Name: measuring reaction rates
Uploaded: 2020-09-24T15:06:50.000Z
Duration: 2 min 53 s
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IPC.7

IPC.7.F

Reaction Rate

Une réaction chimique implique la transformation chimique des réactifs en produits. Au fur et à mesure que la réaction progresse, la concentration des réactifs diminue tandis que la concentration des produits augmente. Cette variance des concentrations de réactifs et de produits peut être tracée dans un graphique en fonction du temps.La vitesse à laquelle une réaction progresse s'appelle la vitesse de réaction. Il mesure le taux de disparition des réactifs ou le taux d'apparence des produits et est exprimé en unités de molarité par seconde. La vitesse de réaction moyenne peut être calculée du changement des concentrations de réactifs ou de produits pendant une période de temps spécifique.La valeur de concentration molaire est indiquée entre parenthèses, t, qui représente la période de temps et le symbole delta qui représente le changement'Étant donné que les réactifs sont épuisés lors d'une réaction chimique, la valeur du changement de concentration des réactifs est toujours négatif. Par conséquent, une vitesse de réaction calculée en termes de réactifs est complété par un signe négatif, pour rendre la valeur globale positive. La vitesse de réaction n'est pas uniforme tout au long d'une réaction.La vitesse au début de la réaction, au temps zéro"s'appelle la vitesse de réaction initiale. Au fur et à mesure que les concentrations de réactifs diminuent, la vitesse de réaction diminue ou la réaction ralentit. La vitesse de réaction à un instant donné, ou la vitesse de réaction instantanée, est mesurée en calculant la pente d'une tangente tracée à la courbe de réaction, soit pour les réactifs, soit pour les produits, au moment opportun.La valeur de la pente ou le taux instantané à un moment donné est égal pour tous les réactifs et produits. La vitesse de réaction d'une réaction chimique reflète également les coefficients stœchiométriques réels de réactifs et de produits. Donc, pour toute réaction équilibrée, où une mole de A réagit avec b moles de B, produisant c moles de C et d moles de D, la vitesse de réaction peut être exprimée et calculée à l'aide de la formule générique:moins 1 a fois le changement de concentration du réactif A avec le temps, égal à moins 1 b fois le changement en concentration de réactif B avec le temps.Ces valeurs sont les mêmes que plus 1 c fois, changement de concentration du produit C par le temps, ce qui équivaut à plus 1 d fois changement de concentration du produit D avec le temps. La détermination des vitesses de réaction est fondamentale pour l'étude de la cinétique chimique, permet de comprendre la rapidité d'action d'un médicament, d'un catalyseur, ou d'une réaction de synthèse pour mieux la contrôler et l'optimiser dans sa fonction.

vitesse de réaction

La vitesse de réaction est la variation de la quantité d'un réactif ou d'un produit par unité de temps. Les vitesses de réaction sont donc déterminées en mesurant la dépendance temporelle de certaines propriétés qui peuvent être liées aux quantités de réactifs ou de produits. Les vitesses des réactions qui consomment ou produisent des substances gazeuses, par exemple, sont facilement déterminées en mesurant les changements de volume ou de pression. 
 La représentation mathématique de la variation de la concentration des réactifs et des produits, au fil du temps, est l'expression de la vitesse de la réaction. Les crochets indiquent des concentrations molaires et le symbole delta (Δ) indique “ variation de ”. 
 Par exemple, la concentration du peroxyde d'hydrogène, H2O2, dans une solution aqueuse varie lentement au fil du temps, car il se décompose selon l'équation : 
  <img alt="Eq1" src="/files/ftp_upload/11373/11373_Equation_1.png" style="height: 50px;" />
 La vitesse à laquelle le peroxyde d'hydrogène se décompose peut être exprimée en termes de vitesse de la variation de sa concentration : 
  <img alt="Eq2" src="/files/ftp_upload/11373/11373_Equation_2.png" style="height: 70px;" />
 Ainsi, [H2O2]t1 représente la concentration molaire du peroxyde d'hydrogène à l'instant t1 ; de même, [H2O2]t2 représente la concentration molaire du peroxyde d'hydrogène à un instant ultérieur t2 ; et Δ[H2O2] représente la variation de la concentration molaire du peroxyde d'hydrogène pendant l'intervalle de temps Δt (c'est-à-dire t2 − t1). Puisque la concentration en réactifs diminue à mesure que la réaction se poursuit, Δ[H2O2] est une quantité négative. Les vitesses de réaction sont, par convention, des quantités positives, cette variation de concentration négative est donc multiplié par −1. 
 Vitesse de réaction moyenne et vitesse de réaction instantanée 
 Les vitesses de réaction varient avec le temps et diminuent au fur et à mesure que la réaction se poursuit. Une vitesse de réaction moyenne sur un intervalle de temps peut être calculée en utilisant les concentrations au début et à la fin de cette période, au cours de laquelle la vitesse de réaction change. À tout moment précis, la vitesse à laquelle une réaction se produit est appelée vitesse instantanée. La vitesse instantanée d'une réaction au “ temps zéro ”, lorsque la réaction débute, est sa vitesse initiale. 
 La vitesse instantanée d'une réaction peut être déterminée de l'une des deux façons. Si les conditions expérimentales permettent de mesurer les variations de concentration sur de courts intervalles de temps, alors le calcul des vitesses moyennes fournit des approximations raisonnablement bonnes des vitesses instantanées. Sinon, une procédure graphique peut être utilisée. Notamment, dans l'exemple de la décomposition du peroxyde d'hydrogène, en traçant la concentration du peroxyde d'hydrogène en fonction du temps, la vitesse instantanée de décomposition de H2O2 peut être calculée à tout instant ‘ t ’ à partir de la pente d'une tangente à la courbe tracée à cet instant-là. 
  <img alt="Image1" src="/files/ftp_upload/11373/11373_Image_1.png" style="height: 300px;" />
 Ce graphique montre un tracé de la concentration par rapport au temps pour une solution de 1,000 M d'H2O2. La vitesse à tout instant est égale à la valeur négative de la pente d'une droite tangente à la courbe à cet instant-là. Les tangentes sont indiquées à t = 0 h (“ vitesse initiale ”) et à t = 12 h (“ vitesse instantanée ” à 12 h). 
 Vitesses de réaction relatives 
 La vitesse d'une réaction peut être exprimée comme étant la variation de concentration de tout réactif ou produit. Pour une réaction donnée, ces expressions de la vitesse sont toutes liées les unes aux autres suivant la stœchiométrie de la réaction. La vitesse de la réaction générale aA ⟶ bB peut être exprimée en termes de diminution de la concentration de A ou d'augmentation de la concentration de B. Ces deux expressions de la vitesse sont liées par la stœchiométrie de la réaction, où : 
  <img alt="Eq3" src="/files/ftp_upload/11373/11373_Equation_3.png" style="height: 70px;" />
 Notez qu'un signe négatif a été inclus comme facteur pour tenir compte des signes opposés des variations des deux quantités (la quantité de réactif diminue alors que la quantité de produit augmente). 
 Ce texte est adapté de<a href="https://openstax.org/books/chemistry-2e/pages/12-introduction"> Openstax, Chimie 2e, Chapitre 12 Introduction</a> et <a href="https://openstax.org/books/chemistry-2e/pages/12-1-chemical-reaction-rates#12163"> Openstax, Chimie 2e, Section 12.1 : Vitesses de réaction chimique. </a>

Vitesse de réaction

La vitesse de réaction est la variation de la quantité d’un réactif ou d’un produit par unité de temps. Les vitesses de réaction sont donc déterminées en mesurant le temps ...

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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 over time. In this expression, brackets mean molar concentration, and the delta symbol means change in amount.

Hydrogen peroxide provides a clear example. As H2O2 decomposes in water, its concentration slowly decreases with time. The reaction rate can be written as the rate of change of the H2O2 concentration. If the concentration at time t1 is compared with the concentration at a later time t2, the change in concentration over the time interval Δt is found.

Because the reactant concentration goes down, Δ[H2O2] is negative. Reaction rates are reported as positive values, so the negative sign is reversed by multiplying by -1. This keeps the reaction rate positive even when the reactant amount is falling.

Reaction rates are not constant. They usually change as the reaction continues, so an average reaction rate is calculated over a time interval using the starting and ending concentrations. The rate at one exact moment is the instantaneous rate, and the instantaneous rate at time zero is the initial rate.

Instantaneous rate can be estimated in two ways. If concentration changes can be measured over very short time intervals, the average rate gives a good approximation. A graph can also be used. When concentration is plotted against time, the instantaneous rate at a given time equals the negative slope of a tangent line to the curve. In the hydrogen peroxide example, tangents can be drawn at t = 0 h for the initial rate and at t = 12 h for the instantaneous rate at 12 h.

The rate of a reaction can be written using either a reactant or a product. For a general reaction aA → bB, the decrease in A and the increase in B are linked by the reaction stoichiometry. The two rate expressions are related by that stoichiometry, and a negative sign is included to account for the reactant decreasing while the product increases.

Measuring Reaction Rates

reaction rate

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...

Tracking Reaction Kinetics with Light and Pressure

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

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...

half-life of a reaction

Reaction Half-Life in Kinetics

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...

temperature dependence on reaction rate

Collision Theory and Reaction Speed

Collision theory explains why reaction speed depends on how particles collide. Atoms, molecules, or ions must come close together before they can react. The theory says the reaction rate depends on the collision rate, the orientation of the colliding species, and whether the collision has enough energy for bonds to change. A gas-phase reaction between carbon monoxide and oxygen shows why orientation matters. The molecules can collide in more than one way. If the oxygen side of carbon monoxide...

arrhenius plots

Determining Activation Energy from Temperature

The Arrhenius equation links activation energy and the rate constant, k, for chemical reactions. It is written as k = Ae−Ea/RT. In this equation, R is the ideal gas constant with a value of 8.314 J/mol·K, T is the temperature on the kelvin scale, Ea is the activation energy in J/mole, e is the constant 2.7183, and A is the frequency factor. The frequency factor depends on collision frequency and the orientation of reacting molecules. Experimental kinetic data can be used to find the activation...

reaction mechanisms

Elementary Steps in Reaction Mechanisms

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...

rate-determining steps

Reaction Bottlenecks in Chemical Kinetics

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 reaction rates are measured using concentration, volume, pressure, average rate, and instantaneous rate with the hydrogen peroxide example.

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Measuring Reaction Rates

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