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

Le velocità di reazione possono essere studiate determinando la variazione delle concentrazioni di reagenti o prodotti in funzione del tempo. Le variazioni di concentrazione possono essere misurate mediante tecniche sperimentali come polarimetria, spettroscopia, o misurazioni della pressione. La polarimetria utilizza la luce polarizzata nel piano con un campo elettrico orientato lungo un solo piano, e misura la capacità dei composti di ruotare la luce polarizzata, che dipende dalla struttura molecolare del composto presente.

Consideriamo l'idrolisi del saccarosio, che produce glucosio e fruttosio. Un polarimetro viene utilizzato per misurare il grado di rotazione della luce polarizzata in un piano che passa attraverso la soluzione di saccarosio reagente. Il saccarosio provoca la rotazione in senso orario, mentre il glucosio e il fruttosio provocano la rotazione in senso antiorario.

Misurando il grado di rotazione della luce ad intervalli di tempo prestabiliti, è possibile calcolare le concentrazioni relative di saccarosio, glucosio o fruttosio, e determinare la velocità di reazione. Le velocità di reazione possono anche essere misurate utilizzando metodi spettrofotometrici, usando la capacità di reagenti o prodotti di assorbire la luce di lunghezze d'onda specifiche. Maggiore è la concentrazione della sostanza di interesse, più intenso sarà il suo assorbimento della luce.

Per esempio, l'idrogeno gassoso incolore reagisce con il vapore di iodio viola per formare ioduro di idrogeno incolore. Il vapore di iodio assorbe la luce nella regione giallo-verde e riflette la luce viola. Durante la reazione, uno spettrofotometro misura la quantità di luce assorbita dal campione e analizza la luce trasmessa.

Pertanto, con il progredire della reazione, la diminuzione della concentrazione dei vapori di iodio viene osservata dalla riduzione dell'assorbimento della luce giallo-verde. Utilizzando la legge Beer-Lambert, è possibile calcolare l'intensità della luce assorbita in diversi momenti e correlarla ai cambiamenti di concentrazione. In alternativa, se uno dei reagenti o dei prodotti è un gas, le misurazioni della pressione sono usate per determinare le velocità di reazione monitorando le variazioni di pressione.

Per esempio, durante la decomposizione del perossido di idrogeno, la velocità di reazione viene studiata utilizzando un manometro per monitorare la pressione del gas di ossigeno rilasciato. Man mano che la reazione progredisce e si evolve più ossigeno gassoso, la pressione aumenta. Utilizzando l'equazione del gas ideale, i valori di pressione registrati in diversi momenti vengono convertiti in concentrazioni.

La variazione della concentrazione in funzione del tempo viene usata per determinare la velocità di reazione. Per reazioni prolungate, campioni, o aliquote, possono essere prelevati dalla miscela di reazione a intervalli di tempo regolari. Le concentrazioni relative vengono dunque misurate utilizzando tecniche strumentali come gascromatografia, spettrometria di massa o titolazione, per calcolare le velocità di reazione.

misurazione della velocità di reazione

La polarimetria trova applicazione nella cinetica chimica per misurare la concentrazione e la cinetica di reazione di sostanze otticamente attive durante una reazione chimica. Le sostanze otticamente attive hanno la capacità di ruotare il piano di polarizzazione della luce polarizzata linearmente che le attraversa, una caratteristica chiamata rotazione ottica. L'attività ottica è attribuita alla struttura molecolare delle sostanze. La normale luce monocromatica non è polarizzata e possiede oscillazioni del campo elettrico in tutti i possibili piani perpendicolari alla direzione della sua propagazione. Quando una luce non polarizzata passa attraverso un polarizzatore, emerge una luce polarizzata linearmente che mantiene le oscillazioni su un piano.

Uno strumento polarimetro determina la direzione di polarizzazione della luce o la rotazione prodotta da una sostanza otticamente attiva. In un polarimetro, la luce polarizzata piana viene introdotta in un tubo contenente la soluzione reagente e la reazione può essere seguita senza disturbare il sistema. Se il campione contiene sostanze otticamente inattive, non ci sarà alcun cambiamento nell'orientamento del piano della luce polarizzata. La luce sarà visibile con la stessa intensità sullo schermo dell'analizzatore e la lettura dell'angolo di rotazione (ɑ) sarà pari a zero gradi.

Tuttavia, la presenza di composti otticamente attivi nel campione reagente provoca la rotazione del piano di passaggio della luce polarizzata. La luce che emergerà sarà meno intensa. L'asse del dispositivo analizzatore dovrà essere ruotato in senso orario (destrogiro) o antiorario (levogiro) per osservare la massima luminosità. La direzione in cui ruotare l'analizzatore dipende dalla natura del composto presente. La rotazione ottica misurata è proporzionale alla concentrazione delle sostanze otticamente attive presenti nel campione. Analizzando le misurazioni dell'angolo di rotazione in diversi momenti, è possibile determinare le concentrazioni dei composti otticamente attivi in funzione del tempo.

Spettrometria

Anche tecniche sperimentali ottiche come la spettrometria vengono spesso impiegate per monitorare le reazioni chimiche e garantire informazioni quantitative sulla cinetica di reazione. Utilizzando la spettrometria, la luce di una lunghezza d'onda specifica viene fatta passare attraverso un campione reagente. Le molecole o i composti (un reagente o un prodotto) all'interno del campione possono assorbire parte della luce mentre trasmettono la quantità rimanente, che viene misurata da un rilevatore. La quantità di luce assorbita dipende dalla concentrazione del composto o della molecola di interesse. Ad esempio, maggiore è la concentrazione di un composto, maggiore è la sua assorbanza. Dall'assorbanza lo strumento sarà in grado di determinare la concentrazione del composto di interesse. In un campione reagente, l'assorbanza misurata a intervalli periodici calcola le concentrazioni del reagente o del prodotto in funzione del tempo.

Misurazioni della pressione

Per le reazioni che coinvolgono sostanze in fase gassosa, la cinetica di reazione viene seguita quantificando le variazioni del numero di moli di gas in funzione delle variazioni di pressione. Le impostazioni sperimentali di una reazione in fase gassosa possono essere collegate a un manometro in grado di misurare la pressione di un reagente o di un prodotto gassoso. Man mano che la reazione procede, la pressione dei reagenti diminuisce e (o) la pressione dei prodotti aumenta. Questo può essere misurato dal manometro in funzione del tempo. Utilizzando la legge dei gas ideali (la concentrazione di un gas è proporzionale alla sua pressione parziale) è possibile calcolare la velocità di una reazione chimica.

Misurazione della velocità di reazione

La polarimetria trova applicazione nella cinetica chimica per misurare la concentrazione e la cinetica di reazione di sostanze otticamente attive durante un ...

Education

Didattica

JoVE Core

Chemistry

Chimica

Chemical Kinetics

Cinetica Chimica

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

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: Chimica illustra concetti chiave di chimica attraverso video animazioni concise e di immediata comprensione. Inoltre, i video della sezione "scienziati in azione" mostrano procedure metodologiche realizzate da ricercatori provenienti dai migliori laboratori internazionali.

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

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

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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Physical and Chemical Properties of Matter

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Physical Properties of Matter

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Alkali Metals

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Halogens

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Noble Gases


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Use patterns within the Periodic Table to predict the relative physical and chemical properties of elements.

IPC.7.B

Classifying Matter by State

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Classifying Matter by Composition

Matter: Pure Substances and Mixtures
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Phase Transitions

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IPC.7.C

Emission Spectra

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Explain how electrons can transition from a high energy level to a low energy state, emitting photons at different frequencies for different energy transitions.

IPC.7.D

The de Broglie Wavelength

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The Uncertainty Principle

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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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