Name: tracking reaction kinetics with light and pressure
Uploaded: 2020-09-24T15:06:50.000Z
Duration: 3 min 9 s
Channel: JoVE

K12 Standards

Texas Essential Knowledge and Skills

High School

Chemistry and Physics

IPC.7

IPC.7.F

Measuring Reaction Rates

Les vitesses de réaction peuvent être étudiés en déterminant le changement de concentrations des réactifs ou des produits en fonction du temps. Les changements de concentration peuvent être mesurés par des techniques expérimentales comme la polarimétrie, la spectroscopie, ou mesures de pression. La polarimétrie utilise une lumière polarisée dans un plan avec un champ électrique orienté dans un seul plan.Il mesure la capacité des composés pour faire pivoter la lumière polarisée, qui dépend sur la structure moléculaire du composé présent. Considérez l'hydrolyse du saccharose, qui donne du glucose et du fructose. Un polarimètre est utilisé pour mesurer le degré de rotation de lumière polarisée dans le plan passant par la solution de saccharose en réaction.Le saccharose provoque une rotation dans le sens horaire, alors que le glucose et le fructose provoquent une rotation antihoraire. En mesurant le degré de rotation de la lumière à intervalles de temps définis, les concentrations relatives de saccharose, glucose ou fructose peuvent être calculés et la vitesse de réaction déterminée. Les vitesses de réaction peuvent également être mesurés en utilisant des méthodes spectrophotométriques, en utilisant la capacité des réactifs ou des produits pour absorber la lumière de longueurs d'onde spécifiques.Plus la concentration de la substance d'intérêt est élevée, plus son absorbance de lumière sera intense. Par exemple, l'hydrogène gazeux incolore réagit avec la vapeur d'iode violette pour former de l'iodire d'hydrogène incolore. La vapeur d'iode absorbe la lumière dans la région jaune-vert et réfléchit la lumière violette.Pendant la réaction, un spectrophotomètre mesure la quantité de lumière absorbée par l'échantillon et analyse la lumière transmise. Ainsi, à mesure que la réaction progresse, la diminution de la concentration de vapeur d'iode s'observe par la réduction de l'absorbance de la lumière jaune-verte. En utilisant la loi de Beer-Lambert, l'intensité de la lumière absorbée à différents moments peuvent être calculés et liés aux changements de concentration.Alternativement, si l'un des réactifs ou produits est un gaz, les mesures de pression sont utilisées pour déterminer les vitesses de réaction en surveillant les changements de pression. Par exemple, lors de la décomposition du peroxyde d'hydrogène, la vitesse de réaction est étudiée à l'aide d'un manomètre pour surveiller la pression de l'oxygène gazeux libéré. Au fur et à mesure que la réaction progresse et que plus d'oxygène gazeux se dégage, la pression augmente.Utilisation de l'équation des gaz parfaits, valeurs de pression enregistrées à différents moments sont converties en concentrations. L'évolution de la concentration en fonction du temps est utilisé pour déterminer la vitesse de réaction. Pour des réactions prolongées, des échantillons ou des aliquotes, peuvent être prélevés dans le mélange réactionnel à intervalles de temps réguliers.Les concentrations relatives sont alors mesurés à l'aide de techniques instrumentales comme la chromatographie en phase gazeuse, la spectrométrie de masse ou le titrage, pour calculer les vitesses de réaction.

mesure des vitesses de réaction

La polarimétrie trouve une application en cinétique chimique pour mesurer la concentration et la cinétique de réaction des substances optiquement actives au cours d'une réaction chimique. Les substances optiquement actives ont la capacité de faire pivoter le plan de polarisation de la lumière polarisée rectilignement qui les traverse, une caractéristique appelée pouvoir rotatoire. L'activité optique est attribuée à la structure moléculaire des substances. La lumière monochromatique normale n'est pas polarisée et possède des oscillations du champ électrique dans tous les plans possibles perpendiculaires à la direction de sa propagation. Lorsqu'une lumière non polarisée traverse un polariseur, une lumière polarisée rectilignement qui maintient les oscillations dans un plan émerge. 
 Un instrument polarimétrique détermine la direction de polarisation de la lumière ou la rotation produite par une substance optiquement active. Dans un polarimètre, la lumière plane polarisée est introduite dans un tube contenant la solution réagissant, et la réaction peut être suivie sans perturber le système. Si l'échantillon contient des substances optiquement inactives, il n'y aura pas de changement dans l'orientation du plan de la lumière polarisée. La lumière est visible à la même intensité sur l'écran de l'analyseur et l'angle de rotation (ɑ) indique zéro degré. 
 Cependant, la présence de composés optiquement actifs dans l'échantillon qui réagit provoque la rotation du plan de la lumière polarisée passant à travers. La lumière émergeant sera moins lumineuse. L'axe de l'analyseur doit être tourné dans le sens des aiguilles d'une montre (dextrogyre) ou dans le sens inverse des aiguilles d'une montre (lévogyre) pour observer la luminosité maximale. Le sens de rotation de l'analyseur dépend de la nature du composé présent. Le pouvoir rotatoire optique mesuré est proportionnel à la concentration des substances optiquement actives présentes dans l'échantillon. En analysant les mesures de l'angle de rotation à différents points dans le temps, les concentrations des composés optiquement actifs peuvent être déterminées en fonction du temps. 
 Spectrométrie 
 Des techniques expérimentales d'optique comme la spectrométrie sont également fréquemment utilisées pour surveiller les réactions chimiques et obtenir des informations quantitatives sur la cinétique de la réaction. La spectrométrie permet de faire passer la lumière d'une longueur d'onde précise à travers un échantillon qui réagit. Les molécules ou composés (un réactif ou un produit) présents dans l'échantillon peuvent absorber de la lumière tout en transmettant la quantité restante, mesurée par un détecteur. La quantité de lumière absorbée dépend de la concentration du composé ou de la molécule d'intérêt. Par exemple, plus la concentration d'un composé est élevée, plus son absorbance est grande. A partir de l'absorbance, l'instrument peut déterminer la concentration du composé d'intérêt. Dans un échantillon qui réagit, l'absorbance mesurée à intervalles réguliers permet de calculer les concentrations du réactif ou du produit en fonction du temps. 
 Mesures de la pression 
 Pour les réactions impliquant des substances en phase gazeuse, la cinétique de la réaction est suivie en quantifiant les variations du nombre de moles des gaz en fonction des variations de pression. Les réglages expérimentaux d'une réaction en phase gazeuse peuvent être connectés à un manomètre qui pourrait mesurer la pression d'un réactif ou d'un produit gazeux. Au fur et à mesure que la réaction progresse, la pression des réactifs diminue et (ou) la pression des produits augmente. Ceci peut être mesuré grâce au manomètre en fonction du temps. En appliquant la loi du gaz parfait #8212; la concentration d'un gaz est proportionnelle à sa pression partielle #8212; la vitesse d'une réaction chimique peut être calculée.

Mesure des vitesses de réaction

La polarimétrie trouve une application dans la cinétique chimique pour mesurer la concentration et la cinétique de réaction de substances optiquement actives au cours d’une ...

Education

Enseignement

JoVE Core

Chemistry

Chimie

Chemical Kinetics

Cinétique chimique

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

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 through a polarizer, it becomes linearly polarized with vibrations in one plane.

In a polarimeter, this plane-polarized light is sent through a tube of reacting solution. The instrument then shows how much the light has been rotated. If the sample contains optically inactive substances, the light keeps the same orientation, the analyzer screen stays equally bright, and the rotation angle is zero degrees. If optically active compounds are present, the light becomes dimmer and the analyzer must be turned clockwise or counter-clockwise to restore maximum brightness. Clockwise rotation is dextrorotatory, and counter-clockwise rotation is levorotatory.

The amount of optical rotation is proportional to the concentration of the optically active substance in the sample. By taking rotation readings at different times, the concentration can be followed as the reaction proceeds. Spectrometry gives similar kinetic information by passing light of a specific wavelength through a reacting sample. Some of the light is absorbed by the reactant or product, and a detector measures the remaining light. Higher concentration gives higher absorbance, so repeated absorbance measurements can be used to find concentration over time.

Pressure measurements are useful for gas-phase reactions. A manometer can be connected to the reaction setup to monitor the pressure of a gaseous reactant or product. As the reaction goes on, reactant pressure may decrease and product pressure may increase. Using the ideal gas law, the concentration of a gas is proportional to its partial pressure, so reaction rate can be calculated from the pressure change.

Tracking Reaction Kinetics with Light and Pressure

tracking-reaction-kinetics-with-light-and-pressure

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...

measuring reaction rates

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

See how polarimetry, spectrometry, and pressure measurements monitor reaction kinetics by tracking optical rotation, absorbance, and gas pressure changes.

Tracking Reaction Kinetics with Light and Pressure | JoVE K12

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north-carolina-k-12-science-standard-course-of-study

NC K-12 Science Standard Course of Study

9-12

Matter and its Interactions

PS.Chm.5.1

Pennsylvania State Academic Standards

Physical Science

Chemical Reactions

3.2.9-12.D

3.2.9-12.E

ohios-learning-standards-and-model-curriculum

Ohio’s Learning Standards and Model Curriculum

Interactions Of Matter

Chemical reactions

Virginia  Standards of Learning

HIgh School

CH.7

CH.7.e

Minnesota Academic Standards in Science

3.2.1 9C.3.1.1.2

Alabama Course of Study Science

Matter and Its Interactions: Energy

9-12.C.3.3.d

Next Generation Science Standards

Matter and Its Interactions

HS-PS1-4

HS-PS1-5

Florida Department of Education

SC.912.P.12

SC.912.P.12.12

new-york-state-science-learning-standards

New York State Science Learning Standards

HS. Chemical Reactions

HS-PS1-5.

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IPC.7.A

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IPC.7.B

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IPC.7.D

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Science concepts. The student knows that relationships exist between the structure and properties of matter. The student is expected to:

Tracking Reaction Kinetics with Light and Pressure

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