Name: reaction order from concentration
Uploaded: 2020-09-24T15:06:50.000Z
Duration: 3 min 3 s
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Concentration and Rate Law

La vitesse d'une réaction dépend souvent sur les concentrations de réactif. Pour toute réaction, la relation entre la vitesse de réaction et les concentrations de réactif peut être exprimé mathématiquement à l'aide d'une loi ou équation de taux. Dans une loi de taux, k est la constante de proportionnalité, ou la constante de vitesse, et n est l'ordre de réaction par rapport à un seul réactif, dont la valeur est souvent un nombre entier.Dans les lois de vitesse pour les réactions multi-réactifs, l'ordre de réaction global est la somme de tous les ordres de réactifs. Pour chaque réactif, la vitesse de réaction, la constante de vitesse, concentration et ordre de réaction sont tous déterminés expérimentalement. La loi des taux exprime la relation entre tous ces paramètres.Les ordres individuels de réactifs prennent généralement les valeurs 0, 1 ou 2 et en fonction de l'ordre global de la réaction, les réactions chimiques peuvent être catégorisées en réactions d'ordre zéro, de premier ordre ou de second ordre. Une réaction chimique à un seul réactif ou unimoléculaire pour lequel 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 de zéro, et selon la loi de vitesse, la concentration de réactif est élevée à la puissance zéro.Puisque la valeur de tout nombre élevé à la puissance zéro est un, la vitesse de réaction d'une réaction d'ordre zéro est égal à la constante de vitesse et, par conséquent, indépendante de la concentration du réactif. Par conséquent, dans une réaction d'ordre zéro, même si la concentration du réactif diminue, la vitesse de réaction ne ralentit pas. 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 taux, la concentration du réactif est élevée à la première puissance. Puisque la valeur de tout nombre élevé à la puissance un reste le même, la vitesse de réaction d'une réaction de premier ordre dépend directement de la concentration du réactif. À mesure que la concentration de 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. L'ordre des réactifs est de deux et la concentration des réactifs est élevée à la deuxième puissance. En conséquence, la vitesse de réaction dans une réaction de second ordre dépend directement du carré de la concentration du réactif.Lorsque la concentration des réactif diminue, la vitesse de réaction diminue de façon exponentielle et quadratique.

concentration et loi des vitesses de réaction

La vitesse d'une réaction est influencée par les concentrations de réactifs. Les lois de vitesse (lois de vitesse différentielles) ou équations de vitesse 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 générale aA + bB ⟶ produits, où a et b sont des coefficients stœchiométriques, la loi de vitesse peut être écrite comme suit : 
  <img alt="Eq1" src="/files/ftp_upload/11375/11375_Equation_1.png" style="height: 50px;" />
 [A] et [B] représentent les concentrations molaires des réactifs, et k est la constante de vitesse, qui est spécifique à une réaction particulière à une température précise. 
 Les exposants m et n sont les ordres de la réaction et sont généralement des nombres entiers positifs, bien qu'ils puissent être des fractions, négatifs ou nuls. 
 La constante de vitesse k et les ordres de réaction m et n sont déterminés expérimentalement en observant comment la vitesse d'une réaction varie au fur et à mesure que les concentrations des réactifs changent. La constante de vitesse k est indépendante des concentrations de réactifs, mais varie en fonction de la température. 
 Les ordres de réaction dans une loi de vitesse décrivent la dépendance mathématique entre la vitesse et les concentrations de réactifs. En se référant à la loi de vitesse générale (vitesse = k[A]m[B]n), la réaction est d'ordre m par rapport à A et d'ordre n par rapport à B. Par exemple, si m = 1 et n = 2, la réaction est du premier ordre pour A et du deuxième ordre pour B. L'ordre global de la réaction est simplement la somme des ordres pour chaque réactif. Pour l'exemple de la loi de vitesse ici, la réaction est du troisième ordre globalement (1 + 2 = 3). 
 Une approche expérimentale courante pour la détermination des lois de vitesse est la méthode des vitesses initiales. Cette méthode consiste à mesurer les vitesses de réaction pour de multiples 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 conjointement servent à 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 expérimentalement seulement et ne sont pas prédites de façon fiable d'après la stœchiométrie de la réaction. 
 L'ordre de la réaction détermine la relation entre la vitesse de réaction et la concentration des réactifs ou des produits. 
 • Dans une réaction d'ordre zéro, la concentration des réactifs n'a aucun effet sur la vitesse de la réaction, qui reste constante tout le long. 
 • Dans une réaction de premier ordre, la vitesse de réaction est directement et linéairement proportionnelle à la variation de la concentration du réactif. Au fur et à mesure que la concentration du réactif diminue, la vitesse de réaction diminue également de façon proportionnelle. 
 • Dans les réactions de deuxième ordre ou d'un 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. 
 Ce texte est adapté de<a href="https://openstax.org/books/chemistry-2e/pages/12-3-rate-laws#57033"> Openstax, Chimie 2e, Section 12.3 : Lois de vitesse</a>.

Concentration et loi des vitesses 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 ...

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reaction order from concentration

Reaction order from concentration shows how reactant amounts affect reaction rate. Rate laws, also called differential rate laws or rate equations, are mathematical expressions that connect reaction rate with the concentrations of reactants. For a generic reaction a A + b B → products, the rate law can be written using the reactant concentrations [A] and [B] and a rate constant k.

In a rate law, k is the rate constant for one specific reaction at one specific temperature. The exponents m and n are the reaction orders. They are usually positive integers, but they can also be fractions, negative numbers, or zero. The values of k, m, and n are found experimentally by measuring how the rate changes when reactant concentrations change.

The rate constant does not depend on the reactant concentrations, but it can change with temperature. The reaction orders show the mathematical link between concentration and rate. In the expression rate = k [A]m [B]n, the reaction is m order with respect to A and n order with respect to B. For example, 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 orders for each reactant. In the example above, the reaction is third order overall because 1 + 2 = 3. A common way to find a rate law is the method of initial rates. In this method, reaction rates are measured in several trials that start with different initial reactant concentrations. Comparing those rates makes it possible to find the reaction orders and then the rate constant.

Rate laws can also include fractional orders. In some cases, a reactant may have a negative order. That means increasing its concentration can lower the reaction rate. Rate laws are determined by experiment only, and they cannot be reliably predicted from reaction stoichiometry. The reaction order also describes how the rate changes as reactant or product concentration changes.

A zero-order reaction has a rate that does not change with reactant concentration. The rate stays constant throughout the reaction. In a first-order reaction, the rate is directly proportional to the change in reactant concentration. As the reactant concentration drops, the rate drops by the same proportion. In second-order or higher-order reactions, the rate depends on the reactants raised to an exponent. As the reaction continues and reactant concentration decreases, the rate falls exponentially.

Reaction Order from Concentration

reaction rate

Measuring Reaction Rates

Measuring reaction rates shows how fast reactants are used up or products are formed over time. The rate of reaction is the change in the amount of a reactant or product per unit time. Because of that, scientists measure how a property changes as the reaction proceeds and connect that change to reactant or product amounts. For reactions that make or use a gas, the rate can be followed by changes in volume or pressure. A rate expression is the mathematical way to write how concentration changes...

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Reaction kinetics can be tracked with polarimetry, spectrometry, and pressure measurements. These methods let scientists follow a reaction without disturbing the sample. They also help determine how reactant or product concentrations change over time. Polarimetry is used for optically active substances. These substances rotate the plane of polarized light, which is called optical rotation. The effect comes from molecular structure. Normal monochromatic light is unpolarized, but after it passes...

concentration and rate law

Reaction order from concentration shows how reactant amounts affect reaction rate. Rate laws, also called differential rate laws or rate equations, are mathematical expressions that connect reaction rate with the concentrations of reactants. For a generic reaction a A + b B → products, the rate law can be written using the reactant concentrations [A] and [B] and a rate constant k. In a rate law, k is the rate constant for one specific reaction at one specific temperature. The exponents m and n...

determining order of reaction

Finding Rate Law from Initial Rates

Rate laws connect the speed of a chemical reaction to the concentration of its reactants. The rate constant, k, and the reaction orders are not taken from the chemical equation. They are found experimentally by watching how the rate changes when the reactant concentrations change. One common way to do this is the method of initial rates. In this method, several trials are run with different starting concentrations. The measured rates are then compared to determine the reaction orders and,...

the integrated rate law: the dependence of concentration on time

the-integrated-rate-law-the-dependence-of-concentration-on-time

Reading Reaction Progress with Integrated Laws

Integrated rate laws connect reaction concentration to time. They help show how much reactant or product is present after a certain period. They can also help estimate how long a reaction must run to reach a given extent. A common use is finding how long radioactive material must be stored before its radioactivity drops to a safe level. These equations come from the differential rate law after it is integrated with respect to time. For a first-order reaction, where rate = k[A], the integrated...

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Reaction half-life is the time needed for half of a reactant to be consumed. After each half-life, half of the remaining reactant is used up. A hydrogen peroxide decomposition example shows this pattern clearly. The concentration drops from 1.000 M to 0.500 M in the first 6.00 hours, then to 0.250 M in the next 6.00 hours, and then to 0.125 M in the third period. Each later 6.00-hour interval removes half of what is left. For first-order reactions, the half-life does not depend on the reactant...

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Chemical reactions can happen in steps, and a balanced equation does not show the full path at the molecular level. A reaction mechanism, also called a reaction path, gives the step-by-step process that explains how reactants become products. The decomposition of ozone is one example of a reaction that can be described by a sequence of steps. Each step in a mechanism is an elementary reaction. These elementary reactions happen in order, and when they are added together, they give the balanced...

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Reaction bottlenecks help explain rate-determining steps in multistep reaction mechanisms. In a mechanism with several elementary steps, one step is often much slower than the others. That slowest step is called the rate-limiting step, or rate-determining step. A reaction cannot move faster than its slowest step, so this step controls the overall reaction rate. The idea is like traffic on a four-lane freeway with one lane closed. Cars can only move as fast as the bottleneck allows. In the same...

catalysis

Catalyst Action in Reaction Rates

Catalysts change the rate of a chemical reaction by giving it a faster path. A catalyst increases reaction rate without being consumed during the process. Its effect is often explained with reaction mechanisms and energy diagrams. For an endothermic reaction, the reactants and products start and end at the same energies in both cases. The uncatalyzed path is a one-step mechanism with one transition state. The catalyzed path follows a different mechanism with two transition states and a lower...

enzymes

Enzyme Action and Inhibition

Enzymes are biological catalysts that speed up many reactions in living organisms. They help control cellular metabolism by lowering the activation energy needed for biochemical reactions. When activation energy drops, reaction rates can increase. Enzyme problems can also lead to serious disease. A genetic abnormality that causes G6PD, or glucose-6-phosphate dehydrogenase, deficiency can disrupt the pathway that supplies NADPH to cells. In red blood cells, this can reduce glutathione and...

key terms and concepts

Learn how concentration affects reaction rate, how rate laws are written, and how initial rates reveal reaction order, the rate constant, and overall order.

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Reaction Order from Concentration

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