Name: elementary steps in reaction mechanisms
Uploaded: 2020-09-24T15:06:50.000Z
Duration: 3 min 6 s
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K12 Standards

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North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.5.1

Reaction Mechanisms

Una reacción química a menudo se representa mediante una ecuación química balanceada general que indica los reactivos y productos. Sin embargo, la reacción real suele ser más compleja y se produce en varios pasos. Por ejemplo, esta reacción de óxido nítrico con hidrógeno que da origen a nitrógeno gaseoso y agua tiene lugar en tres pasos sucesivos distintos.Estos pasos se denominan mecanismo de reacción. Cada paso en el mecanismo de reacción se denomina reacción elemental y representa la interacción, como la rotura o formación de enlaces, entre las especies que reaccionan. Las moléculas específicas, como el dióxido de dinitrógeno y el óxido nitroso, se forman durante un paso elemental y se consumen durante otro.Estas especies se denominan productos intermedios de la reacción. Los intermedios de la reacción son productos de baja energía de una reacción elemental. Suelen ser de corta duración, lo que explica su ausencia en la mezcla de productos.Los intermedios de reacción no son lo mismo que los complejos activados. Los complejos activados son estados de transición de alta energía que existen solo durante la transformación de los reactivos en productos. La combinación de los pasos elementales da la ecuación para la reacción química general.Aquí, los intermedios de la reacción se eliminan y, por lo tanto, no aparecen en la ecuación química general. Las diferentes reacciones elementales pueden progresar a diferentes velocidades. El paso elemental más lento determina la velocidad de la reacción general.Aquí, la reacción del dióxido de dinitrógeno con gas hidrógeno es el paso que limita la velocidad. Las reacciones elementales se pueden caracterizar comúnmente como de tres tipos, según el número de moléculas reaccionantes o la molecularidad. En una reacción unimolecular, una sola molécula de reactivo se transforma en uno o más productos.En una reacción bimolecular, reaccionan dos moléculas distintas. Una reacción termolecular, aunque es poco frecuente, involucra tres moléculas individuales que reaccionan para producir intermedios o productos. A diferencia de la ley de velocidad para una reacción química general, que se determina experimentalmente, las leyes de velocidad para reacciones elementales se pueden predecir a partir de los coeficientes estequiométricos de sus reactivos.En resumen, la molecularidad de una reacción elemental corresponde al orden de reacción general del paso elemental. Por lo tanto, las reacciones unimoleculares son a menudo reacciones de primer orden, las reacciones bimoleculares son de segundo orden y las reacciones termoleculares son de tercer orden. El conocimiento de la cinética y los mecanismos de reacción.ayuda a los químicos a identificar y optimizar las reacciones químicas.

mecanismos de reacción

Las reacciones químicas a menudo ocurren de manera gradual, involucrando dos o más reacciones distintas que ocurren en una secuencia. Una ecuación balanceada indica las especies que reaccionan y las especies del producto, pero no revela detalles sobre cómo ocurre la reacción a nivel molecular. El mecanismo de reacción (o trayectoria de reacción) proporciona detalles sobre el proceso preciso paso a paso por el que se produce una reacción.
 Por ejemplo, la descomposición del ozono parece seguir un mecanismo de dos pasos:
 <img alt="Eq1" src="/files/ftp_upload/11381/11381_Equation_1.png" style="text-align: center; height: 70px;" />
 Cada uno de los pasos de un mecanismo de reacción se denomina reacción elemental. Estas reacciones elementales ocurren en secuencia, como se representan en las ecuaciones de pasos, y se suman para producir la ecuación química balanceada que describe la reacción general:
 <img alt="Eq2" src="/files/ftp_upload/11381/11381_Equation_2.png" style="text-align: center; height: 50px;" />
 Observe que el átomo de oxígeno producido en el primer paso se consume durante el segundo y no aparece como un producto en la reacción general. Tales especies que se producen en un paso y se consumen en un paso posterior se llaman intermediarios de la reacción.
 Mientras que la ecuación general de la reacción indica que dos moléculas de ozono reaccionan para producir tres moléculas de oxígeno, el mecanismo de reacción real no involucra la colisión y reacción directa de dos moléculas de ozono. En cambio, un O3 se descompone para producir O2 y un átomo de oxígeno, y una segunda molécula de O3 reacciona posteriormente con el átomo de oxígeno para producir dos moléculas adicionales de O2.
 A diferencia de las ecuaciones balanceadas que representan una reacción general, las ecuaciones para las reacciones elementales son representaciones explícitas del cambio químico. Una ecuación de reacción elemental muestra el reactante real que está en proceso de ruptura/fabricación de enlaces, y el producto o productos formados. Por lo tanto, la ley de velocidad para una reacción elemental puede derivarse directamente de su ecuación química balanceada. Sin embargo, este no es el caso de las reacciones químicas típicas, para las cuales las leyes de velocidad pueden determinarse de manera fiable sólo a través de la experimentación.
 Reacciones elementales unimoleculares
 La molecularidad de una reacción elemental es el número de especies reactantes (átomos, moléculas o iones). Por ejemplo, una reacción unimolecular implica la reacción de un solo reactivo para producir una o más moléculas de producto:
 <img alt="Eq3" src="/files/ftp_upload/11381/11381_Equation_3.png" style="text-align: center; height: 50px;" />
 La ley de velocidad para una reacción unimolecular es de primer orden; velocidad = k [A].
 Una reacción unimolecular puede ser una de varias reacciones elementales en un mecanismo de reacción complejo. Por ejemplo, la reacción (O3 (g) → O2 (g) + O) ilustra una reacción elemental unimolecular que ocurre como parte de un mecanismo de reacción de dos pasos. Sin embargo, algunas reacciones unimoleculares pueden ser el único paso de un mecanismo de reacción de un solo paso. (En otras palabras, una reacción “general” puede ser también una reacción elemental en algunos casos.) Por ejemplo, la descomposición en fase gaseosa del ciclobutano, C4H8, al etileno, C2H4, se representa mediante la ecuación química:
 <img alt="Eq3A" src="/files/ftp_upload/11381/11381_Equation_3A.png" style="text-align: center; height: 50px;" />
 Esta ecuación representa la reacción general, describiendo un proceso elemental unimolecular. La ley de velocidad predicha a partir de esta ecuación, suponiendo que es una reacción elemental, resulta ser la misma que la ley de velocidad obtenida experimentalmente para la reacción general, mostrando un comportamiento de primer orden:
 <img alt="Eq4" src="/files/ftp_upload/11381/11381_Equation_4.png" style="text-align: center; height: 70px;" />
 Este acuerdo entre las leyes de velocidad observadas y previstas indica que el proceso unimolecular propuesto en un solo paso es un mecanismo razonable para la reacción del butadieno.
 Reacciones elementales bimoleculares
 Una reacción bimolecular involucra dos especies reactantes. Por ejemplo:
 <img alt="Eq5" src="/files/ftp_upload/11381/11381_Equation_5.png" style="text-align: center; height: 70px;" />
 En el primer tipo, donde las dos moléculas de reactivo son diferentes, la ley de velocidad es de primer orden en A y de primer orden en B (en general de segundo orden)
 <img alt="Eq6" src="/files/ftp_upload/11381/11381_Equation_6.png" style="text-align: center; height: 50px;" />
 En el segundo tipo, en el que dos moléculas idénticas colisionan y reaccionan, la ley de velocidad es de segundo orden en A:
 <img alt="Eq7" src="/files/ftp_upload/11381/11381_Equation_7.png" style="text-align: center; height: 50px;" />
 Algunas reacciones químicas ocurren por mecanismos que consisten en una sola reacción elemental bimolecular. Un ejemplo es la reacción del dióxido de nitrógeno con el monóxido de carbono:
 <img alt="Eq8" src="/files/ftp_upload/11381/11381_Equation_8.png" style="text-align: center; height: 50px;" />
 Las reacciones elementales bimoleculares también pueden estar involucradas como pasos en un mecanismo de reacción de varios pasos. La reacción del oxígeno atómico con el ozono es el segundo paso de un mecanismo de descomposición del ozono en dos pasos:
 <img alt="Eq9" src="/files/ftp_upload/11381/11381_Equation_9.png" style="text-align: center; height: 50px;" />
 Reacciones elementales termoleculares 
 Una reacción termolecular elemental implica la colisión simultánea de tres átomos, moléculas o iones. Las reacciones elementales termoleculares son poco comunes porque la probabilidad de que tres partículas colisionen simultáneamente es muy rara. Sin embargo, existen algunas reacciones elementales termoleculares establecidas. La reacción del óxido nítrico con oxígeno parece implicar pasos termoleculares:
 <img alt="Eq10" src="/files/ftp_upload/11381/11381_Equation_10.png" style="text-align: center; height: 70px;" />
 Asimismo, la reacción del óxido nítrico con el cloro parece implicar pasos termoleculares:
 <img alt="Eq11" src="/files/ftp_upload/11381/11381_Equation_11.png" style="text-align: center; height: 70px;" />
 A menudo uno de los pasos elementales en un mecanismo de reacción de varios pasos es significativamente más lento que los otros. Debido a que una reacción no puede proceder más rápido que su paso más lento, este paso limitará la velocidad a la que se produce la reacción general. El paso elemental más lento se llama, por lo tanto, el paso de limitante de la velocidad (o paso determinante de la velocidad) de la reacción.
 Este texto está adaptado de<a href="https://openstax.org/books/chemistry-2e/pages/12-6-reaction-mechanisms"> Openstax, Química 2e, 12.6: Mecanismos de reacción.</a>

Mecanismos de Reacción

Las reacciones químicas a menudo ocurren de manera escalonada, involucrando dos o más reacciones distintas que tienen lugar en una secuencia. Una ecuación equilibrada indica ...

Education

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Chemical Kinetics

Cinética Química

KEY TERMS AND CONCEPTS

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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 equation for the overall reaction. In the ozone example, an oxygen atom is made in the first step and used up in the second step, so it does not appear in the final equation. A species made in one step and used up in a later step is called a reaction intermediate.

The overall equation may show two ozone molecules forming three oxygen molecules, but the mechanism shows that the two ozone molecules do not react directly with each other. Instead, one O3 molecule breaks down to form O2 and an oxygen atom. Then a second O3 molecule reacts with that oxygen atom to make two more O2 molecules. This is why elementary reaction equations give a more direct picture of bond breaking and bond making than the overall equation does.

For an elementary reaction, the rate law can often be written directly from the balanced equation for that step. For most chemical reactions, the rate law must be found by experiment. That is why elementary reactions are useful when chemists build a mechanism that matches the observed behavior of a reaction.

Elementary reactions are described by molecularity, which is the number of reactant species in the step. A unimolecular reaction has one reactant species and forms one or more products. Its rate law is first order, written as rate = k[A]. A unimolecular step may be part of a larger mechanism, such as O3(g) → O2(g) + O in ozone decomposition. Some unimolecular reactions are also the whole reaction, not just one step. The gas-phase decomposition of cyclobutane, C4H8, to ethylene, C2H4, is one example, and its first-order experimental rate law agrees with the rate law predicted for an elementary single-step process.

A bimolecular reaction involves two reactant species. If the two reactants are different, the rate law is first order in A and first order in B, for a second-order overall rate. If the two reacting molecules are the same, the rate law is second order in A. Some reactions occur by a single bimolecular elementary reaction, such as the reaction of nitrogen dioxide with carbon monoxide. Bimolecular steps can also appear in multistep mechanisms, including the reaction of atomic oxygen with ozone in ozone decomposition.

A termolecular reaction involves the simultaneous collision of three atoms, molecules, or ions. These reactions are uncommon because three particles colliding at the same time is very unlikely. Still, a few termolecular elementary reactions are known. The reaction of nitric oxide with oxygen and the reaction of nitric oxide with chlorine both appear to involve termolecular steps.

In many multistep mechanisms, one elementary step is much slower than the others. A reaction cannot proceed faster than its slowest step, so that step controls the overall rate. This slowest step is called the rate-limiting step, or rate-determining step.

Elementary Steps in Reaction Mechanisms

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

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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...

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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

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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...

temperature dependence on reaction rate

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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

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

Explore how chemical reactions happen in steps, including elementary reactions, reaction intermediates, molecularity, rate laws, and the rate-limiting step.

Elementary Steps in Reaction Mechanisms | JoVE K12

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Chemical Reactions

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Elementary Steps in Reaction Mechanisms

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