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

Bir kimyasal reaksiyon genellikle reaktanları ve ürünleri gösteren genel dengeli bir kimyasal denklem ile temsil edilir.
Bununla birlikte, gerçek reaksiyon genellikle daha karmaşıktır ve birden fazla adımda ortaya çıkar. Örneğin, nitrik oksidin nitrojen gazı ve su oluşturan hidrojen ile bu reaksiyonu, birbirini izleyen üç farklı adımda gerçekleşir. Bu adımlara reaksiyon mekanizması denir.
Reaksiyon mekanizmasındaki her adıma temel reaksiyon denir ve reaksiyona giren türler arasındaki bağ kırılması veya oluşumu gibi etkileşimi temsil eder.
Dinitrojen dioksit ve azot oksit gibi spesifik moleküller, bir temel adım sırasında oluşur ve başka bir aşamada tüketilir. Bu tür türlere reaksiyon ara ürünleri denir.
Reaksiyon ara ürünleri, yalnızca reaktanların ürünlere dönüşümü sırasında var olan geçiş durumlarıyla aynı değildir.
Reaksiyon mekanizmaları, dengeli kimyasal denklemlerine ve her bir temel adımın deneysel olarak belirlenmiş hız yasalarına dayanarak hipotez edilir.
Her adımın belirli bir reaksiyon hızı, hız sabiti ve aktivasyon enerjisi vardır. En yavaş adım, hız belirleyici adım olarak adlandırılır ve net reaksiyon hızını etkiler. Genel reaksiyon için hız yasasını doğrulamak ve önerilen bir reaksiyon mekanizmasını doğrulamak için kullanılabilir.
Azot oksidin nitrojen ve oksijene ayrışmasını düşünün. Deneysel olarak belirlenen hız yasası, gözlemlenen oksijen atomlarının varlığı ile desteklenen tek adımlı bir reaksiyonun hız ifadesine karşılık gelmez - bir reaksiyon ara maddesi.
Bu nedenle, genel reaksiyonu vermek için tüm adımların biriktiği bir reaksiyon mekanizması önerilmiştir.
İlk olarak, hız sabitleri, ilk adımın hız sınırlama adımı olduğunu gösterir. En yavaş olanıdır ve bu nedenle genel reaksiyon hızını etkiler. Bu adımdan önerilen bir oran yasası, genel oran yasasına eşit olarak ayarlanabilir.
Doğrudan temel reaktantın moleküler konsantrasyonundan türetilen bu önerilen hız yasası, deneysel hız yasasıyla eşleşir ve tahmin edilen reaksiyon mekanizmasını doğrular.
 Bu metin <a href="https://openstax.org/books/chemistry-2e/pages/12-6-reaction-mechanisms"> Openstax, Chemistry 2e, 12.6: Reaction Mechanisms adlı kaynaktan uyarlanmıştır</a>.

Çok adımlı reaksiyonlar

Kimyasal reaksiyonlar genellikle iki veya daha fazla farklı reaksiyonun bir sırayla gerçekleşmesini içeren adım adım gerçekleşir. Dengeli bir denklem, reaksiyona giren türleri ve ürün türlerini belirtir, ancak reaksiyonun moleküler düzeyde nasıl gerçekleştiğine ilişkin hiçbir ayrıntıyı ortaya çıkarmaz. Reaksiyon mekanizması (veya reaksiyon yolu), bir reaksiyonun meydana geldiği kesin, adım adım sürece ilişkin ayrıntıları sağlar. Bir reaksiyon mekanizmasındaki adımların her birine temel reaksiyon denir. Bu temel reaksiyonlar, adım denklemlerinde gösterildiği gibi sırayla meydana gelir ve genel reaksiyonu tanımlayan dengeli kimyasal denklemi verecek şekilde toplanırlar. Çok adımlı bir reaksiyon mekanizmasında, temel adımlardan biri diğerlerinden daha yavaş, bazen de önemli ölçüde daha yavaş ilerler. Bu en yavaş adıma hız sınırlayıcı adım (veya hız belirleyici adım) adı verilir. Bir reaksiyon en yavaş adımından daha hızlı ilerleyemez ve dolayısıyla hız belirleyen adım genel reaksiyon hızını sınırlar.

Genel bir reaksiyonu temsil eden dengeli denklemlerden farklı olarak, temel reaksiyonlara ilişkin denklemler, kimyasal değişimin net temsilcileridir. Temel bir reaksiyon denklemi, bağ kırılması/yapılması geçiren gerçek reaktanları ve oluşan ürünleri gösterir. Hız yasaları, temel reaksiyonlar için doğrudan dengeli kimyasal denklemlerden türetilebilir. Bununla birlikte, dengeli denklemlerin genellikle çok adımlı reaksiyon mekanizmalarından kaynaklanan kimyasal sistemdeki genel değişikliği temsil ettiği çoğu kimyasal reaksiyon için durum böyle değildir. Bu nedenle hız yasası deneysel verilerden belirlenmeli ve reaksiyon mekanizması daha sonra hız yasasından çıkarılmalıdır.

Örneğin NO_2 ve CO'nun reaksiyonunu düşünün:

<img alt="Figure1" src="/files/ftp_upload/11699/11699_Equation1.svg" style="text-align: center; height: 22px;">

225 °C'nin üzerindeki sıcaklıklarda bu reaksiyon için deneysel hız yasası şöyledir:

<img alt="Figure2" src="/files/ftp_upload/11699/11699_Equation2.svg" style="text-align: center; height: 19px;">

Hız kanununa göre reaksiyon, NO_2'ye göre birinci dereceden, CO'ya göre birinci derecedendir. Ancak 225 °C'nin altındaki sıcaklıklarda reaksiyon, göre ikinci dereceden farklı bir hız kanunu ile tanımlanır. NO_2'ye:

<img alt="Figure3" src="/files/ftp_upload/11699/11699_Equation3.svg" style="text-align: center; height: 21px;">

Bu oran kanunu tek adımlı mekanizmayla tutarlı değildir ancak aşağıdaki iki adımlı mekanizmayla tutarlıdır:

<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;">

Hızı belirleyen (daha yavaş) adım, NO_2 konsantrasyonuna ikinci dereceden bağımlılığı gösteren bir hız yasasını verir ve iki temel denklemin toplamı net genel reaksiyonu verir.

 Genel olarak hız belirleyici (daha yavaş) adım, reaksiyon mekanizmasının ilk adımı olduğunda, genel reaksiyonun hız yasası, bu adımın hız yasasıyla aynıdır. Bununla birlikte, hız belirleme adımından önce hızlı bir şekilde geri dönüşümlü bir reaksiyon içeren temel bir adım geldiğinde, genel reaksiyon için hız yasasının türetilmesi, genellikle reaksiyon ara ürünlerinin varlığından dolayı daha zor olabilir. Bu gibi durumlarda, ileri ve geri süreçlerin hızları eşit olduğunda tersinir bir reaksiyonun dengede olduğu kavramı kullanılabilir.

Çok Adımlı Reaksiyonlar

Kimyasal reaksiyonlar genellikle bir dizide meydana gelen iki veya daha fazla farklı reaksiyonu içeren aşamalı bir şekilde meydana gelir. Dengeli bir denklem gösterir ...

Education

JoVE Core

Organic Chemistry

Organik Kimya

Unit 1

Ünite 1

Thermodynamics and Chemical Kinetics

Termodinamik ve Kimyasal Kinetik

KEY TERMS AND CONCEPTS

TEMEL TERİMLER VE KAVRAMLAR

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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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

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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

Temperature changes can decide whether a process is spontaneous. This idea applies to phase changes and to some chemical reactions. The key link is Gibbs free energy, which connects free energy change with enthalpy and entropy. The sign of the Gibbs free energy change, or ΔG, depends on the signs of enthalpy change (ΔH) and entropy change (ΔS), and sometimes on the absolute temperature. Because temperature is measured in kelvin, it is always a positive value. That means there are four main...

chemical and solubility equilibria

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rate law and reaction order

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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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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

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energy diagrams, transition states, and intermediates

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predicting reaction outcomes

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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 Organik Kimya, kısa ve anlaşılması kolay animasyonlu video dersleri kullanarak stereokimya, reaksiyonlar ve mekanizmaların temel kavramlarını açıklar. Buna ek olarak, bilim insanlarının yer aldığı videolar, ilgili kavramların günümüz laboratuvarlarında gerçekleştirilen klasik ve orijinal araştırma deneylerinde nasıl uygulandığını göstermektedir.

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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The Periodic Table And Organismal Elements

Atomic Structure

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Electron Configurations

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Electronic Structure of Atoms


An atom comprises protons and neutrons, which are contained inside the dense, central&#160;core called the nucleus, with electrons present around the ...

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Electron Configuration of Multielectron Atoms

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Atomic Radii and Effective Nuclear Charge

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Sources and Properties of Electric Charge

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Electric Fields

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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

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Subatomic Particles

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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

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Stoichiometry, Product Yield, and Limiting Reactants

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Physical Properties of Matter

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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

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

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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

Activation energy is the minimum amount of energy necessary for a chemical reaction to move forward. The higher the activation energy, the slower the rate ...

Energy Transfer in Chemical Reactions

Chemical reactions require sufficient energy to cause the matter to collide with enough precision and force that old chemical bonds can be broken and new ...

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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

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Temperature Dependence

Chemical Kinetics 
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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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