Name: measuring reaction rates
Uploaded: 2020-09-24T15:06:50.000Z
Duration: 2 min 53 s
Channel: JoVE

K12 Standards

Texas Essential Knowledge and Skills

High School

Chemistry and Physics

IPC.7

IPC.7.F

Reaction Rate

Una reacción química implica la transformación química de reactivos en productos. A medida que la reacción avanza, la concentración de reactivos disminuye mientras que aumenta la concentración del producto. Esta variación en las concentraciones de reactivos y productos se puede representar en un gráfico como una función de tiempo.La velocidad a la que progresa una reacción se denomina velocidad de reacción. Mide la velocidad de desaparición de los reactivos o la velocidad de aparición de los productos y se expresa en unidades de molaridad por segundo. La velocidad de reacción promedio se puede calcular a partir del cambio en las concentraciones de reactivos o productos durante un período de tiempo específico.El valor de concentración molar se indica entre paréntesis, la t representa el período de tiempo y el símbolo delta representa cambia a"Dado que los reactivos se agotan durante una reacción química, el valor del cambio de concentración de los reactivos es siempre negativo. Por lo tanto, una velocidad de reacción calculada en términos de reactivos se complementa con un signo negativo, para hacer que el valor general sea positivo. La velocidad de reacción no es uniforme en toda la reacción.La velocidad al inicio de la reacción, en la hora cero"se denomina velocidad de reacción inicial. A medida que se reducen las concentraciones de reactivos, la velocidad de reacción disminuye o la reacción se ralentiza. La velocidad de reacción en un punto de tiempo dado, o la velocidad de reacción instantánea, se mide calculando la pendiente de una tangente trazada a la curva de reacción, ya sea para reactivos o productos, en el momento de interés.El valor de la pendiente o la velocidad instantánea en un punto de tiempo específico es igual para todos los reactivos y productos. La velocidad de reacción de una reacción química también refleja los coeficientes estequiométricos reales de los reactivos y productos. Entonces, para cualquier reacción balanceada, donde a moles de A reacciona con b moles de B, produciendo c moles de C y d moles de D, la velocidad de reacción se puede expresar y calcular usando la fórmula genérica:menos 1/a veces el cambio en concentración del reactivo A con el tiempo, igual a menos 1/b veces el cambio de concentración del reactivo B con el tiempo.Estos valores son los mismos que más 1/c veces, el cambio en la concentración del producto C con el tiempo, que es lo mismo que más 1/d veces que cambia la concentración del producto D con el tiempo. Determinar las velocidades de reacción es fundamental para estudiar la cinética química, lo que ayuda a comprender la rapidez con la que funciona un fármaco, un catalizador o una reacción sintética para que pueda controlarse y optimizarse mejor en su función.

velocidad de reacción

La velocidad de reacción es el cambio en la cantidad de un reactivo o producto por unidad de tiempo. Por lo tanto, las velocidades de reacción se determinan midiendo la dependencia del tiempo de alguna propiedad que puede estar relacionada con las cantidades de reactivo o producto. Las velocidades de reacciones que consumen o producen sustancias gaseosas, por ejemplo, se determinan convenientemente midiendo los cambios de volumen o presión.
 La representación matemática del cambio en la concentración de reactivos y productos, a lo largo del tiempo, es la expresión de la velocidad de la reacción. Los corchetes indican las concentraciones molares y el símbolo delta (Δ) indica “cambio”.
 Por ejemplo, la concentración de peróxido de hidrógeno, H2O2, en una solución acuosa cambia lentamente con el tiempo a medida que se descompone según la ecuación:
  <img alt="Eq1" src="/files/ftp_upload/11373/11373_Equation_1.png" style="height: 50px;" />
 La velocidad a la que se descompone el peróxido de hidrógeno puede expresarse en términos de la velocidad de cambio de su concentración:
  <img alt="Eq2" src="/files/ftp_upload/11373/11373_Equation_2.png" style="height: 70px;" />
 Así, [H2O2]t1 representa la concentración molar del peróxido de hidrógeno en el tiempo t1; del mismo modo, [H1O2]t2 representa la concentración molar del peróxido de hidrógeno en un momento posterior t2; y Δ[H2O2] representa el cambio en la concentración molar del peróxido de hidrógeno durante el intervalo de tiempo Δt (es decir, t2 − t1). Dado que la concentración de reactivo disminuye a medida que la reacción avanza, Δ[H2O2] es una cantidad negativa. Las velocidades de reacción son, por convención, cantidades positivas, por lo que este cambio negativo en la concentración se multiplica por −1.
 Velocidad de reacción media y velocidad de reacción instantánea
 Las velocidades de reacción varían con el tiempo y disminuyen a medida que avanza la reacción. Se puede calcular una velocidad de reacción media durante un intervalo de tiempo utilizando las concentraciones al principio y al final de este período, durante las cuales la velocidad de reacción está cambiando. En cualquier momento específico, la velocidad a la que está procediendo una reacción se conoce como su velocidad instantánea. La velocidad instantánea de una reacción en “tiempo cero”, cuando comienza la reacción, es su velocidad inicial.
 La velocidad instantánea de una reacción puede ser determinada por una de las dos maneras. Si las condiciones experimentales permiten la medición de los cambios de concentración en intervalos de tiempo cortos, el cálculo de las velocidades medias proporciona aproximaciones razonablemente buenas de las velocidades instantáneas. Alternativamente, se puede utilizar un procedimiento gráfico. Por ejemplo, en el ejemplo de descomposición del peróxido de hidrógeno, al graficar la concentración de peróxido de hidrógeno frente al tiempo, la velocidad instantánea de descomposición del H2O2 puede calcularse en cualquier momento ‘t’ desde la pendiente de una tangente trazada a la curva en ese momento.
  <img alt="Image1" src="/files/ftp_upload/11373/11373_Image_1.png" style="height: 300px;" />
 Este es un gráfico de la concentración frente al tiempo para una solución de H2O2 de 1.000 M. La velocidad en cualquier momento es igual al negativo de la pendiente de una línea tangente a la curva en ese momento. Las tangentes se muestran en t = 0 h (“velocidad inicial”) y en t = 12 h (“velocidad instantánea” a 12 h).
 Velocidades relativas de reacción
 La velocidad de una reacción puede expresarse como el cambio en la concentración de cualquier reactivo o producto. Para cualquier reacción dada, estas expresiones de velocidad están todas relacionadas entre sí según la estequiometría de la reacción. La velocidad de la reacción general aA ⟶ bB puede expresarse en términos de la disminución de la concentración de A o el aumento de la concentración de B. Estas dos expresiones de velocidad están relacionadas por la estequiometría de la reacción, donde:
  <img alt="Eq3" src="/files/ftp_upload/11373/11373_Equation_3.png" style="height: 70px;" />
 Tenga en cuenta que se ha incluido un signo negativo como factor para tener en cuenta los signos opuestos de los dos cambios de cantidad (la cantidad de reactivo disminuye mientras que la cantidad de producto aumenta).
 Este texto es adaptado de<a href="https://openstax.org/books/chemistry-2e/pages/12-introduction"> Openstax, Química 2e, Capítulo 12 Introducción</a> y <a href="https://openstax.org/books/chemistry-2e/pages/12-1-chemical-reaction-rates#12163"> Openstax, Química 2e, Sección 12.1: Velocidades de reacción química.</a>

Velocidad de Reacción

La velocidad de reacción es el cambio en la cantidad de un reactivo o producto por unidad de tiempo. Por lo tanto, las velocidades de reacción se determinan midiendo el tiempo ...

Education

Educación

JoVE Core

Chemistry

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Chemical Kinetics

Cinética Química

KEY TERMS AND CONCEPTS

TÉRMINOS Y CONCEPTOS CLAVE

measuring reaction rates

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 over time. In this expression, brackets mean molar concentration, and the delta symbol means change in amount.

Hydrogen peroxide provides a clear example. As H2O2 decomposes in water, its concentration slowly decreases with time. The reaction rate can be written as the rate of change of the H2O2 concentration. If the concentration at time t1 is compared with the concentration at a later time t2, the change in concentration over the time interval Δt is found.

Because the reactant concentration goes down, Δ[H2O2] is negative. Reaction rates are reported as positive values, so the negative sign is reversed by multiplying by -1. This keeps the reaction rate positive even when the reactant amount is falling.

Reaction rates are not constant. They usually change as the reaction continues, so an average reaction rate is calculated over a time interval using the starting and ending concentrations. The rate at one exact moment is the instantaneous rate, and the instantaneous rate at time zero is the initial rate.

Instantaneous rate can be estimated in two ways. If concentration changes can be measured over very short time intervals, the average rate gives a good approximation. A graph can also be used. When concentration is plotted against time, the instantaneous rate at a given time equals the negative slope of a tangent line to the curve. In the hydrogen peroxide example, tangents can be drawn at t = 0 h for the initial rate and at t = 12 h for the instantaneous rate at 12 h.

The rate of a reaction can be written using either a reactant or a product. For a general reaction aA → bB, the decrease in A and the increase in B are linked by the reaction stoichiometry. The two rate expressions are related by that stoichiometry, and a negative sign is included to account for the reactant decreasing while the product increases.

Measuring Reaction Rates

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...

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...

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

Learn how reaction rates are measured using concentration, volume, pressure, average rate, and instantaneous rate with the hydrogen peroxide example.

Measuring Reaction Rates | 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.

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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

Louisiana Student Standards for Science

Matter And Its Interactions

HS-PS1-4

HS-PS1-6

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Ohio’s Learning Standards and Model Curriculum

Interactions Of Matter

Chemical reactions

Virginia  Standards of Learning

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CH.7

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Minnesota Academic Standards in Science

3.2.1 9C.3.1.1.2

Arizona Science Standards

HS.P1

HS.P1U1.3

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New Jersey Student Learning Standards for Science

Matter and Its Interactions

HS-PS1-5

Alabama Course of Study Science

Matter and Its Interactions: Energy

9-12.C.3.3.d

Next Generation Science Standards

Florida Department of Education

SC.912.P.12

SC.912.P.12.12

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New York State Science Learning Standards

HS. Chemical Reactions

HS-PS1-5.

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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

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Measuring Reaction Rates

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