Name: stoichiometry, rates, and equilibrium
Uploaded: 2023-04-30T18:28:32.000Z
Duration: 2 min 26 s
Channel: JoVE

K12 Standards

Texas Essential Knowledge and Skills

High School

Chemistry and Physics

IPC.8

IPC.8.A

Chemical Reactions

Envisagez une réaction chimique équilibrée, telle que la combustion de l’hydrogène gazeux. Ici, la relation quantitative entre les réactifs et les produits – H2, O2 et H2O – est que deux molécules de H2 réagissent avec une molécule de O2 pour produire deux molécules de H2O. Cette relation quantitative est connue sous le nom de stœchiométrie.
Dans une réaction chimique, les réactifs interagissent les uns avec les autres pour former des produits. Le réactif qui est complètement consommé est le réactif limitant, et le réactif présent en quantité supérieure à celle nécessaire pour réagir complètement avec le réactif limitant est le réactif en excès.
Une réaction chimique implique la transformation chimique de réactifs en produits. Au fur et à mesure que la réaction progresse, la concentration des réactifs diminue tandis que la concentration des produits augmente. Cette variance des concentrations de réactifs et de produits peut être représentée sur un graphique en fonction du temps.
La vitesse à laquelle une réaction progresse est appelée vitesse de réaction. Il mesure le taux de disparition des réactifs ou le taux d’apparition des produits et est exprimé en unités de molarité par seconde.
Une réaction chimique réversible est une réaction dans laquelle la conversion des réactifs en produits et la conversion des produits en réactifs se produisent simultanément. Une double flèche entre les réactifs et les produits signifie son caractère réversible.
Lorsque le taux de formation des produits, ou le taux de réaction directe, est égal au taux de formation des réactifs, ou au taux de la réaction inverse, l’équilibre chimique est atteint.
L’expression de la constante d’équilibre s’écrit comme les concentrations molaires des produits, C et D, sur les réactifs, A et B, à l’équilibre, chacun élevé à ses coefficients stœchiométriques respectifs. Lorsqu’elle est résolue, l’expression est égale à la constante d’équilibre, Kc.
Une expression sous la même forme peut également être écrite pour les réactifs et les produits à n’importe quelle concentration, et la quantité calculée est connue sous le nom de quotient de réaction, Qc.

réactions chimiques

Une équation chimique équilibrée fournit les informations sur les formules chimiques des réactifs et des produits impliqués dans le changement chimique. La stœchiométrie d'une réaction permet de prédire combien de réactif est nécessaire pour produire la quantité désirée de produit, ou dans certains cas, quelle quantité de produit sera formée à partir d'une quantité spécifique de réactif.

Les quantités relatives de réactifs et de produits représentées dans une équation chimique équilibrée sont souvent appelées quantités stœchiométriques. Cependant, en réalité, les réactifs ne sont pas toujours présents dans les quantités stœchiométriques indiquées par l'équation équilibrée.

Dans une réaction chimique, le réactif qui est consommé en premier, et qui limite la quantité de produit formé, est le réactif limitant, tandis que l'autre substance devient le réactif en excès. Un excès d'un ou plusieurs réactifs est souvent utilisé pour assurer la conversion complète de l'autre réactif en produit.

Considérez la réaction de formation de l'eau représentée par l'équation :

<img alt="Eq1" src="/files/ftp_upload/11691/11691_Equation1.svg" style="text-align: center; height: 20px;">

La stœchiométrie indique que deux moles d'hydrogène et une mole d'oxygène réagissent pour produire deux moles d'eau ; c'est-à-dire que l'hydrogène et l'oxygène se combinent dans un rapport de 2:1.

Imaginez si 5 moles d'hydrogène et 2 moles d'oxygène sont présentes. Le rapport des réactifs est maintenant de 5:2 (ou 2,5:1), ce qui est supérieur au rapport stœchiométrique de 2:1. L'hydrogène est donc présent en excès, et l'oxygène est le réactif limitant. La réaction de tout l'oxygène fourni (2 mol) consommera 4 mol des 5 mol d'hydrogène fournis, laissant 1 mol d'hydrogène non réagi. Le calcul des quantités molaires de chaque réactif fourni et leur comparaison avec les quantités stœchiométriques représentées dans l'équation chimique équilibrée est une façon d'identifier les réactifs limitants et en excès.

Le taux de réaction est le changement dans la quantité d'un réactif ou d'un produit par unité de temps. Les taux de réaction sont donc déterminés en mesurant la dépendance temporelle d'une propriété qui peut être reliée aux quantités de réactifs ou de produits.

<img alt="Figure1" src="/files/ftp_upload/11691/11691_Figure1.png" style="text-align: center; height: 300px;">

Le taux d'une réaction chimique peut être représenté dans un graphique comme la variance dans les concentrations des réactifs et des produits en fonction du temps.

Une réaction chimique réversible représente un processus chimique qui se déroule à la fois dans les directions avant (de gauche à droite) et arrière (de droite à gauche). L'état d'une réaction réversible est facilement évalué en évaluant son quotient de réaction (Q). Pour une réaction réversible décrite par

<img alt="Eq2" src="/files/ftp_upload/11691/11691_Equation2.svg" style="text-align: center; height: 20px;">

le quotient de réaction est dérivé directement de la stœchiométrie de l'équation équilibrée comme

<img alt="Eq3" src="/files/ftp_upload/11691/11691_Equation3.svg" style="text-align: center; height: 44px;">

où le subscript c indique l'utilisation de concentrations molaires dans l'expression.

<img alt="Figure2" src="/files/ftp_upload/11691/11691_Figure2.png" style="text-align: center; height: 300px;">

Lorsque les taux des réactions avant et arrière sont égaux, les concentrations des espèces de réactifs et de produits restent constantes dans le temps et le système est à l'équilibre. Une double flèche spéciale est utilisée pour souligner la nature réversible de la réaction.

Ce texte est adapté de <a href="https://openstax.org/books/chemistry-2e/pages/4-3-reaction-stoichiometry">OpenStax Chemistry 2e, Section 4.3: Reaction Stoichiometry</a>; <a href="https://openstax.org/books/chemistry-2e/pages/4-4-reaction-yields">Section 4.4: Reaction Yield</a>; <a href="https://openstax.org/books/chemistry-2e/pages/12-1-chemical-reaction-rates#12163">Section 12.1: Chemical Reaction Rates</a>; <a href="https://openstax.org/books/chemistry-2e/pages/13-1-chemical-equilibria">Section 13.1 Chemical Equilibria</a>, <a href="https://openstax.org/books/chemistry-2e/pages/13-2-equilibrium-constants">Section 13.2 Equilibrium Constants</a><a href="https://openstax.org/books/chemistry-2e/pages/13-2-equilibrium-constants">.</a>

Réactions chimiques

Une équation chimique équilibrée fournit des informations sur les formules chimiques des réactifs et des produits impliqués dans le changement chimique. Un...

Education

Enseignement

JoVE Core

Organic Chemistry

Chimie organique

Unit 1

Unité 1

Thermodynamics and Chemical Kinetics

Thermodynamique et cinétique chimique

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

stoichiometry, rates, and equilibrium

Stoichiometry, reaction rates, and equilibrium explain how chemical reactions are measured and predicted. A balanced chemical equation shows the chemical formulas of the reactants and products in a chemical change. It also gives the relative amounts of each substance involved.

Reaction stoichiometry helps predict how much reactant is needed to make a desired amount of product. It can also show how much product forms from a given amount of reactant. The amounts shown by a balanced equation are called stoichiometric amounts.

In real reactions, reactants are not always present in the exact amounts shown by the equation. The limiting reactant is the substance that gets used up first. It limits how much product can form. The other substance is the excess reactant. Extra reactant is often added so the other reactant can be fully converted into product.

Water formation is a clear example. The balanced equation shows that 2 moles of hydrogen react with 1 mole of oxygen to make 2 moles of water. This is a 2:1 ratio. If 5 moles of hydrogen and 2 moles of oxygen are available, the ratio is 5:2, or 2.5:1. Hydrogen is then in excess, and oxygen is the limiting reactant. All 2 moles of oxygen use 4 moles of hydrogen, leaving 1 mole of hydrogen unreacted.

Reaction rate is the change in the amount of a reactant or product per unit time. Scientists find reaction rates by measuring how a property changes over time if that property can be linked to reactant or product amounts. A rate graph can show how reactant and product concentrations vary with time.

Some reactions are reversible, which means they can proceed in both the forward and reverse directions. The reaction quotient, Q, helps assess the status of a reversible reaction. It is based on the stoichiometry of the balanced equation, and the subscript c shows that molar concentrations are used in the expression. When the forward and reverse rates are equal, concentrations stay constant and the system is at equilibrium. A double arrow is used to show that the reaction is reversible.

Stoichiometry, Rates, and Equilibrium

chemical reactions

Stoichiometry, reaction rates, and equilibrium explain how chemical reactions are measured and predicted. A balanced chemical equation shows the chemical formulas of the reactants and products in a chemical change. It also gives the relative amounts of each substance involved. Reaction stoichiometry helps predict how much reactant is needed to make a desired amount of product. It can also show how much product forms from a given amount of reactant. The amounts shown by a balanced equation are...

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Reaction rate and reaction pathway are key ideas in chemical kinetics. Thermodynamics, by contrast, focuses on the state of a system. It describes the properties, behavior, and components of reactants and products. It does not describe the path of the process or the speed of the reaction. Kinetics gives information on the atomic or molecular level, while thermodynamics focuses on the energetics of the reactants and products. Thermodynamics can show what may happen in a reaction, but it does...

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JoVE Core Chimie Organique explique les concepts de base de la stéréochimie, des réactions et des mécanismes à l'aide de leçons vidéo animées concises et faciles à comprendre. De plus, les vidéos de scientifiques à l'action présentent l'application de ces concepts grâce à des expériences de recherche classiques et originales réalisées dans des laboratoires modernes.

IPC.8.B

Louisiana Student Standards for Science

Chemistry

Matter And Its Interactions

HS-PS1-6

Tennessee Academic Standards

Matter and Its Interactions

CHEM1.PS1.3

CHEM1.PS1.4

Virginia  Standards of Learning

HIgh School

CH.3

CH.3.f

Florida Department of Education

Physical Science

SC.912.P.10

SC.912.P.10.6

SC.912.P.10.12

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Construct and communicate an evidence-based explanation of the environmental impact of the end-products of chemical reactions such as those that may result in degradation of water, soil, air quality, and global climate change.

IPC.8.D

Science concepts. The student knows that changes in matter affect everyday life. The student is expected to:

Stoichiometry, Rates, and Equilibrium

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