Name: reaction mechanisms and rate-determining steps
Uploaded: 2023-04-30T18:28:32.000Z
Duration: 2 min 31 s
Channel: JoVE

K12 Standards

Louisiana Student Standards for Science

High School

Chemistry

Matter And Its Interactions

HS-PS1-6

Multi-Step Reactions

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 nicht dasselbe wie Übergangszustände, die nur bei der Umwandlung von Reaktanten in Produkte existieren.
Die Reaktionsmechanismen werden auf der Grundlage ihrer ausgewogenen chemischen Gleichungen und experimentell ermittelten Geschwindigkeitsgesetze jedes Elementarschritts hypothetisiert.
Jeder Schritt hat eine bestimmte Reaktionsgeschwindigkeit, eine bestimmte Geschwindigkeitskonstante und Aktivierungsenergie. Der langsamste Schritt wird als geschwindigkeitsbestimmender Schritt bezeichnet und beeinflusst die Nettoreaktionsgeschwindigkeit. Es kann verwendet werden, um das Geschwindigkeitsgesetz für die Gesamtreaktion zu verifizieren und einen vorgeschlagenen Reaktionsmechanismus zu validieren.
Denken Sie an die Zersetzung von Distickstoffmonoxid zu Stickstoff und Sauerstoff. Das experimentell ermittelte Geschwindigkeitsgesetz entspricht nicht der Geschwindigkeitsausprägung einer einstufigen Reaktion, die durch das beobachtete Vorhandensein von Sauerstoffatomen – einem Reaktionszwischenprodukt – bestätigt wird.
Daher wird ein Reaktionsmechanismus vorgeschlagen, bei dem sich alle Schritte kumulieren, um die Gesamtreaktion zu ergeben.
Erstens geben die Kurskonstanten an, dass der erste Schritt der ratenbegrenzende Schritt ist. Sie ist am langsamsten und beeinflusst somit die Gesamtreaktionsgeschwindigkeit. Ein aus diesem Schritt vorgeschlagenes Tarifgesetz kann gleich dem Gesamttarifgesetz gesetzt werden.
Dieses vorgeschlagene Geschwindigkeitsgesetz, das direkt aus der molekularen Konzentration des Elementarreaktanten abgeleitet wird, stimmt mit dem experimentellen Geschwindigkeitsgesetz überein und verifiziert den vorhergesagten Reaktionsmechanismus.
Dieser Text wurde übernommen von <a href="https://openstax.org/books/chemistry-2e/pages/12-6-reaction-mechanisms">Openstax, Chemistry 2e, 12.6: Reaction Mechanisms</a>.

mehrstufige reaktionen

Chemische Reaktionen laufen oft schrittweise ab, wobei zwei oder mehr unterschiedliche Reaktionen nacheinander ablaufen. Eine ausgeglichene Gleichung gibt die reagierende Spezies und die Produktspezies an, verrät jedoch keine Details darüber, wie die Reaktion auf molekularer Ebene abläuft. Der Reaktionsmechanismus (oder Reaktionsweg) liefert Einzelheiten über den genauen, schrittweisen Prozess, durch den eine Reaktion abläuft. Jeder Schritt eines Reaktionsmechanismus wird als Elementarreaktion bezeichnet. Diese Elementarreaktionen laufen nacheinander ab, wie in den Stufengleichungen dargestellt, und ergeben in ihrer Summe die ausgewogene chemische Gleichung, die die Gesamtreaktion beschreibt. Bei einem mehrstufigen Reaktionsmechanismus verläuft einer der Elementarschritte langsamer als die anderen – manchmal sogar deutlich langsamer. Dieser langsamste Schritt wird als geschwindigkeitsbegrenzender Schritt (oder geschwindigkeitsbestimmender Schritt) bezeichnet. Eine Reaktion kann nicht schneller ablaufen als ihr langsamster Schritt, und daher begrenzt der geschwindigkeitsbestimmende Schritt die Gesamtreaktionsgeschwindigkeit.

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. Geschwindigkeitsgesetze können direkt aus den ausgeglichenen chemischen Gleichungen für Elementarreaktionen abgeleitet werden. Dies ist jedoch bei den meisten chemischen Reaktionen nicht der Fall, bei denen ausgewogene Gleichungen häufig die Gesamtänderung im chemischen System darstellen, die sich aus mehrstufigen Reaktionsmechanismen ergibt. Daher muss das Geschwindigkeitsgesetz aus experimentellen Daten bestimmt und anschließend der Reaktionsmechanismus aus dem Geschwindigkeitsgesetz abgeleitet werden.

Betrachten Sie zum Beispiel die Reaktion von NO_2 und CO:

<img alt="Figure1" src="/files/ftp_upload/11699/11699_Equation1.svg" style="text-align: center; height: 22px;">

Das experimentelle Geschwindigkeitsgesetz für diese Reaktion bei Temperaturen über 225 °C lautet:

<img alt="Figure2" src="/files/ftp_upload/11699/11699_Equation2.svg" style="text-align: center; height: 19px;">

Nach dem Geschwindigkeitsgesetz ist die Reaktion erster Ordnung in Bezug auf NO_2 und erster Ordnung in Bezug auf CO. Bei Temperaturen unter 225 °C wird die Reaktion durch ein anderes Geschwindigkeitsgesetz beschrieben, das in Bezug auf NO_2 eine zweite Ordnung aufweist:

<img alt="Figure3" src="/files/ftp_upload/11699/11699_Equation3.svg" style="text-align: center; height: 21px;">

Dieses Geschwindigkeitsgesetz steht nicht im Einklang mit dem einstufigen Mechanismus, wohl aber im Einklang mit dem folgenden zweistufigen Mechanismus:

<img alt="Figure4" src="/files/ftp_upload/11699/11699_Equation4.svg" style="text-align: center; height: 21px;">

<img alt="Figure5" src="/files/ftp_upload/11699/11699_Equation5.svg" style="text-align: center; height: 21px;">

Der geschwindigkeitsbestimmende (langsamere) Schritt ergibt ein Geschwindigkeitsgesetz, das eine Abhängigkeit zweiter Ordnung von der NO_2 Konzentration zeigt, und die Summe der beiden Elementargleichungen ergibt die Nettogesamtreaktion.

Wenn der geschwindigkeitsbestimmende (langsamere) Schritt der erste Schritt im Reaktionsmechanismus ist, ist das Geschwindigkeitsgesetz für die Gesamtreaktion im Allgemeinen dasselbe wie das Geschwindigkeitsgesetz für diesen Schritt. Wenn dem geschwindigkeitsbestimmenden Schritt jedoch ein Elementarschritt mit einer schnell reversiblen Reaktion vorausgeht, kann es schwieriger sein, das Geschwindigkeitsgesetz für die Gesamtreaktion abzuleiten, was häufig auf das Vorhandensein von Reaktionszwischenprodukten zurückzuführen ist.

In solchen Fällen kann das Konzept genutzt werden, dass eine reversible Reaktion im Gleichgewicht ist, wenn die Geschwindigkeiten des Vorwärts- und Rückwärtsprozesses gleich sind.

Mehrstufige Reaktionen

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

Education

Lehre

JoVE Core

Organic Chemistry

Organische Chemie

Unit 1

Einheit 1

Thermodynamics and Chemical Kinetics

Thermodynamik und chemische Kinetik

KEY TERMS AND CONCEPTS

KERNBEGRIFFE UND KONZEPTE

reaction mechanisms and rate-determining steps

Chemical reactions can happen in a sequence of steps. A balanced chemical equation shows the reactants and products, but it does not show how the reaction occurs at the molecular level. The reaction mechanism, or reaction path, gives that step-by-step detail.

Each step in a mechanism is an elementary reaction. These elementary reactions happen in order, and together they add up to the balanced equation for the overall reaction. In a multistep mechanism, one step is often much slower than the others. This slowest step is the rate-limiting step, also called the rate-determining step.

The rate-determining step controls how fast the whole reaction can go. A reaction cannot move faster than its slowest step. Unlike an overall balanced equation, an elementary reaction shows the actual reactants that break and form bonds.

For elementary reactions, the rate law can be written directly from the chemical equation. For most reactions, that is not possible because the balanced equation represents the overall change, not each step. In those cases, the rate law must be found from experiments, and the mechanism is then deduced from that rate law.

The reaction between NO2 and CO shows how this works. At temperatures above 225 °C, the experimental rate law is first-order in NO2 and first-order in CO. At temperatures below 225 °C, the rate law is different and is second-order in NO2. That lower-temperature rate law does not fit a single-step mechanism, but it does fit a two-step mechanism.

In that two-step mechanism, the slower step produces the rate law with second-order dependence on NO2. The two elementary equations then combine to give the net overall reaction. In general, when the rate-determining step is the first step, the overall rate law matches that step.

The rate law can be harder to work out when a fast reversible step comes before the slow step. This can happen because intermediates form during the reaction. In those cases, the idea of equilibrium is useful, where the forward and reverse rates are equal.

Reaction Mechanisms and Rate-Determining Steps

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rate law and reaction order

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key terms and concepts

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organic chemistry

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Next Generation Science Standards

Physical Science

Matter and Its Interactions

HS-PS1-2

HS-PS1-7

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South Carolina College and Career Ready Science Standards

C-PS1-4

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C-PS1-6

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HS-PS1-1

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HS-PS1-3

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Reaction Mechanisms and Rate-Determining Steps

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