Name: how reactant concentration sets reaction rate
Uploaded: 2025-05-22T09:05:33.000Z
Duration: 2 min 33 s
Channel: JoVE

K12 Standards

Florida Department of Education

High School

Physical Science

SC.912.P.12

SC.912.P.12.12

Rate Law and Reaction Order

Para cualquier reacción, la relación entre la velocidad de reacción y las concentraciones de reactivo se puede expresar matemáticamente utilizando una ley de velocidad o la ecuación de velocidad. 
En una ley de tasas, 'k' es la constante de proporcionalidad, o la constante de tasa, 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 multirreactivo, el orden general de la reacción 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.
Los órdenes de reactivos individuales suelen tomar los valores 0, 1 o 2 y, en función del orden general de la reacción, 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 del reactivo en una reacción de orden cero es cero, y la concentración del reactivo 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 del reactivo.
Una reacción química unimolecular en la que la velocidad de reacción es directamente proporcional a la concentración del reactivo es una reacción de primer orden. El orden del reactivo para una reacción de primer orden es uno, y según la ley de tasa, la concentración del reactivo se eleva a la primera potencia.
Dado que el valor de cualquier número elevado a la potencia de uno permanece igual, la velocidad de reacción de una reacción de primer orden depende directamente de la concentración del reactivo. A medida que disminuye la concentración de reactivo, la velocidad de reacción disminuye proporcionalmente de manera lineal.
Una reacción química unimolecular en la que la velocidad de reacción depende del cuadrado de la concentración del reactivo es una reacción de segundo orden. A medida que disminuye la concentración de reactivo, la velocidad de reacción disminuye exponencialmente de manera cuadrática.

ley de la velocidad y orden de reacción

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 ⟶ productos, donde a y b son coeficientes estequiométricos, la ley de velocidad se puede escribir como:


 tasa = k[A]^m[B]^n


 [A] y [B] representan las concentraciones molares de reactivos, 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 reacción y, normalmente, son números enteros positivos, aunque pueden ser fracciones, valores negativos o cero.


 La constante de velocidad k y los órdenes de 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 reactivos pero varía con la temperatura.


 Los órdenes de reacción en una ley de velocidad describen la dependencia matemática de la velocidad de las concentraciones de reactivos. Refiriéndose a la ley de velocidad genérica (velocidad = k[A]^m[B]^n), la reacción es de orden mº con respecto a A y de orden nº 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 general de la reacción es, simplemente, la suma de los órdenes de cada reactivo. Para el ejemplo de ley de velocidad aquí, la reacción es de tercer orden en general (1 + 2 = 3).


 Un enfoque experimental común para la determinación de leyes de tasas es el método de tasas iniciales. Este método implica medir las velocidades de reacción para múltiples ensayos experimentales realizados utilizando diferentes concentraciones iniciales de reactivo. La comparación de las velocidades medidas para estos ensayos permite determinar los órdenes de reacción y, posteriormente, la constante de velocidad, que en conjunto se utilizan para formular una ley de velocidad.


 Las leyes de velocidad pueden exhibir órdenes fraccionarios para algunos reactivos y, a veces, se observan órdenes de reacción negativos cuando un aumento en la concentración de un reactivo provoca una disminución en la velocidad de reacción. Es importante señalar que las leyes de velocidad se determinan únicamente mediante experimentos y no se predicen 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.


 <ul>
 <li>En una reacción de orden cero, la concentración de los reactivos no tiene ningún efecto sobre la velocidad de la reacción, que permanece constante en todo momento.</li>
 <li>En una reacción de primer orden, la velocidad de reacción es directa y linealmente proporcional al cambio en la concentración del reactivo. A medida que disminuye la concentración del reactivo, la velocidad de reacción también disminuye proporcionalmente.</li>
 <li>En reacciones de segundo orden o de orden superior, la velocidad de reacción es proporcional al valor exponencial de los reactivos. Por tanto, a medida que avanza la reacción y disminuye la concentración de los reactivos, la velocidad de reacción disminuye exponencialmente.</li>
 </ul>

Ley de la velocidad y orden de reacción

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 ...

Education

Educación

JoVE Core

Organic Chemistry

Química Orgánica

Unit 1

Unidad 1

Thermodynamics and Chemical Kinetics

Termodinámica y cinética química

KEY TERMS AND CONCEPTS

TÉRMINOS Y CONCEPTOS CLAVE

how reactant concentration sets reaction rate

Reaction rate depends on reactant concentration, and rate laws show that relationship with a mathematical equation. A rate law, also called a differential rate law or rate equation, links the rate of a chemical reaction to the concentrations of its reactants.

For a generic reaction, a A + b B  products, the rate law can be written as rate = k [A]^m [B]^n. Here, [A] and [B] are the molar concentrations of the reactants. The constant k is the rate constant, and it is specific to one reaction at a specific temperature.

The exponents m and n are the reaction orders. They are usually positive integers, but they can also be fractions, negative values, or zero. 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 separate orders, so this example is third order overall.

Rate law values are found by experiment, not by stoichiometry. Scientists change the initial reactant concentrations in several trials and measure the reaction rate. This method, called the method of initial rates, helps determine the reaction orders and then the rate constant.

Some reactions show zero-order behavior. In a zero-order reaction, changes in reactant concentration do not change the rate, so the rate stays constant. In a first-order reaction, the rate changes in direct proportion to the reactant concentration. In second-order and higher-order reactions, the rate depends on the concentration raised to an exponent, so the rate drops more sharply as reactants are used up.

Rate laws can also show fractional orders, and some can even show negative orders. A negative order means that increasing one reactant can decrease the reaction rate. These patterns are useful for describing how reaction rate changes in real experiments. They are not reliably predicted from the reaction equation alone, so the rate law must be determined experimentally.

How Reactant Concentration Sets Reaction Rate

how-reactant-concentration-sets-reaction-rate

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rate law and reaction order

Reaction rate depends on reactant concentration, and rate laws show that relationship with a mathematical equation. A rate law, also called a differential rate law or rate equation, links the rate of a chemical reaction to the concentrations of its reactants. For a generic reaction, a A + b B  products, the rate law can be written as rate = k [A]^m [B]^n. Here, [A] and [B] are the molar concentrations of the reactants. The constant k is the rate constant, and it is specific to one reaction at...

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effect-of-temperature-change-on-reaction-rate

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Reaction rate and reaction pathway are key ideas in chemical kinetics. Thermodynamics, by contrast, focuses on the state of a system. It describes the properties, behavior, and components of reactants and products. It does not describe the path of the process or the speed of the reaction. Kinetics gives information on the atomic or molecular level, while thermodynamics focuses on the energetics of the reactants and products. Thermodynamics can show what may happen in a reaction, but it does...

key terms and concepts

Learn how rate laws connect reactant concentration to reaction speed, how reaction orders work, and how the method of initial rates is used to find them.

How Reactant Concentration Sets Reaction Rate | JoVE K12

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How Reactant Concentration Sets Reaction Rate

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