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

Las velocidades de reacción se pueden estudiar determinando el cambio en las concentraciones de reactivos o productos en función del tiempo. Los cambios de concentración se pueden medir mediante técnicas experimentales como la polarimetría, la espectroscopia o con mediciones de presión. La polarimetría utiliza luz polarizada en un plano con un campo eléctrico orientado a lo largo de un solo plano.Mide la capacidad de los compuestos para rotar la luz polarizada, que depende de la estructura molecular del compuesto presente. Considere la hidrólisis de sacarosa, que produce glucosa y fructosa. Se utiliza un polarímetro para medir el grado de rotación de la luz polarizada plana que entra a través de la solución de sacarosa en reacción.La sacarosa provoca la rotación en el sentido de las agujas del reloj, mientras que la glucosa y la fructosa provocan la rotación en el sentido contrario a las agujas del reloj. Si se mide el grado de rotación de la luz a intervalos de tiempo establecidos, se pueden calcular las concentraciones relativas de sacarosa, glucosa o fructosa y determinar la velocidad de reacción. Las velocidades de reacción también se pueden medir utilizando métodos espectrofotométricos, utilizando la capacidad de los reactivos o productos para absorber luz de longitudes de onda específicas.Cuanto mayor sea la concentración de la sustancia de interés, más intensa será su absorbancia de luz. Por ejemplo, el gas hidrógeno incoloro reacciona con el vapor de yodo violeta para formar yoduro de hidrógeno incoloro. El vapor de yodo absorbe la luz en la región amarillo-verde y refleja la luz violeta.Durante la reacción, un espectrofotómetro mide la cantidad de luz absorbida por la muestra y analiza la luz transmitida. Así, a medida que avanza la reacción, la disminución de la concentración de vapor de yodo se observa mediante la reducción de la absorbancia de la luz amarillo-verde. Si empleamos la ley de Beer-Lambert, la intensidad de la luz absorbida en diferentes puntos de tiempo se puede calcular y relacionar con cambios en la concentración.Alternativamente, si uno de los reactivos o productos es un gas, las mediciones de presión se utilizan para determinar las velocidades de reacción controlando los cambios de presión. Por ejemplo, durante la descomposición del peróxido de hidrógeno, la velocidad de reacción se estudia usando un manómetro para controlar la presión del gas oxígeno liberado. A medida que avanza la reacción y se genera más oxígeno, la presión aumenta.Usando la ecuación del gas ideal, los valores de presión registrados en diferentes momentos se convierten en concentraciones. El cambio de concentración en función del tiempo se utiliza para determinar la velocidad de reacción. Para reacciones prolongadas, se pueden tomar muestras o alícuotas de la mezcla de reacción a intervalos de tiempo regulares.Las concentraciones relativas se miden luego utilizando técnicas instrumentales como la cromatografía de gases, la espectrometría de masas o la titulación, para calcular las velocidades de reacción.

midiendo las velocidades de reacción

La polarimetría encuentra aplicación en la cinética química para medir la concentración y la cinética de reacción de sustancias ópticamente activas durante una reacción química. Las sustancias ópticamente activas tienen la capacidad de rotar el plano de polarización de la luz linealmente polarizada que pasa a través de ellas, una característica llamada rotación óptica. La actividad óptica se atribuye a la estructura molecular de las sustancias. La luz monocromática normal no está polarizada y posee oscilaciones del campo eléctrico en todos los planos posibles perpendiculares a la dirección de su propagación. Cuando una luz no polarizada pasa a través de un polarizador, emerge una luz polarizada linealmente que mantiene oscilaciones en un plano.
 Un instrumento polarímetro determina la dirección de polarización de la luz o la rotación producida por una sustancia ópticamente activa. En un polarímetro, la luz polarizada en plano se introduce en un tubo que contiene la solución de reacción, y la reacción puede examinarse sin alterar el sistema. Si la muestra contiene sustancias ópticamente inactivas, no se producirá ningún cambio en la orientación del plano de la luz polarizada. La luz será visible en la misma intensidad en la pantalla del analizador y la lectura del ángulo de rotación (ɑ) indicará cero grados.
 Sin embargo, la presencia de compuestos ópticamente activos en la muestra que reacciona provoca la rotación del plano de luz polarizada que la atraviesa. La luz que salga será menos brillante. El eje del dispositivo analizador tendrá que girarse en sentido horario (dextrógiro) o en sentido antihorario (levógiro) para observar el brillo máximo. La dirección en la que se debe girar el analizador depende de la naturaleza del compuesto presente. La rotación óptica medida es proporcional a la concentración de las sustancias ópticamente activas presentes en la muestra. Al analizar las mediciones del ángulo de rotación en diferentes puntos de tiempo, las concentraciones de los compuestos ópticamente activos pueden determinarse en función del tiempo.
 Espectrometría
 Las técnicas experimentales ópticas como la espectrometría también se emplean con frecuencia para controlar las reacciones químicas y asegurar información cuantitativa sobre la cinética de las reacciones. Mediante la espectrometría, la luz de una longitud de onda específica se hace pasar a través de una muestra que reacciona. Las moléculas o compuestos (ya sea un reactivo o producto) dentro de la muestra pueden absorber algo de luz mientras transmiten la cantidad restante, que es medida por un detector. La cantidad de luz absorbida depende de la concentración del compuesto o molécula de interés. Por ejemplo, cuanto mayor sea la concentración de un compuesto, mayor será su absorbancia. A partir de la absorbancia, el instrumento podrá determinar la concentración del compuesto de interés. En una muestra que reacciona, la absorbancia medida a intervalos periódicos calcula las concentraciones del reactivo o producto en función del tiempo.
 Mediciones de presión
 En el caso de las reacciones que implican sustancias en fase gaseosa, la cinética de reacción va seguida de la cuantificación de los cambios en el número de moles de gases en función de los cambios de presión. Los ajustes experimentales de una reacción en fase gaseosa pueden conectarse a un manómetro que podría medir la presión de un reactivo gaseoso o de un producto. A medida que la reacción progresa, la presión de los reactivos disminuye y/o la presión de los productos aumenta. El manómetro puede medir este valor en función del tiempo. Al emplear la ley de los gases ideales—la concentración de un gas es proporcional a su presión parcial—se puede calcular la velocidad de una reacción química.

Midiendo las Velocidades de Reacción

La polarimetría encuentra aplicación en la cinética química para medir la concentración y la cinética de reacción de sustancias ópticamente activas ...

Education

Educación

JoVE Core

Chemistry

Química

Chemical Kinetics

Cinética Química

KEY TERMS AND CONCEPTS

TÉRMINOS Y CONCEPTOS CLAVE

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 da vida a los conceptos de química general mediante lecciones de vídeo basadas en animaciones, concisas y fáciles de entender. Además, en los vídeos de científicos en acción, se muestran experimentos de investigación clásicos y originales realizados en laboratorios de todo el 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.

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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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Ionization Energy

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Electron Affinity

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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


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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
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

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Phase Transitions: Sublimation and Deposition

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IPC.7.C

Emission Spectra

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Atomic Orbitals

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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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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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