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

Reactiesnelheden kunnen worden bestudeerd door de verandering in concentraties van reactanten of producten als functie van de tijd te bepalen. Concentratieveranderingen kunnen worden gemeten met experimentele technieken zoals polarimetrie, spectroscopie of drukmetingen. Polarimetrie maakt gebruik van vlakgepolariseerd licht met een elektrisch veld dat slechts langs één vlak is georiënteerd.Het meet het vermogen van verbindingen om gepolariseerd licht te roteren, wat afhangt van de moleculaire structuur van de aanwezige verbinding. Overweeg de hydrolyse van sucrose, die glucose en fructose oplevert. Een polarimeter wordt gebruikt om de mate van rotatie te meten van vlak gepolariseerd licht dat door de reagerende sucrose-oplossing valt.Sucrose veroorzaakt rotatie met de klok mee, terwijl glucose en fructose rotatie tegen de klok in veroorzaken. Door de mate van rotatie van licht op vaste tijdsintervallen te meten, kunnen de relatieve concentraties van sucrose, glucose of fructose worden berekend en de reactiesnelheid worden bepaald. Reactiesnelheden kunnen ook worden gemeten met behulp van spectrofotometrische methoden, waarbij gebruik wordt gemaakt van het vermogen van reactanten of producten om licht van specifieke golflengten te absorberen.Hoe hoger de concentratie van de betreffende stof, hoe intenser de lichtabsorptie ervan zal zijn. Kleurloos waterstofgas reageert bijvoorbeeld met violette jodiumdamp om kleurloos waterstofjodide te vormen. Jodiumdamp absorbeert licht in het geelgroene gebied en reflecteert violet licht.Tijdens de reactie meet een spectrofotometer de hoeveelheid licht die door het monster wordt geabsorbeerd en analyseert het doorgelaten licht. Dus naarmate de reactie vordert, wordt de afname van de jodiumdampconcentratie waargenomen door de vermindering van de geelgroene lichtabsorptie. Met behulp van de wet van Lambert-Beer kan de intensiteit van het op verschillende tijdstippen geabsorbeerde licht worden berekend en gerelateerd aan concentratieveranderingen.Als een van de reactanten of producten een gas is, worden drukmetingen gebruikt om reactiesnelheden te bepalen door drukveranderingen te volgen. Tijdens de ontleding van waterstofperoxide wordt bijvoorbeeld de reactiesnelheid bestudeerd met een manometer om de druk van het vrijkomende zuurstofgas te volgen. Naarmate de reactie vordert en er meer zuurstofgas vrijkomt, stijgt de druk.Met behulp van de ideale gasvergelijking worden drukwaarden die op verschillende tijdstippen zijn geregistreerd, omgezet in concentraties. De concentratieverandering als functie van de tijd wordt gebruikt om de reactiesnelheid te bepalen. Voor langdurige reacties kunnen met regelmatige tussenpozen monsters of aliquots uit het reactiemengsel worden genomen.De relatieve concentraties worden vervolgens gemeten met behulp van instrumentele technieken zoals gaschromatografie, massaspectrometrie of titratie om reactiesnelheden te berekenen.

reactiesnelheden meten

Polarimetrie vindt toepassing in de chemische kinetiek om de concentratie en reactiekinetiek van optisch actieve stoffen tijdens een chemische reactie te meten. Optisch actieve stoffen hebben het vermogen om het polarisatievlak van lineair gepolariseerd licht dat erdoorheen gaat te roteren - een kenmerk dat optische rotatie wordt genoemd. Optische activiteit wordt toegeschreven aan de moleculaire structuur van stoffen. Normaal monochromatisch licht is ongepolariseerd en bezit oscillaties van het elektrische veld in alle mogelijke vlakken loodrecht op de voortplantingsrichting. Wanneer ongepolariseerd licht door een polarisator gaat, komt er lineair gepolariseerd licht naar buiten dat oscillaties in één vlak handhaaft. Een polarimeterinstrument bepaalt de polarisatierichting van het licht of de rotatie geproduceerd door een optisch actieve stof. In een polarimeter wordt het vlakgepolariseerde licht in een buis gebracht die de reagerende oplossing bevat, en de reactie kan worden gevolgd zonder het systeem te verstoren. Als het monster optisch inactieve stoffen bevat, zal er geen verandering optreden in de oriëntatie van het vlak van het gepolariseerde licht. Het licht zal met dezelfde intensiteit zichtbaar zijn op het scherm van de analysator, en de rotatiehoek (ɑ) zal nul graden aangeven. De aanwezigheid van optisch actieve verbindingen in het reagerende monster veroorzaakt echter de rotatie van het vlak van het gepolariseerde licht dat erdoorheen gaat. Het licht dat naar buiten komt, zal minder helder zijn. De as van het analyseapparaat zal met de klok mee (rechtsdraaiend) of tegen de klok in (linksomdraaiend) moeten worden gedraaid om de maximale helderheid waar te nemen. De richting waarin de analysator moet worden gedraaid, is afhankelijk van de aard van de aanwezige verbinding. De gemeten optische rotatie is evenredig met de concentratie van de optisch actieve stoffen die in het monster aanwezig zijn. Door de rotatiehoekmetingen op verschillende tijdstippen te analyseren, kunnen de concentraties van de optisch actieve verbindingen als functie van de tijd worden bepaald. Spectrometrie Optische experimentele technieken zoals spectrometrie worden ook vaak gebruikt om chemische reacties te monitoren en kwantitatieve informatie over reactiekinetiek veilig te stellen. Met behulp van spectrometrie wordt het licht van een specifieke golflengte door een reagerend monster geleid. De moleculen of verbindingen (een reactant of product) in het monster kunnen wat licht absorberen terwijl ze de resterende hoeveelheid doorlaten, wat wordt gemeten door een detector. De hoeveelheid geabsorbeerd licht hangt af van de concentratie van de verbinding of het molecuul in kwestie. Hoe hoger de concentratie van een verbinding, hoe groter de absorptie ervan. Op basis van de absorptie kan het instrument de concentratie van de betreffende verbinding bepalen. In een reagerend monster berekent de met periodieke intervallen gemeten absorptie de concentraties van de reactant of het product als functie van de tijd. Drukmetingen Voor reacties waarbij stoffen in de gasfase betrokken zijn, wordt de reactiekinetiek gevolgd door het kwantificeren van de veranderingen in het aantal mol gassen als functie van de drukveranderingen. De experimentele instellingen van een gasfasereactie kunnen worden aangesloten op een manometer die de druk van een gasvormig reagens of product kan meten. Naarmate de reactie vordert, neemt de druk van de reactanten af en (of) neemt de druk van de producten toe. Dit kan worden gemeten door de manometer als functie van de tijd. Door gebruik te maken van de ideale gaswet – de concentratie van een gas is evenredig met de partiële druk ervan – kan de snelheid van een chemische reactie worden berekend.

Reactiesnelheden meten

Polarimetry finds application in chemical kinetics to measure the concentration and reaction kinetics of optically active substances during a chemical ...

Education

JoVE Core

Chemistry

Scheikunde

Eén

Chemical Kinetics

Chemische kinetiek

KEY TERMS AND CONCEPTS

BELANGRIJKE TERMEN EN CONCEPTEN

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

chemistry

JoVE Core Chemistry brings general chemistry concepts to life using concise and easy-to-understand animated video lessons. Plus, in the scientist-in-action videos, we demonstrate classical and original research experiments performed in today's laboratories worldwide.

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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

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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.B

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IPC.7.D

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IPC.7.E

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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