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

Een omkeerbare chemische reactie is een reactie waarbij de omzetting van reactanten in producten en de omzetting van producten terug in reactanten gelijktijdig plaatsvinden. Een dubbele pijl tussen de reactanten en producten duidt op de omkeerbare aard ervan. Wanneer de snelheid van vorming van producten, of de snelheid van de voorwaartse reactie, gelijk is aan de snelheid van vorming van reactanten, of de snelheid van de omgekeerde reactie, wordt een chemisch evenwicht bereikt.Chemisch evenwicht is in feite een dynamisch proces. Van buitenaf lijkt het hele systeem statisch, maar op moleculair niveau is het zeer actief. Denk aan de thermische ontleding van fosforpentachloride.Bij verhitting in een gesloten container ontleedt gasvormig fosforpentachloride in een omkeerbare reactie in een mengsel van fosfortrichloride en chloorgas. Naarmate de reactie vordert, neemt de concentratie van fosforpentachloride af, terwijl fosfortrichloride en chloorgas toenemen. Als resultaat neemt de snelheid van de voorwaartse reactie af en die van de omgekeerde reactie toe.Uiteindelijk worden de snelheden van de voorwaartse en achterwaartse reacties gelijk en bereikt het systeem een chemisch evenwicht. In deze toestand veranderen de relatieve concentraties van fosforpentachloride, fosfortrichloride en chloor niet meer. Zo'n evenwichtstoestand wordt ook wel dynamisch evenwicht genoemd, omdat zowel de voorwaartse als de omgekeerde reacties blijven plaatsvinden, maar met dezelfde snelheid.Omdat de reactanten en producten met dezelfde snelheden worden gevormd als ze worden verbruikt, blijven hun concentraties constant, hoewel niet noodzakelijk gelijk. Een chemisch evenwicht is homogeen als alle reactanten en producten in een gegeven omkeerbare reactie zich in dezelfde fase bevinden:gasvormig of waterig. In een heterogeen evenwicht bestaan reactanten en producten in verschillende fasen.

dynamisch evenwicht

Een omkeerbare chemische reactie vertegenwoordigt een chemisch proces dat zowel in voorwaartse (van links naar rechts) als in omgekeerde richting (van rechts naar links) verloopt. Wanneer de snelheden van de voorwaartse en achterwaartse reacties gelijk zijn, blijven de concentraties van de reactanten en productsoorten in de loop van de tijd constant en bevindt het systeem zich in evenwicht. Er wordt een speciale dubbele pijl gebruikt om de omkeerbare aard van de reactie te benadrukken. De relatieve concentraties van reactanten en producten in evenwichtssystemen variëren sterk; sommige systemen bevatten voornamelijk producten in evenwicht, sommige bevatten voornamelijk reactanten, en sommige bevatten aanzienlijke hoeveelheden van beide. Dynamisch evenwicht bij chemische reacties Beschouw de omkeerbare ontleding van kleurloos distikstoftetroxide, waarbij bruin stikstofdioxide ontstaat, beschreven door de vergelijking: <img alt="Eq1" src="/files/ftp_upload/11385/11385_Equation_1.png" style="text-align: center; height: 50px;"> Wanneer de reactie begint (tijd = 0), is de concentratie van de N2O4-reactant eindig en die van het NO2-product nul, dus de voorwaartse reactie verloopt met een eindige snelheid terwijl de omgekeerde reactiesnelheid nul is. Naarmate de tijd verstrijkt, wordt N2O4 verbruikt en neemt de concentratie ervan af, terwijl NO2 wordt geproduceerd en de concentratie ervan toeneemt. De afnemende concentratie van de reactant vertraagt de voorwaartse reactiesnelheid, en de toenemende productconcentratie versnelt de omgekeerde reactiesnelheid. Dit proces gaat door totdat de voorwaartse en achterwaartse reactiesnelheden gelijk worden, op welk moment de reactie een evenwicht heeft bereikt. Het is belangrijk om te benadrukken dat chemische evenwichten dynamisch zijn; een reactie bij evenwicht is niet ‘gestopt’, maar verloopt met dezelfde snelheid in voorwaartse en achterwaartse richting. Bij evenwicht veranderen de concentraties van N2O4 en NO2 dus niet meer, omdat de vormingssnelheid van NO2 exact gelijk is aan de consumptiesnelheid van NO2, en de snelheid van de vorming van N2O4 precies gelijk is aan de consumptiesnelheid van N2O4. Homogene en heterogene evenwichten Een homogeen evenwicht is een evenwicht waarin alle reactanten en producten (en eventuele katalysatoren, indien van toepassing) aanwezig zijn in dezelfde fase, hetzij de waterfase, hetzij de gasfase, zoals geïllustreerd door de volgende voorbeelden: <img alt="Eq2" src="/files/ftp_upload/11385/11385_Equation_2.png" style="text-align: center; height: 90px;"> Bij een heterogeen evenwicht zijn reactanten en producten betrokken in twee of meer verschillende fasen, zoals geïllustreerd door het volgende voorbeeld: <img alt="Eq3" src="/files/ftp_upload/11385/11385_Equation_3.png" style="text-align: center; height: 130px;"> Deze tekst is aangepast van <a href="https://openstax.org/books/chemistry-2e/pages/13-1-chemical-equilibria">Openstax, Chemistry 2e, Sectie </a><a href="https://openstax.org/books/chemistry-2e/pages/13-1-chemical-equilibria">13.1 Chemische Evenwichten</a>.

Dynamisch evenwicht

A reversible chemical reaction represents a chemical process that proceeds in both forward (left to right) and reverse (right to left) directions. When ...

Education

JoVE Core

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Chemical Equilibrium

Chemisch evenwicht

KEY TERMS AND CONCEPTS

BELANGRIJKE TERMEN EN CONCEPTEN

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...

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 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

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

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Pennsylvania State Academic Standards

Physical Science

Chemical Reactions

3.2.9-12.F

Louisiana Student Standards for Science

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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