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

K12 Standards

north-carolina-k-12-science-standard-course-of-study

NC K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.5.2

Dynamic Equilibrium

Обратимая химическая реакция это реакция, при которой превращение исходных веществ в продукты и превращение продуктов обратно в исходные вещества происходят одновременно. Двойная стрелка между исходными веществами и продуктами означает обратимый характер реакции. Когда скорость образования продуктов или скорость прямой реакции равна скорости образования исходных веществ или скорости обратной реакции, достигается химическое равновесие.На самом деле химическое равновесие это динамический процесс. Извне вся система выглядит статичной, однако на молекулярном уровне она очень активна. Рассмотрим термическое разложение пентахлорида фосфора.При нагревании в закрытой емкости газообразный пентахлорид фосфора разлагается на смесь трихлорида фосфора и газообразного хлора в обратимой реакции. По мере протекания реакции концентрация пентахлорида фосфора уменьшается, в то время как трихлорид фосфора и газообразный хлор увеличиваются. В результате скорость прямой реакции уменьшается, а скорость обратной реакции увеличивается.В конце концов скорости прямой и обратной реакций становятся равными, и система достигает химического равновесия. В этом состоянии относительные концентрации пентахлорида фосфора, трихлорида фосфора и хлора больше не изменяются. Такое состояние равновесия также называется динамическим равновесием, потому что и прямая, и обратная реакции продолжают протекать, но с одинаковой скоростью.Поскольку исходные вещества и продукты образуются с той же скоростью, что и расходуются, их концентрации остаются постоянными, хотя и не обязательно равными. Химическое равновесие является гомогенным, если все исходные вещества и продукты реакции в данной обратимой реакции находятся в одной и той же фазе газообразной или жидкой. В гетерогенном равновесии исходные вещества и продукты реакции существуют в разных фазах.

Динамическое равновесие

Обратимая химическая реакция представляет собой химический процесс, который протекает как в прямом (слева направо), так и в обратном (справа налево) направлениях. Когда скорость реакции переднего и заднего хода равна, концентрации реагент и видов продукции остаются постоянными с течением времени и система находится в равновесии. Специальная двойная стрелка используется для того, чтобы подчеркнуть обратимое значение реакции. Относительные концентрации реагенты и продуктов в равновесных системах сильно различаются; некоторые системы содержат в основном продукты, которые находятся в равновесии, некоторые содержат в основном реагенты, а некоторые содержат значительные количества обоих. 
 Динамическое равновесие в химических реакциях 
 Рассмотрим обратимое разложение бесцветного тетрароксида динитрогена на выход бурного диоксида азота, описанного уравнением: 
 <img alt="Eq1" src="/files/ftp_upload/11385/11385_Equation_1.png" style="text-align: center; height: 50px;" />
 По мере начала реакции (время = 0) концентрация N2O4 реагент является конечной, а продукт NО2 равна нулю, поэтому реакция движения вперед продолжается с конечной скоростью, а скорость обратной реакции равна нулю. По мере прохождения времени N2O4 потребляется и его концентрация падает, при этом NО2 производится, и его концентрация увеличивается. Снижение концентрации реагент замедляет скорость реакции переднего хода, а увеличение концентрации продукта ускоряет скорость реакции заднего хода. Этот процесс продолжается до тех пор, пока скорость реакции переднего и заднего хода не станет равной, в это время реакция достигла равновесия. Важно подчеркнуть, что химическое равновесие является динамичным; реакция равновесия не “остановилась”, но идет в прямом и обратном направлениях с одинаковой скоростью. Таким образом, при равновесии концентрации N2O4 и NО2 больше не изменяются, так как скорость образования NО2 точно равна скорости потребления NО2, а скорость образования N2O4 точно равна скорости потребления N2O4. 
 Однородное и неоднородное равновесие 
 Однородное равновесие — это равновесие, в котором все реагенты и продукты (и любые катализаторы, если применимо) присутствуют на одной и той же фазе, водной или газообразной, как показано на следующих примерах: 
 <img alt="Eq2" src="/files/ftp_upload/11385/11385_Equation_2.png" style="text-align: center; height: 90px;" />
 Гетерогенное равновесие включает в себя реагенты и продукты на двух или более разных этапах, как показано на следующем примере: 
 <img alt="Eq3" src="/files/ftp_upload/11385/11385_Equation_3.png" style="text-align: center; height: 130px;" />
 Этот текст был адаптирован к Openstax<a href="https://openstax.org/books/chemistry-2e/pages/13-1-chemical-equilibria">, Химия 2е изд., раздел</a><a href="https://openstax.org/books/chemistry-2e/pages/13-1-chemical-equilibria"> 13.1 «Химическое равновесие</a>».

Обратимая химическая реакция представляет собой химический процесс, который протекает как в прямом (слева направо), так и в обратном (справа налево) направлениях. Когда...

Education

JoVE Core

Химия

Один

Chemical Equilibrium

Химическое равновесие

KEY TERMS AND CONCEPTS

ОСНОВНЫЕ ТЕРМИНЫ И ПОНЯТИЯ

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 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

ohios-learning-standards-and-model-curriculum

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

new-york-state-science-learning-standards

New York State Science Learning Standards

HS. Chemical Reactions

HS-PS1-6.

Scientific Laws and Theories

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PS.Chm.1.2

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The Energies of Atomic Orbitals

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PS.Chm.1.3

Periodic Classification of the Elements

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Molar Mass

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Use the Periodic Table as a model to predict the relative properties of elements based on the pattern of valence electrons in the outermost energy levels of atoms.

PS.Chm.2.1

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Ionic Radii

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Construct an explanation to infer the atomic size, reactivity, electronegativity, and ionization energy of an element based on its position in the Periodic Table.

PS.Chm.2.2

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PS.Chm.3.1

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PS.Chm.3.2

Analyze the structure of atoms and isotopes.

Chemical Equilibrium in Reversible Reactions

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