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

As taxas de reação podem ser estudadas determinando a mudança em concentrações de reagentes ou produtos como função do tempo. Mudanças de concentração podem ser medidas por técnicas experimentais como polarimetria, espectroscopia, ou medições de pressão. Usos da polarimetria, luz polarizada plana com um campo elétrico orientado ao longo de apenas um plano.Mede o capacidade dos compostos para rotar a luz polarizada, o que depende da estrutura molecular do complexo presente. Considerando a hidrólise de sacarose, que produz glicose e frutose. Um polarímetro é utilizado para medir o grau de rotação de luz polarizada plana a chegar através da sacarose reagente na solução.Causas da rotação da sacarose no sentido dos ponteiros do relógio, enquanto que a glicose e frutose causam rotação no sentido contrário ao dos ponteiros do relógio. Ao medir o grau de rotação da luz em intervalos de tempo definidos, as concentrações relativas de sacarose, glicose, ou a frutose podem ser calculadas e a taxa da reação determinada. As taxas de reação podem também ser medidas usando métodos espectrofotométricos, utilizando a capacidade de reagentes ou produtos para absorver a luz de comprimentos de onda específicos.Quanto maior for a concentração da substância de interesse, mais intensa é a sua absorção de luz. Por exemplo, o gás hidrogênio incolor reage com vapor de iodo violeta para formar iodeto de hidrogénio sem cor. O vapor de iodo absorve a luz na região amarelo-esverdeada e reflete a luz violeta.Durante a reação, um espectrofotómetro mede o montante de luz absorvida pela amostra e faz análises na luz transmitida. Assim, como a reação avança, a diminuição na concentração de vapor de iodo é observada pela redução da absorvência de luz amarelo-esverdeada. Usando a lei de Beer Lambert, a intensidade da luz absorvida em diferentes pontos de tempo pode ser calculada e relacionada a mudanças na concentração.Em alternativa, se um dos reagentes ou produtos é um gás, são utilizadas medições de pressão para determinar as taxas de reação através da monitorização das mudanças de pressão. Por exemplo, durante a decomposição do peróxido de hidrogênio, a taxa de reação é estudada utilizando um manómetro para monitorizar a pressão de oxigénio gasoso libertado. À medida que a reação avança e mais oxigénio gasoso evolui, a pressão aumenta.Usando a equação de gás ideal, valores de pressão registado em pontos de tempo diferentes são convertidos em concentrações. A mudança na concentração em função do tempo é utilizada para determinar a taxa de reação. Para reações prolongadas, amostras, ou alíquotas, podem ser retiradas da mistura em reação em intervalos regulares de tempo.A semelhança nas concentrações são então medidas usando técnicas instrumentais como a cromatografia de gás, espectroscopia de massa, ou titulação, para calcular as taxas de reação.

medição das velociades de reação

A polarimetria encontra aplicação em cinética química para medir a concentração e a cinética de reação de substâncias opticamente ativas durante uma reação química. As substâncias opticamente ativas têm a capacidade de rodar o plano de polarização da luz polarizada linear que passa por elas—uma característica chamada rotação óptica. A atividade óptica é atribuída à estrutura molecular das substâncias. A luz monocromática normal não é polarizada e possui oscilações do campo elétrico em todos os planos possíveis perpendiculares à direção da sua propagação. Quando uma luz não polarizada passa através de um polarizador, surge uma luz polarizada linearmente que mantém as oscilações em um único plano.
Um instrumento de polarímetro determina o sentido de polarização da luz ou a rotação produzida por uma substância opticamente ativa. Em um polarímetro, a luz polarizada no plano é introduzida em um tubo que contém a solução de reação e a reação pode ser seguida sem perturbar o sistema. Se a amostra contiver substâncias opticamente inativas, não haverá qualquer alteração na orientação do plano da luz polarizada. A luz será visível à mesma intensidade no ecrã do analisador, e a leitura do ângulo de rotação (ɑ) será de zero graus.
No entanto, a presença de compostos opticamente ativos na amostra a reagir provoca a rotação do plano da luz polarizada que passa. A luz que sai será menos brilhante. O eixo do dispositivo analisador terá de ser rodado no sentido dos ponteiros do relógio (dextrógiro) ou no sentido contrário ao dos ponteiros do relógio (levógiro) para observar o brilho máximo. A direção em que o analisador tem de ser rodado depende da natureza do composto presente. A rotação óptica medida é proporcional à concentração das substâncias opticamente ativas presentes na amostra. Ao analisar o ângulo das medições de rotação em diferentes pontos temporais, as concentrações dos compostos opticamente ativos podem ser determinadas em função do tempo.
Espectrometria
Técnicas experimentais ópticas como a espectrometria também são frequentemente utilizadas para monitorizar reações químicas e garantir informações quantitativas sobre a cinética de reação. Utilizando a espectrometria, a luz de um comprimento de onda específico é feita passar através de uma amostra a reagir. As moléculas ou compostos (um reagente ou produto) dentro da amostra podem absorver alguma luz enquanto transmitem a quantidade restante, que é medida por um detector. A quantidade de luz absorvida depende da concentração do composto ou molécula de interesse. Por exemplo, quanto maior for a concentração de um composto, maior será a sua absorvância. A partir da absorvância, o instrumento será capaz de determinar a concentração do composto de interesse. Em uma amostra a reagir, a absorvância medida a intervalos periódicos calcula as concentrações do reagente ou do produto em função do tempo.
Medições de Pressão
Para reações que envolvam substâncias em fase gasosa, a cinética de reação é seguida pela quantificação das alterações no número de moles de gases em função das alterações de pressão. Os parâmetros experimentais de uma reação em fase gasosa podem ser ligados a um manómetro que possa medir a pressão de um reagente ou produto gasoso. À medida que a reação progride, a pressão dos reagentes diminui, e(ou) a pressão dos produtos aumenta. Isto pode ser medido pelo manómetro em função do tempo. Ao empregar a lei de gás ideal—a concentração de um gás é proporcional à sua pressão parcial—a velocidade de uma reação química pode ser calculada.

Medição das Velociades de Reação

A polarimetria encontra aplicação na cinética química para medir a concentração e a cinética de reação de substâncias opticamente ativas durante um processo químico...

Education

JoVE Core

Chemistry

Química

Chemical Kinetics

Cinética Química

KEY TERMS AND CONCEPTS

TERMOS-CHAVE E CONCEITOS

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 - Química dá vida a conceitos gerais de química usando vídeos de animação fáceis de entender. Além disso, nos vídeos Cientistas em ação, demonstramos experimentos de pesquisa clássicos e originais realizados atualmente por cientistas em laboratórios em todo o mundo.

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.

The Atomic Theory of Matter

The earliest recorded discussion of the basic structure of matter comes from ancient Greek philosophers. Leucippus and Democritus argued that all matter ...

Subatomic Particles

Dalton was only partially correct about the particles that make up matter. All matter is composed of atoms, and atoms are composed of three smaller ...

Elements: Chemical Symbols and Isotopes

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Ions and Ionic Charges

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The Periodic Table

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Atomic Mass

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Molar Mass

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Periodic Classification of the Elements

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Types of Chemical Bonds

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Electronegativity

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Bond Polarity

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Lewis Structures of Molecular Compounds and Polyatomic Ions

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Bond Energies and Bond Lengths

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Bonding in Metals

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Model basic atomic structure and relate an element's atomic structure to its bonding, reactivity, and placement on the Periodic Table.

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

A physical property is ...

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Atomic Radii and Effective Nuclear Charge

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Ionic Radii

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Ionization Energy

The amount of energy required to remove the most loosely bound electron from a gaseous atom in its ground state is called its first ionization energy ...

Electron Affinity

The electron affinity (EA) is the energy change for adding an electron to a gaseous atom to form an anion (negative ion).



This process can be either ...

Alkali Metals

Group 1 elements are soft and shiny metallic solids. They are malleable, ductile, and good conductors of heat and electricity. The melting points of the ...

Halogens

Group 17 elements, known as halogens, are nonmetals. At room temperature, fluorine and chlorine are gases, bromine is a liquid, and iodine a solid. ...

Noble Gases


The elements in group 18 are noble gases (helium, neon, argon, krypton, xenon, and radon). They earned the name &ldquo;noble&rdquo; because they were ...

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

IPC.7.B

Classifying Matter by State

Chemistry is the study of matter and the changes it undergoes. Matter is anything that has mass and occupies space. Matter is all around us; the air, ...

Classifying Matter by Composition

Matter: Pure Substances and Mixtures
According to its composition, the matter can be classified into two broad categories &mdash; pure substances and ...

Phase Transitions

Whether solid, liquid, or gas, a substance&#39;s state depends on the order and arrangement of its particles (atoms, molecules, or ions). Particles in the ...

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Phase Transitions: Vaporization and Condensation

The physical form of a substance changes on changing its temperature. For example, raising the temperature of a liquid causes the liquid to vaporize ...

Phase Transitions: Melting and Freezing

Heating a crystalline solid increases the average energy of its atoms, molecules, or ions, and the solid gets hotter. At some point, the added energy ...

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Phase Transitions: Sublimation and Deposition

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Melting Points

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Boiling Points

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IPC.7.C

Emission Spectra

When solids, liquids, or condensed gases are heated sufficiently, they radiate some of the excess energy as light. Photons produced in this manner have a ...

Metal Flame Emission

Metal Flame Emission
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Atomic Orbitals

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The Pauli Exclusion Principle

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The Energies of Atomic Orbitals

In an atom, the negatively charged electrons are attracted to the positively charged nucleus. In a multielectron atom, electron-electron repulsions are ...

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

In the macroscopic world, objects that are large enough to be seen by the naked eye follow the rules of classical physics. A billiard ball moving on a ...

The Uncertainty Principle

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The Quantum-Mechanical Model of an Atom

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Quantum Numbers

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Balmer Series

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Explain how atomic energy levels and emission spectra present evidence for the wave particle duality.

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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