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

K12 Standards

north-carolina-k-12-science-standard-course-of-study

North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.5.1

Reaction Mechanisms

Eine chemische Reaktion wird oft durch eine insgesamt ausgeglichene chemische Gleichung dargestellt, die die Reaktanten und Produkte angibt.
Die eigentliche Reaktion ist jedoch oft komplexer und läuft in mehreren Schritten ab. So läuft diese Reaktion von Stickstoffmonoxid mit Wasserstoff unter Bildung von Stickstoffgas und Wasser in drei verschiedenen, aufeinander aufbauenden Schritten ab. Diese Schritte werden als Reaktionsmechanismus bezeichnet.
Jeder Schritt im Reaktionsmechanismus wird als Elementarreaktion bezeichnet und stellt die Wechselwirkung, wie z. B. Bindungsbruch oder -bildung, zwischen den reagierenden Spezies dar.
Bestimmte Moleküle, wie Distickstoffdioxid und Distickstoffmonoxid, werden in einem Elementarschritt gebildet und in einem anderen verbraucht. Solche Spezies werden als Reaktionszwischenprodukte bezeichnet.
Reaktionszwischenprodukte sind niederenergetische Produkte einer Elementarreaktion. Sie sind oft kurzlebig, was ihr Fehlen in der Produktmischung erklärt. Reaktionszwischenprodukte sind nicht dasselbe wie aktivierte Komplexe. Aktivierte Komplexe sind hochenergetische Übergangszustände, die nur bei der Umwandlung von Reaktanten in Produkte existieren. 
Die Kombination der Elementarschritte ergibt die Gleichung für die gesamte chemische Reaktion. Hier werden die Reaktionszwischenprodukte eliminiert und tauchen daher nicht in der chemischen Gesamtgleichung auf.
Die verschiedenen Elementarreaktionen können unterschiedlich schnell ablaufen. Der langsamste Elementarschritt bestimmt die Gesamtreaktionsgeschwindigkeit. Dabei ist die Reaktion von Distickstoffdioxid mit Wasserstoffgas der geschwindigkeitsbegrenzende Schritt.
Elementarreaktionen können allgemein in drei Typen unterteilt werden, abhängig von der Anzahl der reagierenden Moleküle oder der Molekularität.
Bei einer unimolekularen Reaktion wandelt sich ein einzelnes Reaktantenmolekül in ein oder mehrere Produkte um. In einer bimolekularen Reaktion reagieren zwei unterschiedliche Moleküle. Bei einer termolekularen Reaktion, wenn auch sehr selten, reagieren drei einzelne Moleküle zu Zwischenprodukten oder Produkten.
Im Gegensatz zum Geschwindigkeitsgesetz für eine gesamte chemische Reaktion, das experimentell bestimmt wird, können Geschwindigkeitsgesetze für Elementarreaktionen aus stöchiometrischen Koeffizienten ihrer Reaktanten vorhergesagt werden. Kurz gesagt, die Molekularität einer Elementarreaktion entspricht der Gesamtreaktionsordnung des Elementarschritts.
Daher sind unimolekulare Reaktionen oft Reaktionen erster Ordnung, bimolekulare Reaktionen zweiter Ordnung und termolekulare Reaktionen dritter Ordnung.
Das Verständnis von Reaktionsmechanismen und Kinetik hilft Chemikern dabei, chemische Reaktionen zu identifizieren und zu optimieren.

reaktionsmechanismen

Chemische Reaktionen laufen oft schrittweise ab, wobei zwei oder mehr unterschiedliche Reaktionen nacheinander ablaufen. Eine ausgewogene Gleichung gibt die reagierende Spezies und die Produktspezies an, verrät jedoch keine Details darüber, wie die Reaktion auf molekularer Ebene abläuft. Der Reaktionsmechanismus (oder Reaktionspfad) liefert Einzelheiten zum genauen, schrittweisen Prozess, nach dem eine Reaktion abläuft. Beispielsweise scheint die Zersetzung von Ozon einem Mechanismus mit zwei Schritten zu folgen: <img alt="Eq1" src="/files/ftp_upload/11381/11381_Equation_1.png" style="text-align: center; height: 70px;"> Jeder Schritt eines Reaktionsmechanismus wird als Elementarreaktion bezeichnet. Diese Elementarreaktionen laufen nacheinander ab, wie in den Stufengleichungen dargestellt, und summieren sich, um die ausgewogene chemische Gleichung zu ergeben, die die Gesamtreaktion beschreibt: <img alt="Eq2" src="/files/ftp_upload/11381/11381_Equation_2.png" style="text-align: center; height: 50px;"> Beachten Sie, dass das im ersten Schritt erzeugte Sauerstoffatom im zweiten Schritt verbraucht wird und nicht als Produkt in der Gesamtreaktion auftritt. Solche Spezies, die in einem Schritt entstehen und in einem darauffolgenden Schritt verbraucht werden, werden Reaktionszwischenprodukte genannt. Während die Gesamtreaktionsgleichung darauf hinweist, dass zwei Ozonmoleküle unter Bildung von drei Sauerstoffmolekülen reagieren, beinhaltet der eigentliche Reaktionsmechanismus nicht die direkte Kollision und Reaktion zweier Ozonmoleküle. Stattdessen zerfällt ein O3 zu O2 und einem Sauerstoffatom, und ein zweites O3-Molekül reagiert anschließend mit dem Sauerstoffatom zu zwei weiteren O2-Molekülen. Im Gegensatz zu ausgeglichenen Gleichungen, die eine Gesamtreaktion darstellen, sind die Gleichungen für Elementarreaktionen explizite Darstellungen der chemischen Veränderung. Eine elementare Reaktionsgleichung zeigt die tatsächlichen Reaktanten, die eine Bindung aufbrechen/bilden, und die gebildeten Produkte. Somit kann das Geschwindigkeitsgesetz für eine Elementarreaktion direkt aus ihrer ausgeglichenen chemischen Gleichung abgeleitet werden. Dies ist jedoch bei typischen chemischen Reaktionen nicht der Fall, bei denen Geschwindigkeitsgesetze nur durch Experimente zuverlässig bestimmt werden können. Unimolekulare Elementarreaktionen Die Molekularität einer Elementarreaktion ist die Anzahl der Reaktantenspezies (Atome, Moleküle oder Ionen). Bei einer unimolekularen Reaktion handelt es sich beispielsweise um die Reaktion eines einzelnen Reaktanten zur Bildung eines oder mehrerer Produktmoleküle: <img alt="Eq3" src="/files/ftp_upload/11381/11381_Equation_3.png" style="text-align: center; height: 50px;"> Das Geschwindigkeitsgesetz für eine unimolekulare Reaktion ist erster Ordnung; Rate = k [A]. Eine unimolekulare Reaktion kann eine von mehreren Elementarreaktionen in einem komplexen Reaktionsmechanismus sein. Beispielsweise veranschaulicht die Reaktion (O3 (g) → O2 (g) + O) eine unimolekulare Elementarreaktion, die als Teil eines zweistufigen Reaktionsmechanismus abläuft. Einige unimolekulare Reaktionen können jedoch der einzige Schritt eines einstufigen Reaktionsmechanismus sein. (Mit anderen Worten, eine „Gesamtreaktion“ kann in manchen Fällen auch eine Elementarreaktion sein.) Beispielsweise wird die Gasphasenzersetzung von Cyclobutan, C4H8, zu Ethylen, C2H4, durch die chemische Gleichung dargestellt: <img alt="Eq3A" src="/files/ftp_upload/11381/11381_Equation_3A.png" style="text-align: center; height: 50px;"> Diese Gleichung stellt die Gesamtreaktion dar und beschreibt einen unimolekularen Elementarprozess. Das aus dieser Gleichung vorhergesagte Geschwindigkeitsgesetz erweist sich unter der Annahme, dass es sich um eine Elementarreaktion handelt, als dasselbe wie das experimentell abgeleitete Geschwindigkeitsgesetz für die Gesamtreaktion und zeigt ein Verhalten erster Ordnung: <img alt="Eq4" src="/files/ftp_upload/11381/11381_Equation_4.png" style="text-align: center; height: 70px;"> Diese Übereinstimmung zwischen beobachteten und vorhergesagten Geschwindigkeitsgesetzen weist darauf hin, dass der vorgeschlagene unimolekulare, einstufige Prozess ein sinnvoller Mechanismus für die Butadienreaktion ist. Bimolekulare Elementarreaktionen An einer bimolekularen Reaktion sind zwei Reaktantenarten beteiligt. Zum Beispiel: <img alt="Eq5" src="/files/ftp_upload/11381/11381_Equation_5.png" style="text-align: center; height: 70px;"> Beim ersten Typ, bei dem die beiden Reaktantenmoleküle unterschiedlich sind, ist das Geschwindigkeitsgesetz in A erster Ordnung und in B erster Ordnung (insgesamt zweite Ordnung). <img alt="Eq6" src="/files/ftp_upload/11381/11381_Equation_6.png" style="text-align: center; height: 50px;"> Beim zweiten Typ, bei dem zwei identische Moleküle kollidieren und reagieren, ist das Geschwindigkeitsgesetz zweiter Ordnung in A: <img alt="Eq7" src="/files/ftp_upload/11381/11381_Equation_7.png" style="text-align: center; height: 50px;"> Einige chemische Reaktionen laufen über Mechanismen ab, die aus einer einzigen bimolekularen Elementarreaktion bestehen. Ein Beispiel ist die Reaktion von Stickstoffdioxid mit Kohlenmonoxid: <img alt="Eq8" src="/files/ftp_upload/11381/11381_Equation_8.png" style="text-align: center; height: 50px;"> Auch bimolekulare Elementarreaktionen können als Schritte an einem mehrstufigen Reaktionsmechanismus beteiligt sein. Die Reaktion von atomarem Sauerstoff mit Ozon ist der zweite Schritt eines zweistufigen Ozonzersetzungsmechanismus: <img alt="Eq9" src="/files/ftp_upload/11381/11381_Equation_9.png" style="text-align: center; height: 50px;"> Termmolekulare Elementarreaktionen Bei einer elementaren thermomolekularen Reaktion kommt es zum gleichzeitigen Zusammenstoß dreier Atome, Moleküle oder Ionen. Termolekulare Elementarreaktionen sind selten, da die Wahrscheinlichkeit, dass drei Teilchen gleichzeitig kollidieren, sehr gering ist. Es gibt jedoch einige etablierte termolekulare Elementarreaktionen. Die Reaktion von Stickstoffmonoxid mit Sauerstoff scheint termolekulare Schritte zu umfassen: <img alt="Eq10" src="/files/ftp_upload/11381/11381_Equation_10.png" style="text-align: center; height: 70px;"> Ebenso scheint die Reaktion von Stickstoffmonoxid mit Chlor termolekulare Schritte zu umfassen: <img alt="Eq11" src="/files/ftp_upload/11381/11381_Equation_11.png" style="text-align: center; height: 70px;"> Oftmals ist einer der Elementarschritte in einem mehrstufigen Reaktionsmechanismus deutlich langsamer als die anderen. Da eine Reaktion nicht schneller als ihr langsamster Schritt ablaufen kann, begrenzt dieser Schritt die Geschwindigkeit, mit der die Gesamtreaktion abläuft. Der langsamste Elementarschritt wird daher als geschwindigkeitsbestimmender Schritt (oder geschwindigkeitsbestimmender Schritt) der Reaktion bezeichnet. Dieser Text ist adaptiert von <a href="https://openstax.org/books/chemistry-2e/pages/12-6-reaction-mechanisms">Openstax, Chemistry 2e, 12.6: Reaction Mechanisms.</a>

Reaktionsmechanismen

Chemische Reaktionen laufen oft schrittweise ab, wobei zwei oder mehr unterschiedliche Reaktionen in einer Sequenz ablaufen. Eine ausgeglichene Gleichung ergibt ...

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Chemical Kinetics

Chemische Kinetik

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

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

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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

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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Pennsylvania State Academic Standards

Physical Science

Chemical Reactions

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Matter And Its Interactions

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SC.912.P.12

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South Carolina College and Career Ready Science Standards

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New York State Science Learning Standards

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Elementary Steps in Reaction Mechanisms

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