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

化学反应通常用一个总的配平化学方程式来表示，该方程式指明了反应物和生成物。
然而，实际的反应通常更为复杂，并分多个步骤进行。例如，一氧化氮与氢气反应生成氮气和水的过程分为三个截然不同的连续步骤。这些步骤称为反应机理。
反应机理中的每一步称为基元反应，代表反应物之间发生的相互作用，例如化学键的断裂或形成。
某些分子，如二氧化二氮和一氧化二氮，在一个基元步骤中生成，在另一个基元步骤中被消耗。这类物质被称为反应中间体。
反应中间体与过渡态不同，过渡态仅在反应物转化为产物的过程中存在。
反应机理是基于其配平的化学方程式以及通过实验确定的各基元步骤的速率定律而提出的假设。
每一步都有特定的反应速率、速率常数和活化能。其中最慢的步骤称为决速步，它决定了总反应速率。通过决速步可验证总反应的速率方程，并用于验证所提出的反应机理。
考虑一氧化二氮分解为氮气和氧气的反应。实验测定的速率定律与单步反应的速率表达式不符，这一现象通过观察到氧原子（一种反应中间体）的存在得到了证实。
因此，提出了一个反应机理，其中所有步骤累积起来得到总反应。
首先，速率常数表明第一步是决速步。该步骤最慢，因此影响整体反应速率。由此步骤提出的速率方程可与整体速率方程相等。 
该提出的速率定律直接源自基元反应物的分子浓度，与实验测得的速率定律一致，验证了所预测的反应机理。
本文改编自 <a href="https://openstax.org/books/chemistry-2e/pages/12-6-reaction-mechanisms">Openstax，《化学》第2版，12.6节：反应机理</a>。

多步反应

化学反应通常以逐步方式发生，涉及按顺序发生的两个或多个不同的反应。 平衡方程表明了反应物质和产物物质，但它没有揭示有关反应如何在分子水平上发生的细节。 反应机理（或反应路径）提供了有关反应发生的精确、逐步过程的详细信息。 反应机理中的每个步骤称为基元反应。 这些基元反应按顺序发生，如步骤方程所示，它们相加得出描述整个反应的平衡化学方程。 在多步反应机制中，其中一个基元反应的速度比其他基元反应慢，有时甚至慢得多。 这个最慢的步骤称为限速步骤（或速率限制步骤）。 反应不能比其最慢的步骤进行得更快，因此，速率限制步骤限制了总反应速率。


 与代表整体反应的平衡方程不同，基元反应方程是化学变化的明确表示。 基元反应方程描述了发生键断裂/形成的实际反应物以及形成的产物。 速率定律可以直接从基元反应的平衡化学方程导出。 然而，大多数化学反应并非如此，平衡方程通常代表多步反应机制导致的化学系统的整体变化。 因此，必须根据实验数据确定速率定律，并随后从速率定律推导出反应机理。


 例如，考虑 NO_2 和 CO 的反应:


 <img alt="Figure1" src="/files/ftp_upload/11699/11699_Equation1.svg" style="text-align: center; height: 22px;">


 该反应在 225 °C 以上温度下的实验速率定律为:


 <img alt="Figure2" src="/files/ftp_upload/11699/11699_Equation2.svg" style="text-align: center; height: 19px;">


 根据速率定律，该反应对于 NO_2 为一级反应，对于 CO 为一级反应。然而，在低于 225 °C 的温度下，该反应由不同的速率定律描述，即对于 CO 为二级反应。对二NO_2::


 <img alt="Figure3" src="/files/ftp_upload/11699/11699_Equation3.svg" style="text-align: center; height: 21px;">


 该速率定律与单步机制不一致，但与以下两步机制一致:


 <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;">


 速率限制（较慢）步骤给出了对 NO2 浓度的依赖性为二阶的速率定律，两个基本方程的总和给出了净总反应。


 一般来说，当限速（较慢）步骤是反应机制中的第一步时，整个反应的速率定律与该步骤的速率定律相同。 然而，当限速步骤之前是涉及快速可逆反应的基本步骤时，通常由于反应中间体的存在，整个反应的速率定律可能更难以推导。


 在这种情况下，可以利用这样的概念:当正向过程和反向过程的速率相等时，可逆反应处于平衡状态。

化学反应通常以分步方式进行，涉及按顺序发生的两个或多个不同反应。一个配平的方程式表明……

Education

教育

JoVE Core

Organic Chemistry

有机化学

Unit 1

第一单元

Thermodynamics and Chemical Kinetics

热力学与化学动力学

KEY TERMS AND CONCEPTS

关键术语与概念

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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effect of temperature change on reaction rate

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multi-step reactions

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energy diagrams, transition states, and intermediates

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Reaction Mechanisms and Rate-Determining Steps | JoVE K12

organic chemistry

JoVE Core 有机化学（Organic Chemistry）使用简明易懂的动画视频课程解释立体化学、反应和机制的基本概念。 此外，科学家在行动视频展示了相关概念在当今世界各地实验室进行的经典和原创研究实验中的应用。

Next Generation Science Standards

Physical Science

Matter and Its Interactions

HS-PS1-2

HS-PS1-7

south-carolina-college-and-career-ready-science-standards

South Carolina College and Career Ready Science Standards

C-PS1-4

C-PS1-5

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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