Name: arrhenius equation in reaction speed
Uploaded: 2023-04-30T18:28:32.000Z
Duration: 2 min 28 s
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effect-of-temperature-change-on-reaction-rate

Effect of Temperature Change on Reaction Rate

La loi de vitesse d'une réaction définit la relation entre la concentration d'un réactif et la vitesse de réaction en fonction d'une constante de vitesse, « k ».
La constante de vitesse décrit la relation entre la température et les paramètres cinétiques liés à la collision, à l’orientation et à l’énergie d’activation des molécules en réaction via l’équation d’Arrhenius. 'A' est une constante appelée facteur d’Arrhenius ou facteur de fréquence. 'e' est un facteur exponentiel intégrant l’énergie d’activation mesurée en joules par mole, la constante de gaz et la température en kelvin.
La dépendance des paramètres à la température peut être expliquée par le modèle de collision, qui stipule que les molécules en réaction doivent entrer en collision avec une énergie suffisante dans la bonne orientation pour initier une réaction chimique.
Le facteur de fréquence comprend deux composantes : la fréquence de collision et le facteur d’orientation. La fréquence de collision est le nombre de collisions moléculaires par unité de temps, tandis que le facteur d’orientation décrit la probabilité de collisions avec une orientation favorable.
Pourtant, seule une petite fraction des collisions conduit à une réaction. En effet, les molécules réactives doivent surmonter une barrière énergétique, appelée énergie d’activation, pour se transformer en produits.
Seules les molécules qui entrent en collision avec une énergie cinétique suffisante auront suffisamment d’énergie potentielle pour plier, étirer ou rompre les liaisons et se transformer en un intermédiaire de haute énergie appelé état de transition, ou complexe activé. Le complexe activé instable et à courte durée de vie perd de l’énergie pour former des produits stables, dont l’énergie totale est inférieure à celle des réactifs.
Le facteur exponentiel dans l’équation d’Arrhenius représente la fraction des collisions réussies aboutissant à des produits. Une augmentation de la température influence à la fois le facteur de fréquence et le facteur exponentiel.
À des températures élevées, les molécules se déplacent plus rapidement, avec plus de force et avec des énergies thermiques plus élevées, ce qui entraîne des collisions plus favorables.
Pour la plupart des réactions, une augmentation de la température entraîne une fréquence plus élevée et des facteurs exponentiels, entraînant une augmentation de la constante de vitesse, ce qui se traduit par une vitesse de réaction accélérée.

effet du changement de température sur la vitesse de réaction

<img alt="Figure1" src="/files/ftp_upload/11698/11698_Equation1.svg" style="text-align: center; height: 30px;">

Dans cette équation, R est la constante des gaz parfaits, qui a une valeur de 8,314 J/mol·K, T est la température en kelvin, Ea est l'énergie d'activation en joules par mole, e est la constante 2,7183 et A est une constante. appelé facteur de fréquence, qui est lié à la fréquence des collisions et à l’orientation des molécules en réaction.

Le facteur de fréquence, A, reflète dans quelle mesure les conditions de réaction favorisent des collisions correctement orientées entre les molécules réactives. Une probabilité accrue de collisions efficacement orientées se traduit par des valeurs plus élevées pour A et des taux de réaction plus rapides.

Le terme exponentiel, e−Eₐ/RT, décrit l'effet de l'énergie d'activation sur la vitesse de réaction. Selon la théorie moléculaire cinétique, la température de la matière est une mesure de l'énergie cinétique moyenne de ses atomes ou molécules constitutifs : une énergie d'activation plus faible se traduit par une fraction plus importante de molécules suffisamment énergétiques et une réaction plus rapide.

Le terme exponentiel décrit également l'effet de la température sur la vitesse de réaction. Une température plus élevée représente une fraction proportionnellement plus grande de molécules possédant suffisamment d’énergie (RT) pour surmonter la barrière d’activation (Ea). Cela donne une valeur plus élevée pour la constante de vitesse et une vitesse de réaction proportionnellement plus rapide.

L'énergie minimale nécessaire pour former un produit lors d'une collision entre réactifs est appelée énergie d'activation (Ea). La différence entre l'énergie d'activation requise et l'énergie cinétique fournie par les molécules réactives en collision est un facteur principal affectant la vitesse d'une réaction chimique. Si l’énergie d’activation est bien supérieure à l’énergie cinétique moyenne des molécules, la réaction se produira lentement, puisque seules quelques molécules se déplaçant rapidement auront suffisamment d’énergie pour réagir. Si l’énergie d’activation est bien inférieure à l’énergie cinétique moyenne des molécules, une grande fraction des molécules sera suffisamment énergétique et la réaction se déroulera rapidement.

Les diagrammes de réaction sont largement utilisés en cinétique chimique pour illustrer diverses propriétés d'une réaction d'intérêt. Il montre comment l'énergie d'un système chimique change au fur et à mesure qu'il subit une réaction, convertissant les réactifs en produits.

Ce texte est adapté de <a href="https://openstax.org/books/chemistry-2e/pages/12-5-collision-theory">Openstax, Chemistry 2e, Section 12.5: Collision Theory.</a>

Effet du changement de température sur la vitesse de réaction

L’équation d’Arrhenius,

Relie l’énergie d’activation et la constante de vitesse, k, pour de nombreuses réactions chimiques.
Dans cette équation, R est le gaz parfait ...

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arrhenius equation in reaction speed

The Arrhenius equation connects activation energy, temperature, and the rate constant, k, for many chemical reactions. It helps explain why some reactions move slowly while others happen faster.

In this equation, R is the ideal gas constant, with a value of 8.314 J/mol·K. T is the temperature in kelvin, Ea is the activation energy in joules per mole, e is the constant 2.7183, and A is the frequency factor.

The frequency factor, A, describes how often reacting molecules collide and how well they are oriented during those collisions. Better orientation increases the chance of effective collisions. That leads to larger A values and faster reaction rates.

The exponential term, e−Ea/RT, shows how activation energy affects the reaction rate. Kinetic molecular theory says that temperature measures the average kinetic energy of atoms or molecules. When activation energy is lower, more molecules have enough energy to react, and the reaction is faster.

The same exponential term also shows the effect of temperature. At higher temperature, a larger fraction of molecules has enough energy, RT, to get over the activation barrier, Ea. This increases the rate constant and speeds up the reaction.

Activation energy is the minimum energy needed to form products during a collision between reactants. If Ea is much larger than the average kinetic energy of the molecules, only a few fast-moving molecules can react, so the reaction is slow. If Ea is much smaller than the average kinetic energy, many molecules can react, so the reaction proceeds quickly.

Reaction diagrams are often used in chemical kinetics to show how a system’s energy changes as reactants turn into products. They help visualize the energy barrier and the path from reactants to products. This text is adapted from Openstax, Chemistry 2e, Section 12.5: Collision Theory.

Arrhenius Equation in Reaction Speed

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effect of temperature change on reaction rate

The Arrhenius equation connects activation energy, temperature, and the rate constant, k, for many chemical reactions. It helps explain why some reactions move slowly while others happen faster. In this equation, R is the ideal gas constant, with a value of 8.314 J/mol·K. T is the temperature in kelvin, Ea is the activation energy in joules per mole, e is the constant 2.7183, and A is the frequency factor. The frequency factor, A, describes how often reacting molecules collide and how well...

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Explore how the Arrhenius equation links temperature, activation energy, and collision orientation to reaction speed in high school chemistry.

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Arrhenius Equation in Reaction Speed

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