Name: reaction order and rate constants
Uploaded: 2020-03-26T07:22:26.000Z
Duration: 4 min 37 s
Channel: JoVE

K12 Standards

Minnesota Academic Standards in Science

9-12

Chemistry

3.2.1 9C.3.1.1.2

Concentration Dependence

La mesure de la vitesse à laquelle une réaction se déroule s’appelle la vitesse de réaction. Pour une réaction en une seule étape, la vitesse est égale à la variation de la concentration de chaque réactif ou produit au fil du temps multipliée par l’inverse du coefficient stœchiométrique correspondant. Vous pouvez considérer la variation de la concentration au fil du temps comme la concentration à l’instant t moins la concentration de départ divisée par t. Les vitesses de réaction sont toujours positives, de sorte que les expressions des réactifs ont des signes négatifs. Ces taux peuvent ne pas être égaux dans les réactions en plusieurs étapes, mais nous pouvons toujours utiliser cette relation comme estimation de la réaction globale.
La loi de vitesse, ou équation de vitesse, décrit la relation entre la vitesse de la réaction et les concentrations de réactifs. Dans cette équation, k est la constante de vitesse, A et B sont les deux réactifs, et m et n sont leurs ordres de réaction respectifs. L’ordre de réaction décrit la relation entre la concentration d’un réactif et la vitesse et n’est pas lié à la stœchiométrie. Il est essentiel de se rappeler que l’ordre de réaction n’est pas le même que le coefficient du réactif dans l’équation équilibrée.
Lorsqu’une réaction implique deux réactifs ou plus, nous devons considérer l’ordre de réaction de chaque réactif. L'ordre de réaction global est égal à la somme des ordres de réaction des réactifs. Par exemple, si le réactif A est de premier ordre et le réactif B est d’ordre zéro, l’ordre global de la réaction est un. Les ordres de réaction les plus courants dans les réactions simples sont l’ordre zéro, le premier ordre et le second ordre. Parcourons-les en utilisant une réaction unimoléculaire comme exemple.
Si la réaction est d’ordre zéro, la concentration du réactif n’a aucun effet sur la vitesse de réaction. Ainsi, la vitesse de réaction est égale à k, la constante de vitesse. Un graphique de la vitesse de réaction par rapport à la concentration est une ligne horizontale.
Dans les relations de premier ordre, la concentration du réactif est linéairement liée à la vitesse de réaction. Ainsi, le taux est égal à la constante de vitesse multipliée par la concentration du réactif. Un graphique de la vitesse de réaction par rapport à la concentration sera linéaire, avec k comme pente.
Si la réaction est de second ordre, il existe une relation quadratique entre la concentration et la vitesse. Ainsi, le taux est égal à la constante de vitesse multipliée par la concentration au carré. Un graphique du taux par rapport à la concentration sera parabolique, avec k comme pente.
La constante de vitesse, k, est une valeur dépendante de la température qui relie l’énergie d’activation de la réaction à la vitesse de réaction. Vous explorerez cela dans la prochaine expérience de laboratoire.
Les vitesses de réaction sont généralement indiquées en moles par litre par seconde. La vitesse a les mêmes unités, quel que soit l’ordre de réaction. Ainsi, la constante de vitesse a des unités différentes, en fonction de l’ordre de réaction. Par exemple, si A est de premier ordre et B est d’ordre zéro, alors nous avons une instance de moles par litre dans l’équation du taux. Par conséquent, la constante de vitesse doit être exprimée en secondes inverses.
Alors, comment détermine-t-on l’ordre de réaction ? L’ordre de réaction doit être déterminé expérimentalement à l’aide d’une série de tests. Si vous avez deux réactifs, une méthode consiste à maintenir la concentration d’un réactif constante, à faire varier l’autre et à chronométrer le temps nécessaire pour fabriquer une certaine quantité de produit. Le même processus est répété pour le deuxième réactif. Vous pouvez ensuite estimer l'ordre de chaque réactif en traçant la vitesse en fonction de la concentration variable des réactifs et en voyant s'il ressemble à un graphique de zéro, de premier ou de second ordre. La correspondance de vos données avec l’équation de taux correspondante vous permettra également de calculer k.
Dans cet atelier, vous allez déterminer l’ordre de réaction de deux réactifs en faisant varier leurs concentrations et en chronométrant le temps nécessaire à la réaction pour rendre la solution opaque.

dépendance à la concentration

La cinétique chimique et la loi de la vitesse de réaction
La cinétique chimique fait référence à la vitesse ou à la vitesse d’une réaction chimique. La vitesse dépend du mécanisme, de la complexité et du nombre de réactifs dans la réaction. La concentration du réactif joue également un rôle important dans la vitesse d’une réaction.
La loi de taux quantifie cette relation par l’expérimentation. Chaque réactif contribue à la vitesse de la réaction par un facteur connu sous le nom d’ordre de réaction. Ce facteur peut varier de zéro à deux, et il dépend de la relation de ce réactif avec la vitesse de la réaction.
r = k[A]m[B]n pour aA + bB + ... → cC
Dans cette équation, r est la vitesse de réaction, k est la constante de vitesse de réaction, [A] et [B] sont les concentrations des réactifs A et B, et m et n sont les ordres de réaction des réactifs A et B, respectivement. La vitesse de réaction, r, est définie comme le changement de concentration du produit avec le temps et a des unités de moles par litre et par seconde.
<h4>Ordre d’une réaction</h4>
L’ordre de la réaction décrit la dépendance de puissance de la concentration de réactifs par rapport à la vitesse de réaction. L’ordre global de réaction est la somme des ordres de réaction de chacun des réactifs. Il est important de se rappeler que l’ordre de réaction n’est PAS lié aux facteurs stœchiométriques de l’équation chimique équilibrée. En d’autres termes, la stœchiométrie ne peut pas être utilisée pour déterminer les ordres de réaction d’une réaction chimique. Les ordres de réaction doivent être déterminés expérimentalement
Pour déterminer l’ordre de réaction, la concentration du deuxième réactif est maintenue constante, tandis que la concentration du premier réactif est modifiée. Le temps nécessaire pour que la réaction se produise est mesuré en secondes ; Cependant, ce temps ne correspond pas au temps qu’il faut à la réaction pour aller jusqu’au bout. Au lieu de cela, c’est le temps qu’il faut pour que la réaction commence. Les temps de réaction pour les différentes concentrations de réactifs sont ensuite comparés pour déterminer l’ordre. La même série de réactions est effectuée pour déterminer l’ordre du deuxième réactif, où la concentration du premier réactif est maintenue constante et la concentration du second réactif est modifiée.
En comparant les temps de réaction, l’ordre de réaction peut être déterminé. Par exemple, si le temps de réaction reste constant malgré les changements de concentration du réactif, alors le réactif est d’ordre zéro. Cela signifie que la vitesse de la réaction est égale au coefficient de vitesse de réaction, k, qui doit avoir des unités de M/s. Si le temps de réaction change linéairement avec les changements de concentration (c’est-à-dire que le doublement de la concentration affecte le temps de réaction d’un facteur 2), alors le réactif est de premier ordre. Cela signifie que la vitesse de la réaction est égale au produit du coefficient de vitesse de réaction et de la concentration du réactif. Dans ce cas, k a des unités de 1/s. Enfin, si le temps de réaction est affecté d’un facteur 4 lorsque la concentration d’un réactif est doublée, le réactif est de second ordre. Cela signifie que la vitesse de la réaction est égale à la constante de vitesse multipliée par le carré de la concentration du réactif, formant une relation quadratique. Dans ce cas, les unités de k doivent être 1/M⋅s.
L’ordre global de la réaction est la somme des ordres de réaction individuels. Par exemple, si la réaction est de premier ordre par rapport à A, alors m = 1. Et si la réaction est d’ordre zéro par rapport à B, alors n = 0. L’ordre général de la réaction est de premier ordre car m + n = 1.
<h4>Constante de vitesse de réaction</h4>
La constante de vitesse, k, est spécifique à la réaction et à la température à laquelle la réaction est effectuée. La constante de vitesse est déterminée à partir de la série d’expériences. En utilisant la vitesse mesurée et les concentrations initiales de réactif, l’équation de vitesse est utilisée pour calculer la constante de vitesse. La constante de vitesse dépend de la température et est définie par l’équation d’Arrhenius. Cette équation décrit la relation entre k et l’énergie d’activation, Ea, la température, T, et la constante des gaz parfaits, R. La constante A est une constante de proportionnalité et ne doit pas être confondue avec le réactif A.
<img src="/CDNSource/lm/labs/50/50_Concepts_2.jpg" font-size>Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). Chimie et réactivité chimique. Belmont, Californie : Brooks/Cole, Cengage Learning.
	<li style="font-size :17px">Silderberg, M.S. (2009). Chimie : la nature moléculaire de la matière et du changement. Boston, MA : McGraw Hill.</li>

Dépendance à la concentration

La cinétique chimique et la loi de la vitesse de réaction
La cinétique chimique fait référence à la vitesse ou à la vitesse d’une réaction chimique. Le taux dépend du mécanisme, ...

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reaction order and rate constants

Chemical kinetics describes the speed of a chemical reaction. The rate depends on the reaction mechanism, the complexity of the reaction, and the number of reactants involved. Reactant concentration also affects how fast a reaction happens. The rate law is the experimental relationship that connects these factors to reaction rate.

In the rate law, each reactant contributes through a reaction order. The general form is r = k[A]^m[B]^n for aA + bB + ... → cC. Here, r is the reaction rate, k is the rate constant, and [A] and [B] are the reactant concentrations. The exponents m and n show the reaction order for each reactant.

The reaction rate is the change in product concentration over time. Its units are moles per liter per second. The overall reaction order is the sum of the individual orders. Reaction order is not the same as the numbers in the balanced chemical equation, so stoichiometric coefficients cannot be used to find it. Reaction orders must be measured experimentally.

To find a reaction order, one reactant concentration is changed while the other is held constant. The reaction time is measured in seconds, but this is the time for the reaction to start, not the time to completion. The times from different concentrations are compared to determine the order. The same process is repeated for the second reactant by switching which concentration is varied.

If the reaction time stays the same even when concentration changes, the reactant is zero-order. In that case, the rate equals k, and k has units of M/s. If the time changes linearly with concentration, the reactant is first order. Then the rate equals k times the reactant concentration, and k has units of 1/s. If doubling the concentration changes the time by a factor of 4, the reactant is second order. Then the rate equals k times the square of the concentration, and k has units of 1/M·s.

The overall order is the sum of the individual orders. For example, if A is first order and B is zero-order, the total order is first order because m + n = 1. The rate constant, k, is specific to the reaction and the temperature used. It is found from the experimental data by using the measured rate and initial concentrations in the rate equation.

The rate constant also depends on temperature through the Arrhenius equation. This equation relates k to activation energy, Ea, temperature, T, and the ideal gas constant, R. The constant A in the equation is a proportionality constant. It should not be confused with reactant A.

Reaction Order and Rate Constants

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Solution Formation

There is no one solvent that can dissolve every type of solute. Some substances that readily dissolve in a certain solvent might be insoluble in a ...

Energetics of Solution Formation

The formation of a solution is an example of a spontaneous process, which is a process that occurs under specified conditions without energy from some ...

General Properties of Solutions

Many common substances around us exist as a solution, such as ocean water, air, and gasoline. All solutions are mixtures of substances that are composed ...

Solutions and Concentrations

Source: Laboratory of Dr. Michael Evans &#8212; Georgia Institute of Technology
A solution is a homogeneous mixture containing some components in small ...

Solubility

Solubility
Solubility describes how much of a solute can dissolve in a given volume of a specific solvent. Solubility is usually reported in terms of ...

Solution, Solubility, and Solubility Equilibrium
A solution is a homogeneous mixture&#160;composed of a solvent, the major component, and a solute, the ...

Solution Equilibrium and Saturation

Imagine adding a small amount of sugar to a glass of water, stirring until all the sugar has dissolved, and then adding a bit more. You can repeat this ...

Intermolecular Forces in Solutions

The formation of a solution is an example of a spontaneous process, a process that occurs under specified conditions without energy from some external ...

Matter and Its Interactions: Develop models to illustrate the changes in the composition of the nucleus of the atom and the energy released during the processes of fission, fusion, and radioactive decay. (P: 2, CC: 5, CI: PS1)

9C.3.1.1.3

Chemical and Solubility Equilibria

The free energy change associated with dissolving a solute in a liter of solvent is called the free energy of a solution, &#916;Gsolution. The overall ...

Matter and Its Interactions: Apply scientific principles and evidence to provide an explanation about the effects of changing the surface area, agitation, temperature, and concentration of the reacting particles on the rate at which the reaction occurs. (P: 6, CC: 1, CI: PS1)

9C.3.2.1.2

3 Developing possible explanations of phenomena or designing solutions to engineering problems
 3.2 Constructing explanations and designing solutions 
  3.2.1 Students will be able to apply scientific principles and empirical evidence (primary or secondary) to explain the causes of phenomena or identify weaknesses in explanations developed by the students or others.

Reaction Order and Rate Constants

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