Name: reaction mechanisms and rate-determining steps
Uploaded: 2023-04-30T18:28:32.000Z
Duration: 2 min 31 s
Channel: JoVE

K12 Standards

Louisiana Student Standards for Science

High School

Chemistry

Matter And Its Interactions

HS-PS1-6

Multi-Step Reactions

Une réaction chimique est souvent représentée par une équation chimique globale équilibrée indiquant les réactifs et les produits.
Cependant, la réaction réelle est souvent plus complexe et se déroule en plusieurs étapes. Par exemple, cette réaction de l’oxyde nitrique avec l’hydrogène, formant de l’azote gazeux et de l’eau, se déroule en trois étapes distinctes et successives. Ces étapes sont appelées mécanisme de réaction.
Chaque étape du mécanisme réactionnel est appelée réaction élémentaire et représente l’interaction, telle que la rupture ou la formation d’une liaison, entre les espèces réactives.
Des molécules spécifiques, comme le dioxyde de diazote et le protoxyde d’azote, se forment au cours d’une étape élémentaire et sont consommées au cours d’une autre. Ces espèces sont appelées intermédiaires de réaction.
Les intermédiaires de réaction ne sont pas les mêmes que les états de transition, qui n’existent que lors de la transformation des réactifs en produits.
Les mécanismes de réaction sont supposés sur la base de leurs équations chimiques équilibrées et des lois de vitesse déterminées expérimentalement de chaque étape élémentaire.
Chaque étape a une vitesse de réaction, une constante de vitesse et une énergie d’activation spécifiques. L’étape la plus lente est appelée étape de détermination de la vitesse et influence la vitesse de réaction nette. Il peut être utilisé pour vérifier la loi de vitesse de la réaction globale et pour valider un mécanisme de réaction proposé.
Considérez la décomposition du protoxyde d’azote en azote et en oxygène. La loi de vitesse déterminée expérimentalement ne correspond pas à l’expression de la vitesse d’une réaction en une seule étape, qui est corroborée par la présence observée d’atomes d’oxygène – une réaction intermédiaire.
Par conséquent, un mécanisme de réaction est proposé où toutes les étapes s’additionnent pour donner la réaction globale.
Tout d’abord, les constantes de vitesse indiquent que la première étape est l’étape de limitation de débit. C’est le plus lent et influence donc la vitesse de réaction globale. Une loi de taux proposée à partir de cette étape peut être égale à la loi de taux globale.
Cette loi de vitesse proposée, directement dérivée de la concentration moléculaire du réactif élémentaire, correspond à la loi de vitesse expérimentale et vérifie le mécanisme de réaction prédit.
Ce texte est adapté de <a href="https://openstax.org/books/chemistry-2e/pages/12-6-reaction-mechanisms">Openstax, Chemistry 2e, 12.6 : Reaction Mechanisms</a>.

réactions en plusieurs étapes

Les réactions chimiques se produisent souvent de manière échelonnée impliquant deux réactions distinctes ou plus se déroulant dans une séquence. Une équation équilibrée indique les espèces réactives et les espèces produites, mais elle ne révèle aucun détail sur la manière dont la réaction se produit au niveau moléculaire. Le mécanisme de réaction (ou chemin de réaction) fournit des détails concernant le processus précis et étape par étape par lequel une réaction se produit. Chacune des étapes dans un mécanisme de réaction est appelée réaction élémentaire. Ces réactions élémentaires se produisent en séquence, comme représenté dans les équations des étapes, et elles se somment pour donner l'équation chimique équilibrée décrivant la réaction globale. Dans un mécanisme de réaction à plusieurs étapes, l'une des étapes élémentaires progresse plus lentement que les autres , parfois significativement plus lentement. Cette étape la plus lente est appelée l'étape limitante (ou étape déterminante de la vitesse). Une réaction ne peut pas se dérouler plus rapidement que son étape la plus lente, et donc, l'étape déterminante de la vitesse limite la vitesse globale de la réaction.

Contrairement aux équations équilibrées représentant une réaction globale, les équations des réactions élémentaires sont des représentations explicites du changement chimique. Une équation de réaction élémentaire représente le ou les réactifs réels subissant la rupture/formation de liaisons et les produits formés. Les lois de vitesse peuvent être déduites directement des équations chimiques équilibrées pour les réactions élémentaires. Cependant, ce n'est pas le cas pour la plupart des réactions chimiques, où les équations équilibrées représentent souvent le changement global dans le système chimique résultant de mécanismes de réaction à plusieurs étapes. Par conséquent, la loi de vitesse doit être déterminée à partir de données expérimentales, et le mécanisme de réaction doit être déduit par la suite à partir de la loi de vitesse.

Par exemple, considérez la réaction de NO2 et CO :

<img alt="Figure1" src="/files/ftp_upload/11699/11699_Equation1.svg" style="text-align: center; height: 22px;">

La loi de vitesse expérimentale pour cette réaction à des températures supérieures à 225 °C est :

<img alt="Figure2" src="/files/ftp_upload/11699/11699_Equation2.svg" style="text-align: center; height: 19px;">

Selon la loi de vitesse, la réaction est d'ordre un par rapport à NO2 et d'ordre un par rapport à CO. Cependant, à des températures inférieures à 225 °C, la réaction est décrite par une loi de vitesse différente qui est du deuxième ordre par rapport à NO2 :

<img alt="Figure3" src="/files/ftp_upload/11699/11699_Equation3.svg" style="text-align: center; height: 21px;">

Cette loi de vitesse n'est pas conforme au mécanisme en une seule étape, mais elle est conforme au mécanisme en deux étapes suivant :

<img alt="Figure4" src="/files/ftp_upload/11699/11699_Equation4.svg" style="text-align: center; height: 21px;">

<img alt="Figure5" src="/files/ftp_upload/11699/11699_Equation5.svg" style="text-align: center; height: 21px;">

L'étape déterminante de la vitesse (la plus lente) donne une loi de vitesse montrant une dépendance du deuxième ordre sur la concentration de NO2, et la somme des deux équations élémentaires donne la réaction globale nette.

En général, lorsque l'étape déterminante de la vitesse (la plus lente) est la première étape dans le mécanisme de réaction, la loi de vitesse pour la réaction globale est la même que la loi de vitesse pour cette étape. Cependant, lorsque l'étape déterminante de la vitesse est précédée par une étape élémentaire impliquant une réaction rapidement réversible, la loi de vitesse pour la réaction globale peut être plus difficile à déduire, souvent en raison de la présence d'intermédiaires réactionnels.

Dans de tels cas, le concept selon lequel une réaction réversible est à l'équilibre lorsque les vitesses des processus direct et inverse sont égales peut être utilisé.

Réactions en plusieurs étapes

Les réactions chimiques se produisent souvent de manière progressive impliquant deux ou plusieurs réactions distinctes se déroulant dans une séquence. Une équation équilibrée indique ...

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

reaction mechanisms and rate-determining steps

Chemical reactions can happen in a sequence of steps. A balanced chemical equation shows the reactants and products, but it does not show how the reaction occurs at the molecular level. The reaction mechanism, or reaction path, gives that step-by-step detail.

Each step in a mechanism is an elementary reaction. These elementary reactions happen in order, and together they add up to the balanced equation for the overall reaction. In a multistep mechanism, one step is often much slower than the others. This slowest step is the rate-limiting step, also called the rate-determining step.

The rate-determining step controls how fast the whole reaction can go. A reaction cannot move faster than its slowest step. Unlike an overall balanced equation, an elementary reaction shows the actual reactants that break and form bonds.

For elementary reactions, the rate law can be written directly from the chemical equation. For most reactions, that is not possible because the balanced equation represents the overall change, not each step. In those cases, the rate law must be found from experiments, and the mechanism is then deduced from that rate law.

The reaction between NO2 and CO shows how this works. At temperatures above 225 °C, the experimental rate law is first-order in NO2 and first-order in CO. At temperatures below 225 °C, the rate law is different and is second-order in NO2. That lower-temperature rate law does not fit a single-step mechanism, but it does fit a two-step mechanism.

In that two-step mechanism, the slower step produces the rate law with second-order dependence on NO2. The two elementary equations then combine to give the net overall reaction. In general, when the rate-determining step is the first step, the overall rate law matches that step.

The rate law can be harder to work out when a fast reversible step comes before the slow step. This can happen because intermediates form during the reaction. In those cases, the idea of equilibrium is useful, where the forward and reverse rates are equal.

Reaction Mechanisms and Rate-Determining Steps

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gibbs free energy and thermodynamic favorability

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rate law and reaction order

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effect of temperature change on reaction rate

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multi-step reactions

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key terms and concepts

Learn how reaction mechanisms work, why the slowest step limits reaction rate, and how rate laws help identify multistep reactions in chemistry.

Reaction Mechanisms and Rate-Determining Steps | JoVE K12

organic chemistry

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.

Next Generation Science Standards

Physical Science

Matter and Its Interactions

HS-PS1-2

HS-PS1-7

south-carolina-college-and-career-ready-science-standards

South Carolina College and Career Ready Science Standards

C-PS1-4

C-PS1-5

C-PS1-6

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HS-PS1-1

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HS-PS1-3

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Develop a model to illustrate that the release or absorption of energy from a chemical reaction system depends upon the changes in total bond energy.  Clarification Statement: Emphasis is on the idea that a chemical reaction is a system that affects the energy change.  Examples of models could include molecular-level drawings and diagrams of reactions, graphs showing the relative energies of reactants and products, and representations showing energy is conserved.

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Reaction Mechanisms and Rate-Determining Steps

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