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

Odwracalna reakcja chemiczna to taka, w której konwersja reagentów w produkty i konwersja produktów z powrotem w reagenty zachodzą jednocześnie. Podwójna strzałka między reagentami a produktami oznacza jego odwracalny charakter.
Gdy szybkość tworzenia produktów lub szybkość reakcji postępowej jest równa szybkości powstawania reagentów lub szybkości reakcji odwrotnej, osiągana jest równowaga chemiczna.
Równowaga chemiczna jest w rzeczywistości procesem dynamicznym. Z zewnątrz cały system wydaje się statyczny, jednak na poziomie molekularnym jest bardzo aktywny.
Rozważ rozkład termiczny pentachlorku fosforu.
Po podgrzaniu w zamkniętym pojemniku gazowy pięciochlorek fosforu rozkłada się na mieszaninę trichlorku fosforu i chloru gazowego w reakcji odwracalnej. W miarę postępu reakcji stężenie pięciochlorku fosforu maleje, podczas gdy wzrasta trójchlorek fosforu i chlor gazowy.
W rezultacie szybkość reakcji do przodu maleje, a szybkość reakcji odwrotnej wzrasta.
W końcu szybkość reakcji do przodu i do tyłu staje się równa, a system osiąga równowagę chemiczną. W tym stanie względne stężenia pięciochlorku fosforu, trichlorku fosforu i chloru już się nie zmieniają.
Taki stan równowagi jest również nazywany równowagą dynamiczną, ponieważ zarówno reakcje do przodu, jak i do tyłu nadal zachodzą, ale w tym samym tempie. Ponieważ reagenty i produkty tworzą się w tym samym tempie, w jakim są zużywane, ich stężenia pozostają stałe, choć niekoniecznie równe.
Równowaga chemiczna jest jednorodna, jeśli wszystkie reagenty i produkty w danej odwracalnej reakcji znajdują się w tej samej fazie - gazowej lub wodnej. W równowadze heterogenicznej reagenty i produkty występują w różnych fazach.

równowaga dynamiczna

Odwracalna reakcja chemiczna reprezentuje proces chemiczny, który przebiega zarówno w kierunku do przodu (od lewej do prawej), jak i do tyłu (od prawej do lewej). Gdy szybkości reakcji do przodu i do tyłu są równe, stężenia reagentów i produktów pozostają stałe w czasie, a układ znajduje się w równowadze. Aby podkreślić odwracalny charakter reakcji, zastosowano specjalną podwójną strzałkę. Względne stężenia reagentów i produktów w układach równowagowych znacznie się różnią. Niektóre układy zawierają głównie produkty w równowadze, niektóre zawierają głównie reagenty, a inne zawierają znaczne ilości obu.

Równowaga dynamiczna w reakcjach chemicznych

Rozważmy odwracalny rozkład bezbarwnego czterotlenku diazotu z wytworzeniem brązowego dwutlenku azotu, opisany równaniem:

<img alt="Eq1" src="/files/ftp_upload/11385/11385_Equation_1.png" style="text-align: center; height: 50px;" />

Gdy reakcja się rozpoczyna (czas = 0), stężenie reagenta N2O4 jest skończone, a produktu NO2 wynosi zero, więc reakcja naprzód przebiega ze skończoną szybkością, podczas gdy szybkość reakcji odwrotnej wynosi zero. W miarę upływu czasu N2O4 jest zużywany i jego stężenie spada, natomiast wytwarzany jest NO2, którego stężenie wzrasta. Malejące stężenie reagenta spowalnia szybkość reakcji do przodu, a rosnące stężenie produktu przyspiesza szybkość reakcji odwrotnej. Proces ten trwa do momentu, gdy szybkości reakcji do przodu i do tyłu zrównają się, kiedy to reakcja osiągnie równowagę. Należy podkreślić, że równowagi chemiczne są dynamiczne; reakcja w stanie równowagi nie „zatrzymała się”, ale przebiega w przód i w tył z tą samą szybkością. Zatem w stanie równowagi stężenia N2O4 i NO2 już się nie zmieniają, ponieważ szybkość tworzenia NO2 jest dokładnie równa szybkości zużycia NO2, a szybkość tworzenia N2O4 jest dokładnie równa szybkości zużycia N2O4.

Równowagi jednorodne i heterogeniczne

Równowaga jednorodna to taka, w której wszystkie reagenty i produkty (oraz wszelkie katalizatory, jeśli mają zastosowanie) znajdują się w tej samej fazie, wodnej lub gazowej, jak ilustrują następujące przykłady:

<img alt="Eq2" src="/files/ftp_upload/11385/11385_Equation_2.png" style="text-align: center; height: 90px;" />

Równowaga heterogeniczna obejmuje reagenty i produkty w dwóch lub większej liczbie różnych faz, co ilustruje następujący przykład:

<img alt="Eq3" src="/files/ftp_upload/11385/11385_Equation_3.png" style="text-align: center; height: 130px;" />

Równowaga dynamiczna

Odwracalna reakcja chemiczna reprezentuje proces chemiczny, który przebiega zarówno w kierunku do przodu (od lewej do prawej), jak i wstecznej (od prawej do lewej). Kiedy...

Education

JoVE Core

Chemia

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Chemical Equilibrium

Równowaga chemiczna

KEY TERMS AND CONCEPTS

Kluczowe terminy i pojęcia

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...

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

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JoVE Core Chemistry brings general chemistry concepts to life using concise and easy-to-understand animated video lessons. Plus, in the scientist-in-action videos, we demonstrate classical and original research experiments performed in today's laboratories worldwide.

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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New York State Science Learning Standards

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Chemical Equilibrium in Reversible Reactions

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