Name: reaction order from concentration
Uploaded: 2020-09-24T15:06:50.000Z
Duration: 3 min 3 s
Channel: JoVE

K12 Standards

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Chemistry and Physics

IPC.7

IPC.7.F

Concentration and Rate Law

La velocidad de una reacción depende a menudo de las concentraciones de reactivo. Para cualquier reacción, la relación entre la velocidad de reacción y las concentraciones de reactivo se puede expresar matemáticamente usando una ley de velocidad o la ecuación de velocidad. En una ley de velocidad, k es la constante de proporcionalidad, o la constante de velocidad, y n es el orden de reacción con respecto a un solo reactivo, cuyo valor suele ser un número entero.En las leyes de velocidad para reacciones de reactivos múltiples, el orden de reacción general es la suma de todos los órdenes de reactivos. Para cada reactivo, la velocidad de reacción, la constante de velocidad, la concentración y el orden de reacción se determinan experimentalmente. La ley de velocidad expresa la relación entre todos estos parámetros.Los órdenes de reactivos individuales comúnmente toman los valores 0, 1 o 2 y, según el orden de reacción general, las reacciones químicas se pueden clasificar como reacciones de orden cero, de primer orden o de segundo orden. Una reacción química de un solo reactivo, o unimolecular, para la cual la velocidad de reacción permanece constante durante toda su duración es una reacción de orden cero. El orden de los reactivos en una reacción de orden cero es cero y, de acuerdo con la ley de velocidad, la concentración de reactivos se eleva a la potencia cero.Dado que el valor de cualquier número elevado a la potencia cero es uno, la velocidad de reacción de una reacción de orden cero es igual a la constante de velocidad y, por lo tanto, independiente de la concentración de reactivo. Por lo tanto, en una reacción de orden cero, incluso cuando la concentración del reactivo disminuye, la velocidad de reacción no disminuye. Una reacción química unimolecular donde la velocidad de reacción es directamente proporcional a la concentración del reactivo es una reacción de primer orden.El orden de los reactivos para una reacción de primer orden es uno y, según la ley de velocidad, la concentración del reactivo se eleva a la primera potencia. Dado que el valor de cualquier número elevado a la potencia de uno sigue siendo el mismo, la velocidad de reacción de una reacción de primer orden depende directamente de la concentración de reactivo. A medida que disminuye la concentración de reactivo, la velocidad de reacción disminuye proporcionalmente de manera lineal.Una reacción química unimolecular donde la velocidad de reacción depende del cuadrado de la concentración del reactivo es una reacción de segundo orden. El orden de los reactivos es dos y la concentración del reactivo se eleva a la segunda potencia. Por consiguiente, la velocidad de reacción en una reacción de segundo orden depende directamente del cuadrado de la concentración de reactivo.A medida que disminuye la concentración de reactivo, la velocidad de reacción disminuye exponencialmente de manera cuadrática.

concentración y ley de velocidad

La velocidad de una reacción se ve afectada por las concentraciones de los reactivos. Las leyes de velocidad (leyes de velocidad diferencial) o ecuaciones de velocidad son expresiones matemáticas que describen la relación entre la velocidad de una reacción química y la concentración de sus reactivos.
 Por ejemplo, en una reacción genérica aA + bB &#10203 productos, donde a y b son coeficientes estequiométricos, la ley de velocidad puede escribirse como:
  <img alt="Eq1" src="/files/ftp_upload/11375/11375_Equation_1.png" style="height: 50px;" />
 [A] y [B] representan las concentraciones molares de los reactantes, y k es la constante de velocidad, que es específica para una reacción particular a una temperatura específica.
 Los exponentes m y n son los órdenes de la reacción y suelen ser enteros positivos, aunque pueden ser fracciones, negativos o cero.
 La constante de velocidad k y los órdenes de la reacción m y n se determinan experimentalmente observando cómo cambia la velocidad de una reacción a medida que cambian las concentraciones de los reactivos. La constante de velocidad k es independiente de las concentraciones de los reactivos, pero varía con la temperatura.
 Los órdenes de la reacción en una ley de velocidad describen la dependencia matemática entre la velocidad y las concentraciones de los reactantes. Refiriéndose a la formula general de la ley de velocidad (velocidad = k[A]m[B]n), la reacción es de orden m con respecto a A y de orden n con respecto a B. Por ejemplo, si m = 1 y n = 2, La reacción es de primer orden en A y de segundo orden en B. El orden de reacción global es simplemente la suma de los órdenes para cada reactivo. Para el ejemplo de la ley de velocidad aquí, la reacción es de tercer orden en general (1 + 2 = 3).
 Un enfoque experimental común para la determinación de las leyes de velocidad es el método de las velocidades iniciales. Este método consiste en medir las velocidades de reacción en múltiples ensayos experimentales realizados utilizando diferentes concentraciones iniciales de reactivos. La comparación de las velocidades medidas para estos ensayos permite determinar los órdenes de reacción y, posteriormente, la constante de velocidad, que juntos se utilizan para formular una ley de velocidad.
 Las leyes de velocidad pueden mostrar órdenes fraccionarios para algunos reactivos, y en ocasiones se observan órdenes de reacción negativos cuando un aumento en la concentración de un reactivo provoca una disminución en la velocidad de la reacción. Es importante notar que las leyes de velocidad son determinadas solamente experimentalmente y no son predecibles de manera confiable mediante la estequiometría de reacción.
 El orden de reacción determina la relación entre la velocidad de reacción y la concentración de reactivos o productos.
 • En una reacción de orden cero, la concentración de los reactivos no tiene ningún efecto sobre la velocidad de la reacción, que permanece constante durante todo el proceso. 
 • En una reacción de primer orden, la velocidad de reacción es directa y linealmente proporcional al cambio en la concentración de los reactivos. A medida que disminuye la concentración de los reactivos, la tasa de reacción también disminuye proporcionalmente. 
 • En las reacciones de segundo orden o de mayor orden, la velocidad de reacción es proporcional al valor exponencial de los reactivos. Por lo tanto, a medida que la reacción progresa y la concentración de los reactivos disminuye, la velocidad de reacción disminuye exponencialmente.
 Este texto es adaptado de<a href="https://openstax.org/books/chemistry-2e/pages/12-3-rate-laws#57033"> Openstax, Química 2e, Sección 12.3: Leyes de velocidad</a>.

Concentración y Ley de Velocidad

La velocidad de una reacción se ve afectada por las concentraciones de reactivos. Las leyes de tasa (leyes de tasa diferencial) o ecuaciones de tasa son expresiones matemáticas ...

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KEY TERMS AND CONCEPTS

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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 are the reaction orders. They are usually positive integers, but they can also be fractions, negative numbers, or zero. The values of k, m, and n are found experimentally by measuring how the rate changes when reactant concentrations change.

The rate constant does not depend on the reactant concentrations, but it can change with temperature. The reaction orders show the mathematical link between concentration and rate. In the expression rate = k [A]m [B]n, the reaction is m order with respect to A and n order with respect to B. For example, if m = 1 and n = 2, the reaction is first order in A and second order in B.

The overall reaction order is the sum of the orders for each reactant. In the example above, the reaction is third order overall because 1 + 2 = 3. A common way to find a rate law is the method of initial rates. In this method, reaction rates are measured in several trials that start with different initial reactant concentrations. Comparing those rates makes it possible to find the reaction orders and then the rate constant.

Rate laws can also include fractional orders. In some cases, a reactant may have a negative order. That means increasing its concentration can lower the reaction rate. Rate laws are determined by experiment only, and they cannot be reliably predicted from reaction stoichiometry. The reaction order also describes how the rate changes as reactant or product concentration changes.

A zero-order reaction has a rate that does not change with reactant concentration. The rate stays constant throughout the reaction. In a first-order reaction, the rate is directly proportional to the change in reactant concentration. As the reactant concentration drops, the rate drops by the same proportion. In second-order or higher-order reactions, the rate depends on the reactants raised to an exponent. As the reaction continues and reactant concentration decreases, the rate falls exponentially.

Reaction Order from Concentration

reaction rate

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...

measuring reaction rates

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 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

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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...

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arrhenius plots

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reaction mechanisms

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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

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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

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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 concentration affects reaction rate, how rate laws are written, and how initial rates reveal reaction order, the rate constant, and overall order.

Reaction Order from Concentration | JoVE K12

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Matter and its Interactions

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New York State Science Learning Standards

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Reaction Order from Concentration

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