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

Matter and its Interactions

PS.Chm.5.1

Reaction Mechanisms

Химическая реакция часто представляется общим сбалансированным химическим уравнением с указанием исходных веществ и продуктов. Однако реальная реакция часто бывает сложнее и протекает в несколько этапов. Например, эта реакция оксида азота с водородом с образованием газообразного азота и воды протекает в три последовательных этапа.Эти этапы называются механизмом реакции. Каждый этап механизма реакции называется элементарной реакцией и представляет собой взаимодействие, такое как разрыв или образование связи между реагирующими частицами. Определенные молекулы, такие как диоксид азота и закись азота, образуются на одном элементарном этапе и расходуются на другом.Такие вещества называются промежуточными продуктами реакции. Промежуточные продукты реакции это низкоэнергетические продукты элементарной реакции. Зачастую они короткоживущие, что объясняет их отсутствие в смеси продуктов.Промежуточные продукты реакции это не то же самое, что активированные комплексы. Активированные комплексы это высокоэнергетические переходные состояния, существующие только при превращении исходных веществ в продукты. Объединение элементарных этапов дает общее уравнение химической реакции.В этом случае промежуточные продукты реакции удаляются и, следовательно, не появляются в общем химическом уравнении. Различные элементарные реакции могут протекать с разной скоростью. Самый медленный элементарный этап определяет общую скорость реакции.Здесь реакция диоксида азота с газообразным водородом является этапом, ограничивающим скорость. Элементарные реакции обычно делятся на три типа, в зависимости от количества реагирующих молекул или молекулярности. В мономолекулярной реакции одна молекула исходного вещества превращается в один или несколько продуктов.В бимолекулярной реакции реагируют две разные молекулы. Тримолекулярная реакция, хотя и очень редкая, включает в себя реакцию трех разных молекул с образованием промежуточных соединений или продуктов. В отличие от кинетического уравнения реакции общей химической реакции, которое определяется экспериментально, кинетические уравнения элементарных реакций могут быть предсказаны на основе стехиометрических коэффициентов их исходных веществ.Короче говоря, молекулярность элементарной реакции соответствует общему порядку реакции элементарного этапа. Следовательно, мономолекулярные реакции часто являются реакциями первого порядка, бимолекулярные реакции реакциями второго порядка, а тримолекулярные реакции реакциями третьего порядка. Понимание механизмов и кинетики реакций помогает химикам выявлять и оптимизировать химические реакции.

Механизмы реакции

Химические реакции часто происходят поэтапно, включая две или более отдельных реакции, происходящих в последовательности. Сбалансированное уравнение указывает на участники реакции и вид продукта, но не показывает подробностей о том, как происходит реакция на молекулярном уровне. Механизм реакции (или путь реакции) предоставляет подробные сведения о точном пошаговом процессе, при котором происходит реакция. 
 Например, распад озона, как представляется, происходит в рамках механизма, который состоит из двух этапов: 
 <img alt="Eq1" src="/files/ftp_upload/11381/11381_Equation_1.png" style="text-align: center; height: 70px;" />
 Каждый из шагов в механизме реакции называется элементарной реакцией. Эти элементарные реакции происходят последовательно, как показано в уравнениях шага, и они суммируют, чтобы получить сбалансированное химическое уравнение, описывающее общую реакцию: 
 <img alt="Eq2" src="/files/ftp_upload/11381/11381_Equation_2.png" style="text-align: center; height: 50px;" />
 Обратите внимание, что атом кислорода, производимый на первом этапе, потребляется во втором и не появляется как продукт в общей реакции. Такие виды, которые производятся за один шаг и потребляются в последующем, называются промежуточными реакциях. 
 В то время как общее уравнение реакции указывает на то, что две молекулы озона реагируют на три молекулы кислорода, механизм фактической реакции не предполагает прямого столкновения и реакции двух молекул озона. Вместо этого, один O3 разлагается для получения O2 и атома кислорода, а вторая молекула O3 впоследствии вступает в реакцию с атомом кислорода, чтобы получить две дополнительные молекулы O2. 
 В отличие от сбалансированных уравнений, представляющих общую реакцию, уравнения для элементарных реакций являются явными представлениями химического изменения. Элементарное реактивное уравнение показывает фактическую реактант (ы), которые подвергаются разрыву/изготовление связей, и сформированные продукты. Таким образом, закон нормы для элементарной реакции может быть получен непосредственно из его сбалансированного химического уравнения. Однако это не относится к типичным химическим реакциям, для которых законы о норме могут быть достоверно определены только путем экспериментов. 
 Элементарные реакции Unimolecular 
 Молекулярная структура элементарной реакции — это количество видов реагент (атомов, молекул или ионов). Например, одномолекулярная реакция включает в себя реакцию одного реагент на создание одной или нескольких молекул продукта: 
 <img alt="Eq3" src="/files/ftp_upload/11381/11381_Equation_3.png" style="text-align: center; height: 50px;" />
 Закон нормы для неимолеккулярной реакции — первый порядок; Rate = k [A]. 
 Одномолекулярная реакция может быть одной из нескольких элементарных реакций в сложном механизме реакции. Например, реакция (O3 (g) → O2 (g) + O) иллюстрирует одномолекулярную элементарную реакцию, которая происходит как часть двухступенчатого механизма реакции. Однако некоторые одномолекулярные реакции могут быть единственным шагом одношагового механизма реакции. (Другими словами, в некоторых случаях «общая» реакция также может быть элементарной.) Например, газфазное разложение циклобутана, C4H8, на этилен, C2H4, представлено химическим уравнением: 
 <img alt="Eq3A" src="/files/ftp_upload/11381/11381_Equation_3A.png" style="text-align: center; height: 50px;" />
 Это уравнение представляет собой общую реакцию, описывающую неимимолекулярный элементарный процесс. Закон ставок, предсказанный из этого уравнения, предполагая, что это элементарная реакция, оказывается таким же, как закон ставок, полученный экспериментально для общей реакции, показывающий поведение первого порядка: 
 <img alt="Eq4" src="/files/ftp_upload/11381/11381_Equation_4.png" style="text-align: center; height: 70px;" />
 Это соглашение между наблюдаемыми и прогнозируемыми законами о норме ставок указывает на то, что предлагаемый одноэтапный одноэтапный процесс является разумным механизмом для реакции бутадиена. 
 Элементарные реакции на бимолекулярные 
 Биметокулярная реакция включает два вида реагент. Например: 
 <img alt="Eq5" src="/files/ftp_upload/11381/11381_Equation_5.png" style="text-align: center; height: 70px;" />
 В первом типе, где две молекулы реагент различаются, закон скорости является первым в A и первым в B (общий второй порядок) 
 <img alt="Eq6" src="/files/ftp_upload/11381/11381_Equation_6.png" style="text-align: center; height: 50px;" />
 Во втором типе, в котором сталкиваются и реагируют две идентичные молекулы, закон скорости является вторым порядком в: 
 <img alt="Eq7" src="/files/ftp_upload/11381/11381_Equation_7.png" style="text-align: center; height: 50px;" />
 Некоторые химические реакции возникают механизмами, состоящим из одной биметолеккулярной элементарной реакции. Одним из примеров является реакция диоксида азота с угарным газом: 
 <img alt="Eq8" src="/files/ftp_upload/11381/11381_Equation_8.png" style="text-align: center; height: 50px;" />
 Биметолеккулярные элементарные реакции также могут быть задействованы в качестве шагов в многошаговом механизме реакции. Реакция атомного кислорода с озоном является вторым шагом двухступенчатого механизма разложения озона: 
 <img alt="Eq9" src="/files/ftp_upload/11381/11381_Equation_9.png" style="text-align: center; height: 50px;" />
 Термолекулярные элементарные реакции 
 Элементарная термолекулярная реакция включает одновременное столкновение трех атомов, молекул или ионов. Термолекулярные элементарные реакции редки, потому что вероятность одновременного столкновения трех частиц очень редка. Однако, есть несколько устоявшихся термолекулярных элементарных реакций. Реакция оксида азота с кислородом, как представляется, включает в себя термолекулярные шаги: 
 <img alt="Eq10" src="/files/ftp_upload/11381/11381_Equation_10.png" style="text-align: center; height: 70px;" />
 Аналогично, реакция оксида азота с хлором, как представляется, включает в себя термолекулярные шаги: 
 <img alt="Eq11" src="/files/ftp_upload/11381/11381_Equation_11.png" style="text-align: center; height: 70px;" />
 Часто один из элементарных шагов в многошаговом механизме реакции значительно медленнее, чем другие. Поскольку реакция не может быть более быстрой, чем ее самый медленный шаг, этот шаг ограничит скорость, с которой происходит общая реакция. Таким образом, самый медленный элементарный шаг называется шагом ограничения скорости (или шагом определения скорости) реакции. 
 Этот текст адаптирован из<a href="https://openstax.org/books/chemistry-2e/pages/12-6-reaction-mechanisms"> Openstax, Химия 2е изд., 12.6: Механизмы реакции. </a>

Химические реакции часто происходят ступенчато, включая две или более различных реакций, происходящих в одной последовательности. Сбалансированное уравнение указывает на ...

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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...

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

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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...

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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

Collision Theory and Reaction Speed

Collision theory explains why reaction speed depends on how particles collide. Atoms, molecules, or ions must come close together before they can react. The theory says the reaction rate depends on the collision rate, the orientation of the colliding species, and whether the collision has enough energy for bonds to change. A gas-phase reaction between carbon monoxide and oxygen shows why orientation matters. The molecules can collide in more than one way. If the oxygen side of carbon monoxide...

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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Electron Configuration of Multielectron Atoms

The alkali metal sodium (atomic number 11) has one more electron than the neon atom. This electron must go into the lowest-energy subshell available, the ...

Use models to explain how electrons are distributed in atoms.

PS.Chm.1.3

Periodic Classification of the Elements

The periodic table arranges atoms based on increasing atomic number so that elements with the same chemical properties recur periodically. When their ...

The Periodic Table

As early chemists discovered more elements, they realized that various elements could be grouped by their similar chemical behaviors. One such grouping ...

Atomic Mass

Atoms &mdash; and the protons, neutrons, and electrons that compose them &mdash; are extremely small. For example, a carbon atom weighs less than 2 ...

Molar Mass

The identity of a substance is defined not only by the types of atoms or ions it contains but by the quantity of each type of atom or ion. For example, ...

Use the Periodic Table as a model to predict the relative properties of elements based on the pattern of valence electrons in the outermost energy levels of atoms.

PS.Chm.2.1

atomic-radii-and-effective-nuclear-charge

Atomic Radii and Effective Nuclear Charge

The elements in groups of the periodic table exhibit similar chemical behavior. This similarity occurs because the members of a group have the same number ...

Ionic Radii

Ionic radius is the measure used to describe the size of an ion. A cation always has fewer electrons and the same number of protons as the parent atom; it ...

Ionization Energy

The amount of energy required to remove the most loosely bound electron from a gaseous atom in its ground state is called its first ionization energy ...

Electron Affinity

The electron affinity (EA) is the energy change for adding an electron to a gaseous atom to form an anion (negative ion).



This process can be either ...

Construct an explanation to infer the atomic size, reactivity, electronegativity, and ionization energy of an element based on its position in the Periodic Table.

PS.Chm.2.2

Types of Chemical Bonds

Chemical bonding theories were pioneered by American chemist Gilbert&#xA0;N. Lewis. He developed a model called the Lewis model to explain the type and ...

Lewis Symbols and the Octet Rule

Chemical bonds are complex interactions between two or more atoms or ions, which reduce the potential energy of the molecule. Gilbert N. Lewis developed a ...

Ionic Bonding and Electron Transfer

Ions are atoms or molecules bearing an electrical charge. A cation (a positive ion) forms when a neutral atom loses one or more electrons from its valence ...

The Born-Haber Cycle

Lattice Energy&#xA0;

An ionic compound is stable because of the electrostatic attraction between its positive and negative ions. The lattice energy of a ...

trends-in-lattice-energy-ion-size-and-charge

Trends in Lattice Energy: Ion Size and Charge

An ionic compound is stable because of the electrostatic attraction between its positive and negative ions. The lattice energy of a compound is a measure ...

Covalent Bonding and Lewis Structures

Compared to ionic bonds, which results from the transfer of electrons between metallic and nonmetallic atoms, covalent bonds result from the mutual ...

Electronegativity

Whether a bond is nonpolar or polar covalent is determined by a property of the bonding atoms called electronegativity.&#xA0;

Electronegativity values of ...

bond-polarity-dipole-moment-and-percent-ionic-character

Bond Polarity, Dipole Moment, and Percent Ionic Character

Bond Polarity

The absolute value of the difference in electronegativity (&Delta;EN) of two bonded atoms provides a rough measure of the polarity to be ...

lewis-structures-of-molecular-compounds-and-polyatomic-ions

Lewis Structures of Molecular Compounds and Polyatomic Ions

To draw Lewis structures for complicated molecules and molecular ions, it is helpful to follow a step-by-step procedure as outlined:


	Determine the ...

Resonance

The Lewis structure of a nitrite anion (NO2&minus;) may actually be drawn in two different ways, distinguished by the locations of the N-O and N=O ...

Formal Charges

In some cases, there are seemingly more than one valid Lewis structures for molecules and polyatomic ions. The concept of formal charges can be used to ...

Exceptions to the Octet Rule

Many covalent molecules have central atoms that do not have eight electrons in their Lewis structures. These molecules fall into three categories:


	...

Bond Energies and Bond Lengths

Stable molecules exist because covalent bonds hold the atoms together. The strength of a covalent bond is measured by the energy required to break it, ...

Analyze and interpret data to explain the mechanisms and properties of the two main types of intramolecular (ionic and covalent) bonds.

PS.Chm.3.1

Analyze the structure of atoms and isotopes.

Elementary Steps in Reaction Mechanisms

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