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

غالبا ما يتم تمثيل التفاعل الكيميائي بمعادلة كيميائية متوازنة شاملة تشير إلى المواد المتفاعلة والمنتجات.
ومع ذلك ، غالبا ما يكون التفاعل الفعلي أكثر تعقيدا ويحدث في خطوات متعددة. على سبيل المثال ، يحدث تفاعل أكسيد النيتريك مع الهيدروجين الذي يشكل غاز النيتروجين والماء في ثلاث خطوات متتالية ومتميزة. تسمى هذه الخطوات آلية التفاعل.
تسمى كل خطوة في آلية التفاعل بالتفاعل الأولي وتمثل التفاعل ، مثل كسر الرابطة أو تكوينها ، بين الأنواع المتفاعلة.
تتشكل جزيئات معينة ، مثل ثاني أكسيد النيتروجين وأكسيد النيتروز ، خلال خطوة أولية وتستهلك خلال خطوة أخرى. تسمى هذه الأنواع بوسيطة التفاعل.
المواد الوسيطة للتفاعل ليست هي نفسها الحالات الانتقالية ، والتي توجد فقط أثناء تحويل المواد المتفاعلة إلى منتجات.
يتم افتراض آليات التفاعل بناء على معادلاتها الكيميائية المتوازنة ، وقوانين المعدل المحددة تجريبيا لكل خطوة أولية.
كل خطوة لها معدل تفاعل محدد ومعدل ثابت وطاقة تنشيط. تسمى أبطأ خطوة بخطوة تحديد المعدل وتؤثر على معدل التفاعل الصافي. يمكن استخدامه للتحقق من قانون المعدل للتفاعل الكلي والتحقق من صحة آلية التفاعل المقترحة.
ضع في اعتبارك تحلل أكسيد النيتروز إلى نيتروجين وأكسجين. لا يتوافق قانون المعدل المحدد تجريبيا مع التعبير عن معدل تفاعل أحادي الخطوة ، والذي يؤكده الوجود الملحوظ لذرات الأكسجين - وهو تفاعل وسيط.
ومن ثم ، يتم اقتراح آلية تفاعل حيث تتراكم جميع الخطوات لإعطاء التفاعل الكلي.
أولا ، تشير ثوابت المعدل إلى أن الخطوة الأولى هي خطوة تحديد المعدل. إنه الأبطأ ، وبالتالي يؤثر على معدل التفاعل الإجمالي. يمكن تعيين قانون سعر الفائدة المقترح من هذه الخطوة مساويا لقانون السعر الإجمالي.
يتطابق قانون المعدل المقترح هذا ، المشتق مباشرة من التركيز الجزيئي للمتفاعل الأولي ، مع قانون المعدل التجريبي ويتحقق من آلية التفاعل المتوقعة.
هذا النص مقتبس من <a href = "https://openstax.org/books/chemistry-2e/pages/12-6-reaction-mechanisms" >Openstax ، الكيمياء 2 ه ، 12.6: آليات التفاعل</a>.

التفاعلات متعددة الخطوات

<div style="direction:rtl">
تحدث التفاعلات الكيميائية غالبًا بطريقة تدريجية تتضمن تفاعلين متميزين أو أكثر يحدثان في تسلسل. تشير المعادلة المتوازنة إلى الأنواع المتفاعلة وأنواع المنتجات، ولكنها لا تكشف عن أي تفاصيل حول كيفية حدوث التفاعل على المستوى الجزيئي. توفر آلية التفاعل (أو مسار التفاعل) تفاصيل تتعلق بالعملية الدقيقة خطوة بخطوة التي يحدث التفاعل من خلالها. تسمى كل خطوة من خطوات آلية التفاعل بتفاعل ابتدائي أولي. تحدث هذه التفاعلات الأولية بالتسلسل، كما هو موضح في المعادلات المرحلية، ومجموعها يعطي المعادلة الكيميائية المتوازنة التي تصف التفاعل الإجمالي. في آلية التفاعل متعددة الخطوات، تكون إحدى الخطوات الأولية أبطأ من الخطوات الأخرى، وأحيانًا أبطأ بشكل ملحوظ. تسمى هذه الخطوة الأبطأ بخطوة تحديد المعدل (أو الخطوة المحددة للمعدل). لا يمكن للتفاعل أن يستمر بشكل أسرع من أبطأ خطواته، وبالتالي فإن الخطوة المحددة للمعدل تحد من سرعة التفاعل الإجمالي. 
على عكس المعادلات المتوازنة التي تمثل التفاعل الشامل، فإن معادلات التفاعلات الأولية هي تمثيل صريح للتغير الكيميائي. تصور معادلة التفاعل الأولية المادة (المواد) المتفاعلة الفعلية التي تخضع لكسر/تكوين الروابط والمنتج (المنتجات) المتكون. يمكن اشتقاق قوانين المعدلات مباشرة من المعادلات الكيميائية المتوازنة للتفاعلات الأولية. ومع ذلك، فإن هذا ليس هو الحال بالنسبة لمعظم التفاعلات الكيميائية، حيث تمثل المعادلات المتوازنة غالبًا التغير الإجمالي في النظام الكيميائي الناتج عن آليات التفاعل متعددة الخطوات. لذلك، يجب تحديد قانون المعدل من البيانات التجريبية، ويجب استنتاج آلية التفاعل لاحقًا من قانون المعدل. 
على سبيل المثال، النظر في تفاعل NO2 وCO: 
<img alt="Figure1" src="/files/ftp_upload/11699/11699_Equation1.svg" style="text-align: center; height: 22px;"> قانون المعدل التجريبي لهذا التفاعل عند درجات حرارة أعلى من 225 درجة مئوية هو:
 <img alt="Figure2" src="/files/ftp_upload/11699/11699_Equation2.svg" style="text-align: center; height: 19px;"> وفقًا لقانون المعدل، يكون التفاعل من الدرجة الأولى بالنسبة إلى NO2 ومن الدرجة الأولى بالنسبة إلى ثاني أكسيد الكربون. ومع ذلك، عند درجات حرارة أقل من 225 درجة مئوية، يوصف التفاعل بمعدل مختلف. القانون من الدرجة الثانية فيما يتعلق بـ NO2: 
<img alt="Figure3" src="/files/ftp_upload/11699/11699_Equation3.svg" style="text-align: center; height: 21px;"> 
لا يتوافق قانون المعدل هذا مع آلية الخطوة الواحدة، ولكنه يتوافق مع آلية الخطوتين التالية:
 <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;"> تعطي خطوة تحديد المعدل (الأبطأ) قانون معدل يوضح الاعتماد من الدرجة الثانية على تركيز NO2، ويعطي مجموع المعادلتين الأوليتين صافي التفاعل الإجمالي. 
بشكل عام، عندما تكون خطوة تحديد المعدل (الأبطأ) هي الخطوة الأولى في آلية التفاعل، فإن قانون سرعة التفاعل الكلي هو نفس قانون معدل هذه الخطوة. ومع ذلك، عندما تكون خطوة تحديد المعدل مسبوقة بخطوة أولية تتضمن تفاعلًا عكسيًا سريعًا، قد يكون من الصعب استخلاص قانون سرعة التفاعل الإجمالي، غالبًا بسبب وجود متفاعلات وسطية. 
في مثل هذه الحالات، يمكن استخدام مفهوم أن التفاعل العكسي يكون في حالة توازن عندما تكون معدلات العمليات الأمامية والعكسية متساوية.
</div>

غالبا ما تحدث التفاعلات الكيميائية بطريقة تدريجية تتضمن تفاعلين أو أكثر متميزين يحدثان في تسلسل. تشير المعادلة المتوازنة إلى ...

Education

JoVE Core

Organic Chemistry

الكيمياء العضوية

Unit 1

الوحدة الأولى

Thermodynamics and Chemical Kinetics

الديناميكا الحرارية والحركية الكيميائية

KEY TERMS AND CONCEPTS

المصطلحات والمفاهيم الأساسية

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

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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

Solvation is the process of surrounding a solute with solvent. It spreads the solute evenly through the solvent. A useful rule is that like dissolves like. A good solvent has molecular features similar to the substance being dissolved. Polar solvents dissolve polar solutes, and apolar solvents dissolve apolar solutes. A polar solvent has a high dielectric constant, with ε greater than or equal to 15. An apolar solvent has a low dielectric constant. The dielectric constant is defined by the...

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

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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

energy-diagrams-transition-states-and-intermediates

Reaction Coordinate Diagrams in Chemical Reactions

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predicting reaction outcomes

How Temperature Changes Reaction Rate

Reaction rate and reaction pathway are key ideas in chemical kinetics. Thermodynamics, by contrast, focuses on the state of a system. It describes the properties, behavior, and components of reactants and products. It does not describe the path of the process or the speed of the reaction. Kinetics gives information on the atomic or molecular level, while thermodynamics focuses on the energetics of the reactants and products. Thermodynamics can show what may happen in a reaction, but it does...

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

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

Periodic Classification of the Elements

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Atomic Structure

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Electron Configurations

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Electronic Structure of Atoms


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Electron Configuration of Multielectron Atoms

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Atomic Radii and Effective Nuclear Charge

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The Aufbau Principle and Hund's Rule

To determine the electron configuration for any particular atom, we can build the structures in the order of atomic numbers. Beginning with hydrogen, and ...

Coulomb's Law

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Electric Charges

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Sources and Properties of Electric Charge

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Types of Forces

In most situations, forces can be grouped into two categories: contact forces and field forces.&#160; Contact forces occur as a result of direct physical ...

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

Atoms and molecules interact through bonds (or forces): intramolecular and intermolecular. The forces are electrostatic as they arise from interactions ...

Van der Waals Interactions

Atoms and molecules interact with each other through intermolecular forces. These electrostatic forces arise from attractive or repulsive interactions ...

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, ...

Temperature Dependence on Reaction Rate

The Collision Theory
Atoms, molecules, or ions must collide before they can react with each other. Atoms must be close together to form chemical bonds. ...

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Stoichiometry, Product Yield, and Limiting Reactants

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

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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

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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



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Electric Field of Two Equal and Opposite Charges

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Electric Field

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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

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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

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Coupled Reactions

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Energy Diagrams, Transition States, and Intermediates

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What is Metabolism?

Overview

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Types of Chemical Reactions: Anabolic and Catabolic

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Non-equilibrium in the Cell

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Activation Energy

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Energy Transfer in Chemical Reactions

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Factors Influencing the Rate of Chemical Reactions

A variety of factors influence the rate of chemical reactions. For a chemical reaction to happen, atoms must collide with enough energy to overcome the ...

Predicting Reaction Outcomes

Kinetics describes the rate and path by which a reaction occurs. In contrast, thermodynamics deals with state functions and describes the properties, ...

States of Matter and Phase Changes

The internal energy of a substance&#8212;the total kinetic energy of all its molecules and the potential energy of their associated forces&#8212;depends ...

Reaction Mechanisms

Chemical reactions often occur in a stepwise fashion, involving two or more distinct reactions taking place in a sequence. A balanced equation indicates ...

Temperature Dependence

Chemical Kinetics 
The reaction rate is the speed at which a chemical reaction occurs. The reaction rate is defined as the change in concentration of a ...

Molecular Kinetic Energy

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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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