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

Reaktionsgeschwindigkeiten können untersucht werden, indem die Änderung der Konzentrationen von Reaktanten oder Produkten in Abhängigkeit von der Zeit bestimmt wird.
Konzentrationsänderungen können durch experimentelle Techniken wie Polarimetrie, Spektroskopie oder Druckmessungen gemessen werden.
Die Polarimetrie verwendet planpolarisiertes Licht mit einem elektrischen Feld, das nur entlang einer Ebene ausgerichtet ist. Es misst die Fähigkeit von Verbindungen, polarisiertes Licht zu drehen, was von der molekularen Struktur der vorhandenen Verbindung abhängt.
Betrachten Sie die Hydrolyse von Saccharose, die Glukose und Fruktose liefert. Ein Polarimeter wird verwendet, um den Rotationsgrad von planpolarisiertem Licht zu messen, das durch die reagierende Saccharoselösung kommt. Saccharose bewirkt eine Drehung im Uhrzeigersinn, während Glukose und Fruktose eine Drehung gegen den Uhrzeigersinn verursachen.
Durch die Messung des Rotationsgrades des Lichts in festgelegten Zeitintervallen können die relativen Konzentrationen von Saccharose, Glukose oder Fruktose berechnet und die Reaktionsgeschwindigkeit bestimmt werden.
Reaktionsgeschwindigkeiten können auch mit spektrophotometrischen Methoden gemessen werden, wobei die Fähigkeit von Reaktanten oder Produkten genutzt wird, Licht bestimmter Wellenlängen zu absorbieren. Je höher die Konzentration der interessierenden Substanz ist, desto intensiver ist ihre Lichtabsorption.
Zum Beispiel reagiert farbloses Wasserstoffgas mit violettem Joddampf zu farblosem Jodidwasserstoff. Joddampf absorbiert Licht im gelb-grünen Bereich und reflektiert violettes Licht.
Während der Reaktion misst ein Spektralphotometer die von der Probe absorbierte Lichtmenge und analysiert das durchgelassene Licht. So wird im Verlauf der Reaktion die Abnahme der Joddampfkonzentration durch die Verringerung der gelb-grünen Lichtabsorption beobachtet.
Mit Hilfe des Beer-Lambert-Gesetzes kann die Intensität des zu verschiedenen Zeitpunkten absorbierten Lichts berechnet und mit Konzentrationsänderungen in Beziehung gesetzt werden.
Wenn es sich bei einem der Reaktanten oder Produkte um ein Gas handelt, werden alternativ Druckmessungen verwendet, um die Reaktionsgeschwindigkeit durch Überwachung von Druckänderungen zu bestimmen.
Zum Beispiel wird bei der Zersetzung von Wasserstoffperoxid die Reaktionsgeschwindigkeit mit einem Manometer untersucht, um den Druck des freigesetzten Sauerstoffgases zu überwachen. Wenn die Reaktion fortschreitet und mehr Sauerstoffgas entsteht, steigt der Druck.
Mit Hilfe der idealen Gasgleichung werden Druckwerte, die zu unterschiedlichen Zeitpunkten aufgezeichnet wurden, in Konzentrationen umgerechnet. Die Konzentrationsänderung in Abhängigkeit von der Zeit wird zur Bestimmung der Reaktionsgeschwindigkeit herangezogen.
Bei längeren Reaktionen können in regelmäßigen Zeitabständen Proben oder Aliquots aus dem Reaktionsgemisch entnommen werden. Die relativen Konzentrationen werden dann mit instrumentellen Techniken wie Gaschromatographie, Massenspektrometrie oder Titration gemessen, um die Reaktionsgeschwindigkeiten zu berechnen.

reaktionsraten messen

Polarimetrie findet in der chemischen Kinetik Anwendung, um die Konzentration und Reaktionskinetik optisch aktiver Substanzen während einer chemischen Reaktion zu messen. Optisch aktive Substanzen haben die Fähigkeit, die Polarisationsebene des durch sie hindurchtretenden linear polarisierten Lichts zu drehen – eine Eigenschaft, die als optische Drehung bezeichnet wird. Die optische Aktivität wird auf die molekulare Struktur von Stoffen zurückgeführt. Normales monochromatisches Licht ist unpolarisiert und weist Schwingungen des elektrischen Feldes in allen möglichen Ebenen senkrecht zu seiner Ausbreitungsrichtung auf. Wenn unpolarisiertes Licht einen Polarisator passiert, tritt linear polarisiertes Licht aus, das Schwingungen in einer Ebene beibehält. Ein Polarimeter-Instrument bestimmt die Polarisationsrichtung des Lichts oder die Rotation, die von einer optisch aktiven Substanz erzeugt wird. In einem Polarimeter wird das linear polarisierte Licht in ein Röhrchen eingeleitet, das die Reaktionslösung enthält, und die Reaktion kann verfolgt werden, ohne das System zu stören. Enthält die Probe optisch inaktive Substanzen, ändert sich die Ausrichtung der Ebene des polarisierten Lichts nicht. Das Licht wird in der gleichen Intensität auf dem Bildschirm des Analysegeräts sichtbar sein und der Drehwinkelwert (ɑ) beträgt null Grad. Das Vorhandensein optisch aktiver Verbindungen in der reagierenden Probe führt jedoch zu einer Drehung der Ebene des durchtretenden polarisierten Lichts. Das austretende Licht wird weniger hell sein. Um die maximale Helligkeit zu beobachten, muss die Achse des Analysegeräts im Uhrzeigersinn (rechtsdrehend) oder gegen den Uhrzeigersinn (linksdrehend) gedreht werden. Die Richtung, in die der Analysator gedreht werden muss, hängt von der Art der vorhandenen Verbindung ab. Die gemessene optische Drehung ist proportional zur Konzentration der in der Probe vorhandenen optisch aktiven Substanzen. Durch die Analyse der Drehwinkelmessungen zu verschiedenen Zeitpunkten können die Konzentrationen der optisch aktiven Verbindungen als Funktion der Zeit bestimmt werden. Spektrometrie Auch optische experimentelle Techniken wie die Spektrometrie werden häufig zur Überwachung chemischer Reaktionen und zur Gewinnung quantitativer Informationen zur Reaktionskinetik eingesetzt. Bei der Spektrometrie wird Licht einer bestimmten Wellenlänge durch eine reagierende Probe geleitet. Die Moleküle oder Verbindungen (entweder ein Reaktant oder ein Produkt) in der Probe absorbieren möglicherweise einen Teil des Lichts, während sie die verbleibende Menge durchlassen, die von einem Detektor gemessen wird. Die absorbierte Lichtmenge hängt von der Konzentration der interessierenden Verbindung oder des interessierenden Moleküls ab. Je höher beispielsweise die Konzentration einer Verbindung ist, desto größer ist ihre Absorption. Anhand der Absorption kann das Gerät die Konzentration der interessierenden Verbindung bestimmen. In einer reagierenden Probe berechnet die in regelmäßigen Abständen gemessene Absorption die Konzentrationen des Reaktanten oder Produkts als Funktion der Zeit. Druckmessungen Bei Reaktionen, an denen Substanzen in der Gasphase beteiligt sind, wird die Reaktionskinetik verfolgt, indem die Änderungen der Molzahl von Gasen als Funktion der Druckänderungen quantifiziert werden. Die experimentellen Einstellungen einer Gasphasenreaktion können mit einem Manometer verbunden werden, das den Druck eines gasförmigen Reaktanten oder Produkts messen kann. Mit fortschreitender Reaktion nimmt der Druck der Reaktanten ab und (oder) der Druck der Produkte steigt. Dies kann mit dem Manometer als Funktion der Zeit gemessen werden. Mithilfe des idealen Gasgesetzes – die Konzentration eines Gases ist proportional zu seinem Partialdruck – kann die Geschwindigkeit einer chemischen Reaktion berechnet werden.

Reaktionsraten messen

Die Polarimetrie findet Anwendung in der chemischen Kinetik, um die Konzentration und Reaktionskinetik von optisch aktiven Substanzen während eines chemischen Prozesses zu messen.

Education

Lehre

JoVE Core

Chemistry

Chemie

Eins

Chemical Kinetics

Chemische Kinetik

KEY TERMS AND CONCEPTS

KERNBEGRIFFE UND KONZEPTE

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 Chemie erweckt allgemeine Konzepte der Chemie durch kompakte und leicht verständliche animierte Videolektionen zum Leben. Außerdem zeigen wir in den Videos Wissenschaftler in Aktion bei klassischen und neuartigen Forschungsexperimenten, wie sie heutzutage in Laboren weltweit durchgeführt wurden.

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North Carolina 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.A

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Physical and Chemical Properties of Matter

The characteristics that enable us to distinguish one substance from another are called properties.

Physical Properties of Matter

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Atomic Radii and Effective Nuclear Charge

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Noble Gases


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IPC.7.B

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Classifying Matter by Composition

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IPC.7.C

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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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