Name: how reactant concentration sets reaction rate
Uploaded: 2025-05-22T09:05:33.000Z
Duration: 2 min 33 s
Channel: JoVE

K12 Standards

Florida Department of Education

High School

Physical Science

SC.912.P.12

SC.912.P.12.12

Rate Law and Reaction Order

Pour toute réaction, la relation entre la vitesse de réaction et les concentrations de réactif peut être exprimée mathématiquement à l’aide d’une loi de vitesse ou de l’équation de vitesse. 
Dans une loi de vitesse, 'k' est la constante de proportionnalité, ou la constante de vitesse, et 'n' est l’ordre de réaction par rapport à un réactif unique, dont la valeur est souvent un entier. Dans les lois de vitesse pour les réactions multiréactives, l’ordre global de réaction est la somme de tous les ordres de réactifs.
Pour chaque réactif, la vitesse de réaction, la constante de vitesse, la concentration et l’ordre de réaction sont tous déterminés expérimentalement.
Les ordres de réactifs individuels prennent généralement les valeurs 0, 1 ou 2, et en fonction de l’ordre de réaction global, les réactions chimiques peuvent être classées en réactions d’ordre zéro, de premier ordre ou de second ordre.
Une réaction chimique à réactif unique, ou unimoléculaire, pour laquelle la vitesse de réaction reste constante pendant toute sa durée est une réaction d’ordre zéro. L’ordre des réactifs dans une réaction d’ordre zéro est égal à zéro et la concentration du réactif est élevée à la puissance zéro.
Comme la valeur de tout nombre élevé à la puissance zéro est un, la vitesse de réaction d’une réaction d’ordre zéro est égale à la constante de vitesse et, par conséquent, indépendante de la concentration du réactif.
Une réaction chimique unimoléculaire où la vitesse de réaction est directement proportionnelle à la concentration du réactif est une réaction de premier ordre. L’ordre des réactifs pour une réaction de premier ordre est un, et selon la loi de vitesse, la concentration du réactif est élevée à la première puissance.
Comme la valeur d’un nombre quelconque élevé à la puissance un reste la même, la vitesse de réaction d’une réaction de premier ordre dépend directement de la concentration du réactif. Au fur et à mesure que la concentration du réactif diminue, la vitesse de réaction diminue proportionnellement de manière linéaire.
Une réaction chimique unimoléculaire où la vitesse de réaction dépend du carré de la concentration du réactif est une réaction de second ordre. Lorsque la concentration du réactif diminue, la vitesse de réaction diminue de manière exponentielle de manière quadratique.

loi sur les taux et ordonnance de réaction

La vitesse d'une réaction est affectée par les concentrations de réactifs. Les lois de taux (lois de taux différentiels) ou équations de taux sont des expressions mathématiques décrivant la relation entre la vitesse d'une réaction chimique et la concentration de ses réactifs.

Par exemple, dans une réaction générique aA + bB ⟶ produits, où a et b sont des coefficients stoechiométriques, la loi de vitesse peut s'écrire :

taux = k[A]m[B]n

[A] et [B] représentent les concentrations molaires des réactifs et k est la constante de vitesse, spécifique à une réaction particulière à une température spécifique.

Les exposants m et n sont les ordres de réaction et sont généralement des entiers positifs, bien qu'ils puissent être des fractions, des valeurs négatives ou zéro.

La constante de vitesse k et les ordres de réaction m et n sont déterminés expérimentalement en observant comment la vitesse d'une réaction change à mesure que les concentrations des réactifs changent. La constante de vitesse k est indépendante des concentrations de réactifs mais varie avec la température.

Les ordres de réaction dans une loi de vitesse décrivent la dépendance mathématique de la vitesse sur les concentrations de réactifs. En se référant à la loi de vitesse générique (taux = k[A]m[B]n), la réaction est du mth ordre par rapport à A et du nth ordre par rapport à B. Par exemple, si m = 1 et n = 2, le la réaction est du premier ordre dans A et du deuxième ordre dans B. L'ordre global de la réaction est simplement la somme des ordres pour chaque réactif. Pour l'exemple de loi de taux ici, la réaction est globalement du troisième ordre (1 + 2 = 3).

Une approche expérimentale courante pour la détermination des lois de taux est la méthode des taux initiaux. Cette méthode consiste à mesurer les taux de réaction pour plusieurs essais expérimentaux réalisés en utilisant différentes concentrations initiales de réactifs. La comparaison des vitesses mesurées pour ces essais permet de déterminer les ordres de réaction et, par la suite, la constante de vitesse, qui sont utilisés ensemble pour formuler une loi de vitesse.

Les lois de vitesse peuvent présenter des ordres fractionnaires pour certains réactifs, et des ordres de réaction négatifs sont parfois observés lorsqu'une augmentation de la concentration d'un réactif entraîne une diminution de la vitesse de réaction. Il est important de noter que les lois de vitesse sont déterminées uniquement par l’expérience et ne sont pas prédites de manière fiable par la stœchiométrie des réactions.

L'ordre de réaction détermine la relation entre la vitesse de réaction et la concentration des réactifs ou des produits.

<ul style="list-style-type: disc; margin-left: 35px;">
	<li>Dans une réaction d'ordre zéro, la concentration des réactifs n'a aucun effet sur la vitesse de la réaction, qui reste constante tout au long.</li>
	<li>Dans une réaction de premier ordre, la vitesse de réaction est directement et linéairement proportionnelle à la modification de la concentration du réactif. À mesure que la concentration du réactif diminue, la vitesse de réaction diminue également proportionnellement.</li>
	<li>Dans les réactions de second ordre ou d'ordre supérieur, la vitesse de réaction est proportionnelle à la valeur exponentielle des réactifs. Par conséquent, à mesure que la réaction progresse et que la concentration des réactifs diminue, la vitesse de réaction diminue de façon exponentielle.</li>
</ul>

Loi sur les taux et ordonnance de réaction

La vitesse d’une réaction est affectée par les concentrations de réactifs. Les lois de taux (lois différentielles de taux) ou les équations de taux sont des expressions mathématiques ...

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

how reactant concentration sets reaction rate

Reaction rate depends on reactant concentration, and rate laws show that relationship with a mathematical equation. A rate law, also called a differential rate law or rate equation, links the rate of a chemical reaction to the concentrations of its reactants.

For a generic reaction, a A + b B  products, the rate law can be written as rate = k [A]^m [B]^n. Here, [A] and [B] are the molar concentrations of the reactants. The constant k is the rate constant, and it is specific to one reaction at a specific temperature.

The exponents m and n are the reaction orders. They are usually positive integers, but they can also be fractions, negative values, or zero. If m = 1 and n = 2, the reaction is first order in A and second order in B. The overall reaction order is the sum of the separate orders, so this example is third order overall.

Rate law values are found by experiment, not by stoichiometry. Scientists change the initial reactant concentrations in several trials and measure the reaction rate. This method, called the method of initial rates, helps determine the reaction orders and then the rate constant.

Some reactions show zero-order behavior. In a zero-order reaction, changes in reactant concentration do not change the rate, so the rate stays constant. In a first-order reaction, the rate changes in direct proportion to the reactant concentration. In second-order and higher-order reactions, the rate depends on the concentration raised to an exponent, so the rate drops more sharply as reactants are used up.

Rate laws can also show fractional orders, and some can even show negative orders. A negative order means that increasing one reactant can decrease the reaction rate. These patterns are useful for describing how reaction rate changes in real experiments. They are not reliably predicted from the reaction equation alone, so the rate law must be determined experimentally.

How Reactant Concentration Sets Reaction Rate

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rate law and reaction order

Reaction rate depends on reactant concentration, and rate laws show that relationship with a mathematical equation. A rate law, also called a differential rate law or rate equation, links the rate of a chemical reaction to the concentrations of its reactants. For a generic reaction, a A + b B  products, the rate law can be written as rate = k [A]^m [B]^n. Here, [A] and [B] are the molar concentrations of the reactants. The constant k is the rate constant, and it is specific to one reaction at...

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Learn how rate laws connect reactant concentration to reaction speed, how reaction orders work, and how the method of initial rates is used to find them.

How Reactant Concentration Sets Reaction Rate | JoVE K12

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How Reactant Concentration Sets Reaction Rate

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