Name: reaction order and rate constants
Uploaded: 2020-03-26T07:22:26.000Z
Duration: 4 min 37 s
Channel: JoVE

K12 Standards

Minnesota Academic Standards in Science

9-12

Chemistry

3.2.1 9C.3.1.1.2

Concentration Dependence

La medida de la rapidez con la que se produce una reacción se denomina velocidad de reacción. Para una reacción de un solo paso, la velocidad es igual al cambio en la concentración de cada reactivo o producto a lo largo del tiempo multiplicado por el inverso del coeficiente estequiométrico correspondiente. Puedes pensar en el cambio en la concentración a lo largo del tiempo como la concentración en el tiempo t menos la concentración inicial dividida por t. Las velocidades de reacción son siempre positivas, por lo que las expresiones reactivas tienen signos negativos. Es posible que estas tasas no sean iguales en reacciones de varios pasos, pero aún podemos usar esta relación como una estimación de la reacción general.
La ley de tasas, o ecuación de tasas, describe la relación entre la velocidad de la reacción y las concentraciones de reactivos. En esta ecuación, k es la constante de velocidad, A y B son los dos reactivos, y m y n son sus respectivos órdenes de reacción. El orden de reacción describe la relación entre la concentración de un reactivo y la velocidad y no está relacionado con la estequiometría. Es vital recordar que el orden de reacción no es el mismo que el coeficiente del reactivo en la ecuación equilibrada.
Cuando una reacción involucra dos o más reactivos, debemos considerar el orden de reacción para cada reactivo. El orden de reacción general es igual a la suma de los órdenes de reacción de los reactivos. Por ejemplo, si el reactivo A es de primer orden y el reactivo B es de orden cero, el orden de reacción general es uno. Los órdenes de reacción más comunes en las reacciones simples son de orden cero, primer orden y segundo orden. Vamos a repasarlos usando una reacción unimolecular como ejemplo.
Si la reacción es de orden cero, la concentración del reactivo no tiene ningún efecto sobre la velocidad de reacción. Por lo tanto, la velocidad de reacción es igual a k, la constante de la velocidad. Un gráfico de la velocidad de reacción con respecto a la concentración es una línea horizontal.
En las relaciones de primer orden, la concentración de reactivo está relacionada linealmente con la velocidad de reacción. Por lo tanto, la tasa es igual a la constante de velocidad por la concentración del reactivo. Un gráfico de la velocidad de reacción con respecto a la concentración será lineal, con k como pendiente.
Si la reacción es de segundo orden, existe una relación cuadrática entre la concentración y la velocidad. Por lo tanto, la tasa es igual a la constante de tasa por la concentración al cuadrado. Un gráfico de la tasa con respecto a la concentración será parabólico, con k como pendiente.
La constante de velocidad, k, es un valor dependiente de la temperatura que relaciona la energía de activación de la reacción con la velocidad de reacción. Explorará esto en el próximo experimento de laboratorio.
Las velocidades de reacción generalmente se dan en moles por litro por segundo. La velocidad tiene las mismas unidades, independientemente del orden de reacción. Por lo tanto, la constante de velocidad tiene diferentes unidades, dependiendo del orden de reacción. Por ejemplo, si A es de primer orden y B es de orden cero, entonces tenemos un ejemplo de moles por litro en la ecuación de tasa. Por lo tanto, la constante de velocidad debe estar en segundos inversos.
Entonces, ¿cómo determinamos el orden de reacción? El orden de reacción debe determinarse experimentalmente mediante una serie de pruebas. Si tiene dos reactivos, un método es mantener constante la concentración de un reactivo, variar el otro y cronometrar el tiempo que se tarda en producir una cierta cantidad de producto. El mismo proceso se repite para el segundo reactivo. A continuación, puede estimar el orden de cada reactivo trazando la velocidad en función de la concentración variable de los reactivos y viendo si parece un gráfico de cero, primer o segundo orden. Hacer coincidir tus datos con la ecuación de tasa correspondiente también te permitirá calcular k.
En este laboratorio, determinará los órdenes de reacción de dos reactivos variando sus concentraciones y cronometrando el tiempo que tarda la reacción en progresar hasta volverse opaca la solución.

dependencia de la concentración

La cinética química y la ley de la velocidad de reacción
La cinética química se refiere a la velocidad de una reacción química. La velocidad depende del mecanismo, la complejidad y el número de reactivos en la reacción. La concentración de reactivos también juega un papel importante en la velocidad de una reacción.
La ley de la tasa cuantifica esta relación a través de la experimentación. Cada reactivo contribuye a la velocidad de la reacción mediante un factor conocido como orden de reacción. Este factor puede oscilar entre cero y dos, y depende de la relación de ese reactivo con la velocidad de la reacción.
r = k[A]m[B]n for aA + bB + ... → cC
En esta ecuación, r es la velocidad de reacción, k es la constante de velocidad de reacción, [A] y [B] son las concentraciones de los reactivos A y B, y m y n son los órdenes de reacción de los reactivos A y B, respectivamente. La velocidad de reacción, r, se define como el cambio en la concentración del producto con el tiempo y tiene unidades de moles por litro por segundo.
<h4>Orden de una reacción</h4>
El orden de la reacción describe la dependencia de potencia que la concentración de reactivos tiene con la velocidad de reacción. El orden de reacción general es la suma de los órdenes de reacción de cada uno de los reactivos. Es importante recordar que el orden de reacción NO está relacionado con los factores estequiométricos de la ecuación química equilibrada. En otras palabras, la estequiometría no se puede utilizar para determinar los órdenes de reacción de una reacción química. Las órdenes de reacción deben determinarse experimentalmente
Para determinar el orden de reacción, la concentración del segundo reactivo se mantiene constante, mientras que la concentración del primer reactivo varía. El tiempo que tarda en producirse la reacción se mide en segundos; Sin embargo, este tiempo no se corresponde con el tiempo que tarda la reacción en completarse. En cambio, este es el tiempo que tarda en comenzar la reacción. A continuación, se comparan los tiempos de reacción de las diferentes concentraciones de reactivos para determinar el orden. La misma serie de reacciones se realiza para determinar el orden del segundo reactivo, donde la concentración del primer reactivo se mantiene constante y la concentración del segundo reactivo varía.
Al comparar los tiempos de reacción, se puede determinar el orden de reacción. Por ejemplo, si el tiempo de reacción permanece constante a pesar de los cambios en la concentración del reactivo, entonces el reactivo es de orden cero. Esto significa que la velocidad de la reacción es igual al coeficiente de velocidad de reacción, k, que debe tener unidades de M/s. Si el tiempo de reacción cambia linealmente con cambios en la concentración (es decir, duplicar la concentración afecta el tiempo de reacción en un factor de 2), entonces el reactivo es de primer orden. Esto significa que la velocidad de la reacción es igual al producto del coeficiente de velocidad de reacción y la concentración del reactivo. En este caso, k tiene unidades de 1/s. Finalmente, si el tiempo de reacción se ve afectado por un factor de 4 cuando la concentración de un reactivo se duplica, el reactivo es de segundo orden. Esto significa que la velocidad de la reacción es igual a la constante de velocidad por el cuadrado de la concentración del reactivo, formando una relación cuadrática. En este caso, las unidades de k deben ser 1/M⋅s.
El orden general de la reacción es la suma de los órdenes de reacción individuales. Por ejemplo, si la reacción es de primer orden con respecto a A, entonces m = 1. Y si la reacción es de orden cero con respecto a B, entonces n = 0. El orden general de la reacción es de primer orden porque m + n = 1.
<h4>Constante de velocidad de reacción</h4>
La constante de velocidad, k, es específica de la reacción y la temperatura a la que se realiza la reacción. La constante de velocidad se determina a partir de la serie de experimentos. Usando la tasa medida y las concentraciones iniciales de reactivo, la ecuación de velocidad se usa para resolver la constante de tasa. La constante de velocidad depende de la temperatura y está definida por la ecuación de Arrhenius. Esta ecuación describe la relación entre k y la energía de activación, Ea, la temperatura, T, y la constante de gas ideal, R. La constante A es una constante de proporcionalidad y no debe confundirse con el reactivo A.
<img src="/CDNSource/lm/labs/50/50_Concepts_2.jpg">
<h4>Referencias</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). Química y reactividad química. Belmont, CA: Brooks/Cole, Cengage Learning.</li>
	<li style="font-size:17px">Silderberg, M.S. (2009). Química: La naturaleza molecular de la materia y el cambio. Boston, MA: McGraw Hill.</li>
</ol>

Dependencia de la concentración

La cinética química y la ley de la velocidad de reacción
La cinética química se refiere a la velocidad de una reacción química. La tasa depende del mecanismo, ...

Education

Educación

JoVE Lab Manual

Química

reaction order and rate constants

Chemical kinetics describes the speed of a chemical reaction. The rate depends on the reaction mechanism, the complexity of the reaction, and the number of reactants involved. Reactant concentration also affects how fast a reaction happens. The rate law is the experimental relationship that connects these factors to reaction rate.

In the rate law, each reactant contributes through a reaction order. The general form is r = k[A]^m[B]^n for aA + bB + ... → cC. Here, r is the reaction rate, k is the rate constant, and [A] and [B] are the reactant concentrations. The exponents m and n show the reaction order for each reactant.

The reaction rate is the change in product concentration over time. Its units are moles per liter per second. The overall reaction order is the sum of the individual orders. Reaction order is not the same as the numbers in the balanced chemical equation, so stoichiometric coefficients cannot be used to find it. Reaction orders must be measured experimentally.

To find a reaction order, one reactant concentration is changed while the other is held constant. The reaction time is measured in seconds, but this is the time for the reaction to start, not the time to completion. The times from different concentrations are compared to determine the order. The same process is repeated for the second reactant by switching which concentration is varied.

If the reaction time stays the same even when concentration changes, the reactant is zero-order. In that case, the rate equals k, and k has units of M/s. If the time changes linearly with concentration, the reactant is first order. Then the rate equals k times the reactant concentration, and k has units of 1/s. If doubling the concentration changes the time by a factor of 4, the reactant is second order. Then the rate equals k times the square of the concentration, and k has units of 1/M·s.

The overall order is the sum of the individual orders. For example, if A is first order and B is zero-order, the total order is first order because m + n = 1. The rate constant, k, is specific to the reaction and the temperature used. It is found from the experimental data by using the measured rate and initial concentrations in the rate equation.

The rate constant also depends on temperature through the Arrhenius equation. This equation relates k to activation energy, Ea, temperature, T, and the ideal gas constant, R. The constant A in the equation is a proportionality constant. It should not be confused with reactant A.

Reaction Order and Rate Constants

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chemistry

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Many common substances around us exist as a solution, such as ocean water, air, and gasoline. All solutions are mixtures of substances that are composed ...

Solutions and Concentrations

Source: Laboratory of Dr. Michael Evans &#8212; Georgia Institute of Technology
A solution is a homogeneous mixture containing some components in small ...

Solubility

Solubility
Solubility describes how much of a solute can dissolve in a given volume of a specific solvent. Solubility is usually reported in terms of ...

Solution, Solubility, and Solubility Equilibrium
A solution is a homogeneous mixture&#160;composed of a solvent, the major component, and a solute, the ...

Solution Equilibrium and Saturation

Imagine adding a small amount of sugar to a glass of water, stirring until all the sugar has dissolved, and then adding a bit more. You can repeat this ...

Intermolecular Forces in Solutions

The formation of a solution is an example of a spontaneous process, a process that occurs under specified conditions without energy from some external ...

Matter and Its Interactions: Develop models to illustrate the changes in the composition of the nucleus of the atom and the energy released during the processes of fission, fusion, and radioactive decay. (P: 2, CC: 5, CI: PS1)

9C.3.1.1.3

Chemical and Solubility Equilibria

The free energy change associated with dissolving a solute in a liter of solvent is called the free energy of a solution, &#916;Gsolution. The overall ...

Matter and Its Interactions: Apply scientific principles and evidence to provide an explanation about the effects of changing the surface area, agitation, temperature, and concentration of the reacting particles on the rate at which the reaction occurs. (P: 6, CC: 1, CI: PS1)

9C.3.2.1.2

3 Developing possible explanations of phenomena or designing solutions to engineering problems
 3.2 Constructing explanations and designing solutions 
  3.2.1 Students will be able to apply scientific principles and empirical evidence (primary or secondary) to explain the causes of phenomena or identify weaknesses in explanations developed by the students or others.

Reaction Order and Rate Constants

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