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

화학 반응은 종종 반응물질과 제품을 나타내는 전반적인 균형 잡힌 화학 방정식으로 표현됩니다.
그러나 실제 반응은 종종 더 복잡하고 여러 단계에서 발생합니다. 예를 들어, 질소 가스와 물을 형성하는 수소와 산화 질소의 이 반응은 세 가지 뚜렷한 연속적인 단계로 일어납니다. 이러한 단계는 반응 메커니즘이라고 합니다.
반응 메커니즘의 각 단계는 초등반응이라고 하며 반응종 간의 결합 파손 또는 형성과 같은 상호작용을 나타낸다.
이산화질소와 아산화질소와 같은 특정 분자는 한 가지 기본 단계에서 형성되어 다른 단계에서 소비됩니다. 이러한 종은 반응 중간제라고합니다.
반응 중간체는 제품에 대한 반응제를 변환하는 동안만 존재하는 전환 상태와 동일하지 않습니다.
반응 메커니즘은 균형 잡힌 화학 방정식에 기초하여 가설, 각 초등학교 단계의 실험적으로 결정 된 속도 법칙.
각 단계마다 특정 반응 속도, 속도 상수 및 활성화 에너지가 있습니다. 가장 느린 단계는 속도 결정 단계라고하며 순 반응 속도에 영향을 미칩니다. 전체 반응에 대한 요율법을 확인하고 제안된 반응 메커니즘을 검증하는 데 사용할 수 있다.
질소와 산소에 아산화 질소의 분해를 고려하십시오. 실험적으로 결정된 속도법은 산소 원자의 관찰된 존재(반응 중간)에 의해 확증되는 단단계 반응의 속도 발현에 해당하지 않는다.
따라서, 모든 단계가 전체적인 반응을 주기 위하여 구체화되는 반응 기계장치가 제안됩니다.
첫째, 속도 상수는 첫 번째 단계가 속도 제한 단계임을 나타냅니다. 그것은 가장 느리고, 따라서 전반적인 반응 비율에 영향을 미칩니다. 이 단계에서 제안 된 요금법은 전체 요금법과 동일하게 설정할 수 있습니다.
이 제안된 비율법은, 초등학교 반응제의 분자 농도에서 직접 파생되고, 실험속도법과 일치하고 예측된 반응 기전을 확인한다.
이 텍스트는 <a href="https://openstax.org/books/chemistry-2e/pages/12-6-reaction-mechanisms">Openstax, 화학 2e, 12.6 : 반응 메커니즘에서 적응된다.</a>

다단계 반응

화학 반응은 종종 두 개 이상의 서로 다른 반응이 순차적으로 일어나는 단계적 방식으로 발생합니다. 균형 잡힌 방정식은 반응하는 종과 생성물 종을 나타내지만 분자 수준에서 반응이 어떻게 일어나는지에 대한 세부 정보는 나타내지 않습니다. 반응 메커니즘(또는 반응 경로)은 반응이 발생하는 정확한 단계별 과정에 관한 세부 정보를 제공합니다. 반응 메커니즘의 각 단계를 기본 반응이라고 합니다. 이러한 기본 반응은 단계 방정식에 표시된 대로 순서대로 발생하며, 이를 합산하여 전체 반응을 설명하는 균형 잡힌 화학 방정식을 생성합니다. 다단계 반응 메커니즘에서 기본 단계 중 하나는 다른 단계보다 느리게 진행되며 때로는 상당히 느리게 진행됩니다. 이 가장 느린 단계를 속도 제한 단계(또는 속도 결정 단계)라고 합니다. 반응은 가장 느린 단계보다 빠르게 진행될 수 없으므로 속도 결정 단계는 전체 반응 속도를 제한합니다.

전체 반응을 나타내는 균형 방정식과 달리 기본 반응의 방정식은 화학 변화를 명시적으로 표현합니다. 기본 반응식은 결합이 끊어지거나 생성되는 실제 반응물과 형성된 생성물을 나타냅니다. 속도 법칙은 기본 반응에 대한 균형 화학 반응식에서 직접 유래될 수 있습니다. 그러나 균형 잡힌 반응식은 종종 다단계 반응 메커니즘으로 인해 발생하는 화학 시스템의 전반적인 변화를 나타내는 대부분의 화학 반응에는 해당되지 않습니다. 따라서, 속도법칙은 실험 데이터로부터 결정되어야 하며, 반응 메커니즘은 후속적으로 속도법칙으로부터 추론되어야 합니다.

예를 들어 NO_2와 CO의 반응을 생각해 보세요.

<img alt="Figure1" src="/files/ftp_upload/11699/11699_Equation1.svg" style="text-align: center; height: 22px;">

225°C 이상의 온도에서 이 반응에 대한 실험 속도 법칙은 다음과 같습니다.

<img alt="Figure2" src="/files/ftp_upload/11699/11699_Equation2.svg" style="text-align: center; height: 19px;">

 속도 법칙에 따르면 반응은 NO_2에 대한 1차 반응이고 CO에 대해서는 1차 반응입니다. 그러나 225°C 미만의 온도에서 반응은 NO_2에 대해 2차에 해당하는 다른 속도 법칙으로 설명됩니다.

<img alt="Figure3" src="/files/ftp_upload/11699/11699_Equation3.svg" style="text-align: center; height: 21px;">

이 속도 법칙은 단일 단계 메커니즘과 일치하지 않지만 다음과 같은 2단계 메커니즘과 일치합니다.

<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;">

속도 결정(느린) 단계는 NO_2 농도에 대한 2차 의존성을 보여주는 속도 법칙을 제공하고 두 기본 방정식의 합은 순 전체 반응을 제공합니다.

일반적으로 속도 결정(느린) 단계가 반응 메커니즘의 첫 번째 단계인 경우 전체 반응의 속도 법칙은 이 단계의 속도 법칙과 동일합니다. 그러나 속도 결정 단계 앞에 빠르게 가역적인 반응을 포함하는 기본 단계가 있을 경우, 종종 반응 중간체의 존재로 인해 전체 반응에 대한 속도 법칙을 도출하기가 더 어려울 수 있습니다.

이러한 경우, 정방향 공정과 역방향 공정의 속도가 같을 때 가역 반응이 평형 상태에 있다는 개념이 활용될 수 있습니다.

화학 반응은 종종 두 개 이상의 별개의 반응이 순서대로 발생하는 단계적 방식으로 발생합니다. 균형 잡힌 방정식은 다음을 나타냅니다.

Education

교육

JoVE Core

Organic Chemistry

유기 화학

Unit 1

제1단원

Thermodynamics and Chemical Kinetics

열역학과 화학 반응 속도론

KEY TERMS AND CONCEPTS

주요 용어 및 개념

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

reaction-mechanisms-and-rate-determining-steps

chemical reactions

Stoichiometry, Rates, and Equilibrium

Stoichiometry, reaction rates, and equilibrium explain how chemical reactions are measured and predicted. A balanced chemical equation shows the chemical formulas of the reactants and products in a chemical change. It also gives the relative amounts of each substance involved. Reaction stoichiometry helps predict how much reactant is needed to make a desired amount of product. It can also show how much product forms from a given amount of reactant. The amounts shown by a balanced equation are...

enthalpy and heat of reaction

Combustion Heat and Hydrocarbon Stability

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energetics of solution formation

Why Some Solutions Form Spontaneously

Solution formation can happen spontaneously under the right conditions. A spontaneous process is one that occurs without energy from an outside source. During dissolution, solute and solvent particles separate and then attract each other in a new way. The energy change depends on the balance of intermolecular forces, which are the attractions between particles. If the solute-solute and solvent-solvent forces are about the same as the new solute-solvent forces, the solution forms with no energy...

entropy and solvation

Hydrophobic Effects in Solvation

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gibbs free energy and thermodynamic favorability

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Temperature Effects on Gibbs Free Energy

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chemical and solubility equilibria

How Solubility Depends on Free Energy

Solubility depends on free energy changes when a solute dissolves in a solvent. The free energy change for this process is called the free energy of solution, ΔGsolution. When ΔGsolution is less than zero, solution formation is favorable. When it is greater than zero, solution formation is unfavorable. For complete dissolution to happen, the Gibbs energy change must be negative. This means the balance of forces in the mixture must favor solution formation. The amount a solute dissolves in a...

rate law and reaction order

How Reactant Concentration Sets Reaction Rate

Reaction rate depends on reactant concentration, and rate laws show that relationship with a mathematical equation. A rate law, also called a differential rate law or rate equation, links the rate of a chemical reaction to the concentrations of its reactants. For a generic reaction, a A + b B  products, the rate law can be written as rate = k [A]^m [B]^n. Here, [A] and [B] are the molar concentrations of the reactants. The constant k is the rate constant, and it is specific to one reaction at...

effect of temperature change on reaction rate

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Arrhenius Equation in Reaction Speed

The Arrhenius equation connects activation energy, temperature, and the rate constant, k, for many chemical reactions. It helps explain why some reactions move slowly while others happen faster. In this equation, R is the ideal gas constant, with a value of 8.314 J/mol·K. T is the temperature in kelvin, Ea is the activation energy in joules per mole, e is the constant 2.7183, and A is the frequency factor. The frequency factor, A, describes how often reacting molecules collide and how well...

multi-step reactions

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...

bond dissociation energy and activation energy

bond-dissociation-energy-and-activation-energy

How Bond Strength Shapes Reaction Speed

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energy diagrams, transition states, and intermediates

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Reaction Coordinate Diagrams in Chemical Reactions

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predicting reaction outcomes

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key terms and concepts

Learn how reaction mechanisms work, why the slowest step limits reaction rate, and how rate laws help identify multistep reactions in chemistry.

Reaction Mechanisms and Rate-Determining Steps | JoVE K12

organic chemistry

JoVE Core Organic Chemistry explains basic concepts of stereochemistry, reactions, and mechanisms using concise and easy-to-understand animated video lessons. Additionally, the scientist-in-action videos demonstrate the application of related concepts in classical and original research experiments performed in today's laboratories worldwide.

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

The Periodic Table

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Coulomb's Law

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Types of Forces

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Electric Fields

Source: Yong P. Chen, PhD, Department of Physics &#38; Astronomy, College of Science, Purdue University, West Lafayette, IN
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Intermolecular Forces

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Van der Waals Interactions

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Chemical Bonds


Atoms participate in a chemical bond formation&#160;to acquire a completed valence-shell electron configuration similar to that of the noble gas nearest ...

Electron Behavior

Electrons are negatively charged subatomic particles attracted to and orbit around the positively-charged nucleus of an atom. They reside in spaces ...

Subatomic Particles

Dalton was only partially correct about the particles that make up matter. All matter is composed of atoms, and atoms are composed of three smaller ...

Ions and Ionic Charges

In ordinary chemical reactions, the nucleus &mdash; which contains the protons and neutrons of each atom and thus identifies the element &mdash; remains ...

Use the periodic table as a model to predict the relative properties of elements based on the patterns of electrons in the outermost energy level and the composition of the nucleus of atoms.  Clarification Statement: Chemistry: Examples of properties that could be predicted from patterns could include reactivity of metals, types of bonds formed, numbers of bonds formed, atomic radius, atomic mass, or reactions with oxygen. Emphasis is on main group elements and qualitative understanding of the relative trends of ionization energy and electronegativity.

HS-PS1-1

Properties of Transition Metals

Transition metals are defined as those elements that have partially filled d orbitals. As shown in Figure 1, the d-block elements in groups 3&#8211;12 are ...

The Atomic Theory of Matter

The earliest recorded discussion of the basic structure of matter comes from ancient Greek philosophers. Leucippus and Democritus argued that all matter ...

Chemical Reactions

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Chemical Equations

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Introduction to Chemical Reactions

All chemical reactions begin with a reactant, the general term for one or more substances entering the reaction. Sodium and chloride ions, for example, ...

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The Collision Theory
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Stoichiometry, Product Yield, and Limiting Reactants

Source: Smaa Koraym at Johns Hopkins University, MD, USA

	Preparation of 25% v/v Ethylenediamine in Deionized WaterExpand
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Physical and Chemical Properties of Matter

The characteristics that enable us to distinguish one substance from another are called properties.

Physical Properties of Matter

A physical property is ...

Dynamic Equilibrium

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Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.  Clarification Statement: Chemistry: Examples of chemical reactions could include the reaction of sodium and chlorine, carbon and oxygen, or carbon and hydrogen. Reaction classification aids in the prediction of products (e.g. synthesis, decomposition, single displacement, double displacement, and acid-base).

Intermolecular vs Intramolecular Forces

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comparison-between-electrical-and-gravitational-forces

Comparison Between Electrical And Gravitational Forces

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intermolecular-forces-and-physical-properties

Intermolecular Forces and Physical Properties



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electric-field-of-two-equal-and-opposite-charges

Electric Field of Two Equal and Opposite Charges

Atoms generally contain the same number of positively and negatively charged particles, protons, and electrons. Hence, they are electrically neutral. ...

Electric Field

Consider two point charges, each exerting Coulomb force on the other. It is possible to describe the Coulomb interaction via an intermediate step by ...

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 ...

Electric Dipoles and Dipole Moment

Consider two charges of equal magnitude but opposite signs. If they cannot be separated by an external electric field, the system is called a permanent ...

Plan and conduct an investigation to gather evidence to compare the structure of substances at the macroscale to infer the strength of electrical forces between particles.  Clarification Statement: Emphasis is on understanding the strengths of forces between particles, not on naming specific intermolecular forces (such as dipole-dipole). Examples of particles could include ions, atoms, molecules, and network solids (such as graphite). Examples of macro-properties of substances could include the melting point and boiling point, vapor pressure, and surface tension.

HS-PS1-3

Bond Dissociation Energy and Activation Energy

Bond energy is the energy required to break a bond homolytically. These values are usually expressed in units of kcal/mol or kJ/mol and are referred to as ...

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, ...

Coupled Reactions

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Develop a model to illustrate that the release or absorption of energy from a chemical reaction system depends upon the changes in total bond energy.  Clarification Statement: Emphasis is on the idea that a chemical reaction is a system that affects the energy change.  Examples of models could include molecular-level drawings and diagrams of reactions, graphs showing the relative energies of reactants and products, and representations showing energy is conserved.

HS-PS1-4

Reaction Mechanisms and Rate-Determining Steps

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