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

一个化学反应通常由 一个整体平衡的化学方程式来表示，该方程式表示反应物和产物。然而，实际的反应往往 更为复杂，而且是分多个步骤进行的。例如，一氧化氮与氢气 形成氮气和水的这个反应，分三个不同的连续步骤进行。这些步骤称为反应机理。反应机理中的每一步 都称为基本反应，代表反应物种 之间的相互作用，如键的断裂或形成。像二氧化二氮和一氧化二氮这样的 特殊分子，在一个基本步骤中形成，在另一个基本步骤中消耗。这种物质被称为反应中间体。反应中间体是基本反应的 低能态产物。它们通常寿命很短，这就解释了它们在 产物混合物中的缺失。反应中间体与活化 络合物不同。活化络合物是仅存在于反应物 转化为产物过程中的高能 过渡态。将这些基本步骤结合起来，得到了 整个化学反应的方程式。在这里，反应中间体被消除，因此不会 出现在整个化学方程式中。不同的基本反应 可以以不同的速度进行。最慢的基本步骤决定了整个反应的 速率。在这里，二氧化二氮与氢气的反应 是限速步骤。根据反应分子的数目或分子数的 不同，基本反应通常可以分为三种 类型。在单分子反应中，一个反应物分子 转化成一个或多个产物。在双分子反应中，两个不同的分子发生反应。三分子反应，虽然很少见，但涉及三个单独的分子 反应生成中间体或产物。与通过实验确定的整个化学反应的 速率定律不同，基本反应的速率定律 可以通过它们的反应物的化学计量系数 来预测。简言之，一个基本反应的分子数 对应于该基本步骤的 总反应级数。因此，单分子反应通常是 一级反应，双分子反应 是二级反应，而三分子反应 是三级反应。对反应机理和动力学的理解 有助于化学家识别和优化 化学反应。

反应机理

化学反应通常是循序渐进发生的，涉及两种或多种不同的反应依次发生。 平衡的方程指示反应物种和生成物物种，但它没有显示反应在分子级别的发生方式的详细信息。 反应机理 (或反应路径) 提供了有关发生反应的精确，分步过程的详细信息。 
 例如，臭氧分解似乎遵循机理，并执行两个步骤： 
 <img alt="Eq1" src="/files/ftp_upload/11381/11381_Equation_1.png" style="text-align: center; height: 70px;" />
 反应机理中的每个步骤都称为基本反应。 这些基本反应按步骤方程式中所示的顺序发生，并与产率描述总体反应的平衡化学方程相加： 
 <img alt="Eq2" src="/files/ftp_upload/11381/11381_Equation_2.png" style="text-align: center; height: 50px;" />
 请注意，第一步产生的氧原子在第二步中被消耗，在整体反应中不会显示为生成物。 这种一步生产并在随后一步中消费的物种被称为反应中间体。 
 虽然整体反应方程表明两个臭氧分子会做出反应以产生三个氧分子，但实际的反应机理并不涉及两个臭氧分子的直接碰撞和反应。 相反，一个 O3 分解为产率 O2 和一个氧原子，另一个 O3 分子随后与氧原子反应为产率两个额外的氧分子。 
 与代表整体反应的平衡方程式不同，基本反应方程式是化学变化的明确表示。 一个基本的反应等式描述了正在进行键拆分 / 制作的实际反应物，以及形成的产品。 因此，一种基本反应的速率定律可以直接从其平衡的化学方程中导出。 但是，典型化学反应的情况并非如此，只有通过实验才能可靠地确定速率定律。 
 单分子基本反应 
 基本反应的分子性是反应物物种 (原子、分子或离子) 的数量。 例如，分子反应涉及单个反应物产生一个或多个生成物分子的反应： 
 <img alt="Eq3" src="/files/ftp_upload/11381/11381_Equation_3.png" style="text-align: center; height: 50px;" />
 单分子反应的速率定律为第一级；率值 = k [A]。 
 分子反应可能是复杂反应机理中的几种基本反应之一。 例如，该反应 (O3 (g) → O2 (g) + O) 说明了作为两级反应机理的一部分发生的单分子基本反应。 但是，一些分子反应可能是单步反应机理的唯一一步。 (换言之，在某些情况下，“整体”反应也可能是基本反应。) 例如，环丁烷 C4H8 至乙烯 C2H4 的气相分解由化学品方程表示： 
 <img alt="Eq3A" src="/files/ftp_upload/11381/11381_Equation_3A.png" style="text-align: center; height: 50px;" />
 此方程代表整体反应，描述了单分子基本过程。 速率定律根据这一方程预测，假设这是一种基本反应，结果与速率定律得出的总体反应实验结果相同，显示出第一级行为： 
 <img alt="Eq4" src="/files/ftp_upload/11381/11381_Equation_4.png" style="text-align: center; height: 70px;" />
 观察到的和预测的速率定律之间达成的这一协议表明，提议的单步单步单步单分子过程是丁二烯反应的合理机理。 
 双分子的初级反应 
 双分子的反应涉及两种反应物物种。 例如： 
 <img alt="Eq5" src="/files/ftp_upload/11381/11381_Equation_5.png" style="text-align: center; height: 70px;" />
 在第一种类型 (两个反应物分子不同) 中，速率定律是 A 级和 B 级 (总体二级) 
 <img alt="Eq6" src="/files/ftp_upload/11381/11381_Equation_6.png" style="text-align: center; height: 50px;" />
 在第二种类型中，两个相同的分子发生碰撞并作出反应，速率定律是 A 中的第二个顺序： 
 <img alt="Eq7" src="/files/ftp_upload/11381/11381_Equation_7.png" style="text-align: center; height: 50px;" />
 一些化学反应是由机理引起的，该双分子的由单个 基本反应组成。 一个例子是二氧化氮与一氧化碳的反应： 
 <img alt="Eq8" src="/files/ftp_upload/11381/11381_Equation_8.png" style="text-align: center; height: 50px;" />
 双分子的基本反应也可能作为多步反应机理中的步骤进行。 原子氧与臭氧的反应是臭氧分解两步机理的第二步： 
 <img alt="Eq9" src="/files/ftp_upload/11381/11381_Equation_9.png" style="text-align: center; height: 50px;" />
 分子初级反应 
 初级特分子反应涉及三个原子，分子或离子同时发生碰撞。 由于三个粒子同时发生碰撞的概率非常低，所以极分子的基本反应非常少见。 但是，有几种既定的特分子基本反应。 氮氧化物与氧气的反应似乎涉及到特分子步骤： 
 <img alt="Eq10" src="/files/ftp_upload/11381/11381_Equation_10.png" style="text-align: center; height: 70px;" />
 同样，一氧化氮与氯的反应似乎涉及到三分子步骤： 
 <img alt="Eq11" src="/files/ftp_upload/11381/11381_Equation_11.png" style="text-align: center; height: 70px;" />
 多步反应机理中的一个基本步骤通常比其他步骤慢得多。 由于反应的进行速度不能超过其最慢的步骤，因此此步骤将限制总体反应发生的率值。 因此，最慢的基本步骤被称为反应的速率限制步骤 (或速率确定步骤)。 
 本文改编自<a href="https://openstax.org/books/chemistry-2e/pages/12-6-reaction-mechanisms"> Openstax, 化学 2e, 第12.6节：反应机理。 </a>

化学反应通常以分步方式进行，涉及按一定顺序发生的两个或多个独立反应。一个配平的方程式表明……

Education

教育

JoVE Core

化学

Chemical Kinetics

化学动力学

KEY TERMS AND CONCEPTS

关键术语与概念

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

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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

chemistry

JoVE Core 化学通过简单明了且通俗易懂的动画视频课程，将普通化学概念带入到现实生活当中。此外，在科学家行动视频中，我们演示了在实验室中进行的经典和原始研究实验。

Pennsylvania State Academic Standards

Physical Science

Chemical Reactions

3.2.9-12.D

3.2.9-12.E

Louisiana Student Standards for Science

High School

Matter And Its Interactions

HS-PS1-4

ohios-learning-standards-and-model-curriculum

Ohio’s Learning Standards and Model Curriculum

Interactions Of Matter

Chemical reactions

Virginia  Standards of Learning

HIgh School

CH.7

CH.7.e

Arizona Science Standards

HS.P1

HS.P1U1.3

Alabama Course of Study Science

Matter and Its Interactions: Energy

9-12.C.3.3.b

Next Generation Science Standards

Matter and Its Interactions

HS-PS1-2

HS-PS1-5

HS-PS1-7

Florida Department of Education

SC.912.P.12

SC.912.P.12.12

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

South Carolina College and Career Ready Science Standards

C-PS1-4

C-PS1-5

new-york-state-science-learning-standards

New York State Science Learning Standards

HS. Chemical Reactions

HS-PS1-5.

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Elementary Steps in Reaction Mechanisms

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