è data dalle concentrazioni molari dei prodotti, diviso le concentrazioni molari dei reagenti, tutto elevato ai loro coefficienti stechiometrici. La costante di equilibrio è spesso espressa con il simbolo K o Kc, dove il pedice c indica la costante di equilibrio rispetto alla concentrazione molare.

Essendo un rapporto di concentrazioni molari, la costante di equilibrio è senza unità. Una grande costante di equilibrio alquanto maggiore di 1 significa che il numeratore, cioè la concentrazione dei prodotti, è maggiore del denominatore, la concentrazione dei reagenti. Ciò indica che l'equilibrio è verso i prodotti e favorisce la reazione in avanti.

Al contrario, una piccola costante di equilibrio alquanto inferiore a 1 significa che il denominatore è maggiore del numeratore. Questo significa che l'equilibrio è verso i reagenti e favorisce la reazione inversa. Per una costante di equilibrio di grandezza uguale o prossima a 1, le concentrazioni relative dei reagenti e dei prodotti sono quasi le stesse.

Ciò indica che né la reazione, né in avanti né all'indietro, è favorita. Le modifiche all'equazione chimica cambiano anche la costante di equilibrio. Se una reazione chimica con una costante di equilibrio x è invertita, l'espressione della costante di equilibrio per la reazione inversa è il reciproco della reazione diretta.

Quindi, la nuova costante di equilibrio sarebbe 1 su x. Se i coefficienti vengono moltiplicati per un altro fattore, n, la costante di equilibrio viene aumentata allo stesso fattore. La nuova costante di equilibrio sarebbe ora x elevata alla potenza di n.

Quando due o più reazioni individuali vengono sommate, la costante di equilibrio per la reazione complessiva è il prodotto delle singole costanti di equilibrio. Quindi, la nuova costante di equilibrio sarebbe y moltiplicata z. Per un equilibrio eterogeneo, l'espressione della costante di equilibrio esclude entità puramente solide o liquide.

Per esempio, l'espressione costante di equilibrio per la decomposizione del nitrato di ammonio solido in protossido di azoto gassoso e vapore acqueo, esclude il nitrato di ammonio un solido puro. Questo perché la concentrazione molare relativa di un solido o liquido puro rimane costante durante la reazione.

la costante di equilibrio

Consideriamo l'ossidazione dell'anidride solforosa:

<img alt="Eq1" src="/files/ftp_upload/11386/11386_Equation_1.png" style="text-align: center; height: 50px;" />

Per una reazione che inizia con una sola miscela di reagenti, la concentrazione del prodotto è inizialmente pari a zero. Man mano che la reazione procede verso l'equilibrio nella direzione in avanti, le concentrazioni dei reagenti diminuiscono e la concentrazione del prodotto aumenta. Quando viene raggiunto l’equilibrio, le concentrazioni di reagenti e prodotti rimangono costanti.

Se la reazione inizia solo con i prodotti presenti, la reazione procede verso l'equilibrio nella direzione inversa. La concentrazione del prodotto diminuisce con il tempo e le concentrazioni dei reagenti aumentano fino a quando le concentrazioni diventano costanti all'equilibrio.

La legge di azione di massa afferma che il rapporto tra la concentrazione dei prodotti e la concentrazione dei reagenti all'equilibrio, elevato ai rispettivi coefficienti stechiometrici, è uguale ad una costante, detta costante di equilibrio, K o K_c.

Pertanto, l'espressione della costante di equilibrio per la reazione di cui sopra è scritta come:

<img alt="Eq2" src="/files/ftp_upload/11386/11386_Equation_2.png" style="text-align: center; height: 100px;" />

dove il pedice “c” indica che la costante di equilibrio considera la concentrazione molare di reagenti e prodotti.

L'entità della costante di equilibrio riflette esplicitamente la composizione di una miscela di reazione all'equilibrio. Una reazione che mostra un K elevato raggiungerà l'equilibrio quando la maggior parte del reagente è stata convertita nel prodotto, mentre un K piccolo indica che la reazione raggiunge l'equilibrio dopo che è stata convertita una quantità minima di reagente. È importante tenere presente che la grandezza di K non indica quanto rapidamente o lentamente verrà raggiunto l’equilibrio. Alcuni equilibri vengono stabiliti così rapidamente da essere quasi istantanei, mentre altri così lentamente che non si osserva alcun cambiamento percettibile nel corso di giorni, anni o anche di più. La costante di equilibrio di una reazione può essere utilizzata per prevedere il comportamento delle miscele contenenti i suoi reagenti e/o prodotti. Come dimostrato dal processo di ossidazione dell'anidride solforosa sopra descritto, una reazione chimica procederà in qualunque direzione sia necessaria per raggiungere l'equilibrio.

Equilibri accoppiati

Molti sistemi di equilibrio coinvolgono due o più reazioni di equilibrio accoppiate, quelle che hanno in comune uno o più specie di reagenti o di prodotti. Il valore K per un sistema che coinvolge equilibri accoppiati può essere correlato ai valori K delle singole reazioni. In questo approccio sono coinvolte tre manipolazioni di base, come descritto di seguito:

<ol>
	<li>Cambiando la direzione di un'equazione chimica si scambiano essenzialmente le identità di “reagenti” e “prodotti”, quindi la costante di equilibrio per l'equazione invertita è semplicemente il reciproco di quella per l'equazione diretta.</li>
	<li>Cambiando i coefficienti stechiometrici in un'equazione di un certo fattore x si ottiene una variazione esponenziale della costante di equilibrio di quello stesso fattore.</li>
	<li>Sommando due o più equazioni di equilibrio si ottiene un'equazione complessiva la cui costante di equilibrio è il prodotto matematico dei valori K della singola reazione.</li>
</ol>

Equilibrio Espressione costante per equilibri eterogenei

Per equilibri eterogenei, che coinvolgono reagenti e prodotti in due o più fasi diverse, le concentrazioni di solidi puri o liquidi puri non sono incluse nell'espressione della costante di equilibrio, come illustrato dal seguente esempio:

<img alt="Eq3" src="/files/ftp_upload/11386/11386_Equation_3.png" style="text-align: center; height: 150px;" />

Questo perché le concentrazioni relative dei liquidi puri e dei solidi puri rimangono costanti durante la reazione.

Questo testo è adattato da <a href="https://openstax.org/books/chemistry-2e/pages/13-2-equilibrium-constants">Openstax, Chemistry 2e, Section 13.2 Equilibrium Constants</a>

La costante di equilibrio

Consideriamo l'ossidazione dell'anidride solforosa:

Per una reazione che inizia con una miscela di soli reagenti, la concentrazione del prodotto è inizialmente pari a ...

Education

Didattica

JoVE Core

Chimica

Chemical Equilibrium

Equilibrio chimico

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

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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JoVE Core: Chimica illustra concetti chiave di chimica attraverso video animazioni concise e di immediata comprensione. Inoltre, i video della sezione "scienziati in azione" mostrano procedure metodologiche realizzate da ricercatori provenienti dai migliori laboratori internazionali.

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

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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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