Name: chemical equilibrium in reversible reactions
Uploaded: 2020-09-24T15:06:50.000Z
Duration: 2 min 20 s
Channel: JoVE

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

Dynamic Equilibrium

Una reazione chimica reversibile è quella in cui la conversione dei reagenti in prodotti e la conversione dei prodotti in reagenti avvengono simultaneamente. Una doppia freccia fra i reagenti e i prodotti indica la sua natura reversibile. Quando la velocità di formazione dei prodotti, o la velocità della reazione diretta, è uguale alla velocità di formazione dei reagenti, o alla velocità della reazione inversa, si ottiene l'equilibrio chimico.

L'equilibrio chimico è infatti un processo dinamico. Dall'esterno, l'intero sistema appare statico tuttavia, a livello molecolare, è altamente attivo. Considerate la decomposizione termica del pentacloruro di fosforo.

Quando viene riscaldato in un contenitore chiuso, il pentacloruro di fosforo gassoso si decompone in una miscela di tricloruro di fosforo e cloro gassoso, in una reazione reversibile. Man mano che la reazione procede, la concentrazione di pentacloruro di fosforo diminuisce mentre aumentano il tricloruro di fosforo e il gas di cloro. Ne consegue che la velocità della reazione diretta diminuisce e quella della reazione inversa aumenta.

Alla fine, le velocità delle reazioni dirette e inverse diventano uguali e il sistema raggiunge l'equilibrio chimico. In questo stato, le concentrazioni relative di pentacloruro di fosforo, tricloruro di fosforo e cloro, non cambiano più. Tale stato di equilibrio è anche detto equilibrio dinamico perché sia le reazioni dirette che quelle inverse continuano a verificarsi, ma alla stessa velocità.

Poiché i reagenti e i prodotti si formano alla stessa velocità con cui vengono consumati, le loro concentrazioni rimangono costanti, sebbene non necessariamente uguali. Un equilibrio chimico è omogeneo se tutti i reagenti e i prodotti in una data reazione reversibile sono nella stessa fase sia gassosa che acquosa. In un equilibrio eterogeneo, reagenti e prodotti esistono in fasi diverse.

equilibrio dinamico

Una reazione chimica reversibile rappresenta un processo chimico che procede sia in direzione avanti (da sinistra a destra) che inversa (da destra a sinistra). Quando le velocità delle reazioni dirette e inverse sono uguali, le concentrazioni delle specie reagente e prodotto rimangono costanti nel tempo e il sistema è in equilibrio. Una speciale doppia freccia viene utilizzata per enfatizzare la natura reversibile della reazione. Le concentrazioni relative di reagenti e prodotti nei sistemi di equilibrio variano notevolmente; alcuni sistemi contengono principalmente prodotti in equilibrio, altri contengono principalmente reagenti e altri ancora contengono quantità apprezzabili di entrambi.

Equilibrio dinamico nelle reazioni chimiche

Consideriamo la decomposizione reversibile del tetrossido di diazoto incolore per produrre biossido di azoto marrone, descritto dall'equazione:

<img alt="Eq1" src="/files/ftp_upload/11385/11385_Equation_1.png" style="text-align: center; height: 50px;" />

Quando la reazione inizia (tempo = 0), la concentrazione del reagente N_2O_4 è finita e quella del prodotto NO_2 è zero, quindi la reazione diretta procede a una velocità finita mentre la velocità della reazione inversa è zero. Col passare del tempo, N_2O4 viene consumato e la sua concentrazione diminuisce, mentre viene prodotto NO_2 e la sua concentrazione aumenta. La diminuzione della concentrazione del reagente rallenta la velocità della reazione diretta, mentre l'aumento della concentrazione del prodotto accelera la velocità della reazione inversa. Questo processo continua finché le velocità di reazione diretta e inversa non diventano uguali, momento in cui la reazione ha raggiunto l'equilibrio. È importante sottolineare che gli equilibri chimici sono dinamici; una reazione in equilibrio non si è “fermata”, ma sta procedendo nelle direzioni avanti e indietro alla stessa velocità. Pertanto, all'equilibrio, le concentrazioni di N_2O_4 e NO_2 non cambiano più perché la velocità di formazione di NO_2 è esattamente uguale alla velocità di consumo di NO_2, e la velocità di formazione di N_2O_4 è esattamente uguale alla velocità di consumo di N_2O_4.

Equilibri omogenei ed eterogenei

Un equilibrio omogeneo è quello in cui tutti i reagenti e i prodotti (ed eventuali catalizzatori, se applicabile) sono presenti nella stessa fase, acquosa o gassosa, come illustrato dai seguenti esempi:

<img alt="Eq2" src="/files/ftp_upload/11385/11385_Equation_2.png" style="text-align: center; height: 90px;" />

Un equilibrio eterogeneo coinvolge reagenti e prodotti in due o più fasi diverse, come illustrato dal seguente esempio:

<img alt="Eq3" src="/files/ftp_upload/11385/11385_Equation_3.png" style="text-align: center; height: 130px;" />

Questo testo è adattato da <a href="https://openstax.org/books/chemistry-2e/pages/13-1-chemical-equilibria">Openstax, Chemistry 2e, Section </a><a href="https://openstax.org/books/chemistry-2e/pages/13-1-chemical-equilibria">13.1 Chemical Equilibria</a>

Equilibrio dinamico

Una reazione chimica reversibile rappresenta un processo chimico che procede sia in avanti (da sinistra a destra) che in senso inverso (da destra a sinistra). Quando...

Education

Didattica

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Chimica

Chemical Equilibrium

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KEY TERMS AND CONCEPTS

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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 N2O4 and no NO2, so the forward reaction begins while the reverse reaction is still zero. As N2O4 is used up, its concentration falls. As NO2 forms, its concentration rises.

These concentration changes affect the reaction rates. Less N2O4 slows the forward reaction, and more NO2 speeds the reverse reaction. The process continues until both rates match. At that point, the system has reached equilibrium.

Chemical equilibrium is dynamic, not stopped. The reaction still proceeds in both directions, but at the same rate. So the amount of NO2 does not change because it is formed and consumed at equal rates, and the amount of N2O4 stays constant for the same reason.

The video also distinguishes homogeneous and heterogeneous equilibria. A homogeneous equilibrium has all reactants and products in the same phase, such as all aqueous or all gaseous species. A heterogeneous equilibrium has reactants and products in two or more different phases.

Chemical Equilibrium in Reversible Reactions

chemical-equilibrium-in-reversible-reactions

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

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 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 reversible reactions reach chemical equilibrium, why it is dynamic, and how homogeneous and heterogeneous equilibria differ in high school chemistry.

Chemical Equilibrium in Reversible Reactions | JoVE K12

chemistry

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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High School

Matter And Its Interactions

HS-PS1-2

HS-PS1-6

HS-PS1-7

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Ohio’s Learning Standards and Model Curriculum

Interactions Of Matter

Chemical reactions

Arizona Science Standards

HS+C.P1

HS+C.P1U1.6

Next Generation Science Standards

Matter and Its Interactions

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-PS1-2

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 in Reversible Reactions

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