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

Una reazione chimica è spesso rappresentata da un'equazione chimica complessivamente bilanciata che indica i reagenti e i prodotti.
Tuttavia, la reazione vera e propria è spesso più complessa e si svolge in più fasi. Ad esempio, questa reazione dell'ossido nitrico con l'idrogeno che forma azoto gassoso e acqua avviene in tre fasi distinte e successive. Questi passaggi sono chiamati meccanismo di reazione.
Ogni fase del meccanismo di reazione è chiamata reazione elementare e rappresenta l'interazione, come la rottura o la formazione di legami, tra le specie reagenti.
Molecole specifiche, come il biossido di diazoto e il protossido di azoto, si formano durante un passaggio elementare e vengono consumate durante un altro. Tali specie sono chiamate intermedi di reazione.
Gli intermedi di reazione non sono gli stessi degli stati di transizione, che esistono solo durante la trasformazione dei reagenti in prodotti.
I meccanismi di reazione sono ipotizzati in base alle loro equazioni chimiche bilanciate e alle leggi di velocità determinate sperimentalmente di ogni passo elementare.
Ogni fase ha una velocità di reazione, una costante di velocità e un'energia di attivazione specifiche. La fase più lenta è chiamata fase che determina la velocità e influenza la velocità di reazione netta. Può essere utilizzato per verificare la legge di velocità per la reazione complessiva e per convalidare un meccanismo di reazione proposto.
Si consideri la decomposizione del protossido di azoto in azoto e ossigeno. La legge della velocità determinata sperimentalmente non corrisponde all'espressione della velocità di una reazione a passo singolo, che è corroborata dalla presenza osservata di atomi di ossigeno, un intermedio di reazione.
Quindi, viene proposto un meccanismo di reazione in cui tutti i passaggi si accumulano per dare la reazione complessiva.
Innanzitutto, le costanti di velocità indicano che il primo passaggio è il passaggio di limitazione della velocità. È il più lento e quindi influenza la velocità di reazione complessiva. Una legge tariffaria proposta a partire da questa fase può essere impostata uguale alla legge tariffaria complessiva.
Questa legge di velocità proposta, derivata direttamente dalla concentrazione molecolare del reagente elementare, corrisponde alla legge di velocità sperimentale e verifica il meccanismo di reazione previsto.
Questo testo è adattato da <a href="https://openstax.org/books/chemistry-2e/pages/12-6-reaction-mechanisms">Openstax, Chimica 2e, 12.6: Meccanismi di reazione</a>.

reazioni a più fasi

Le reazioni chimiche spesso si verificano in modo graduale coinvolgendo due o più reazioni distinte che si svolgono in sequenza. Un'equazione bilanciata indica le specie reagenti e le specie prodotte, ma non rivela dettagli su come avviene la reazione a livello molecolare. Il meccanismo di reazione (o percorso di reazione) fornisce dettagli riguardanti il processo preciso e graduale attraverso il quale avviene una reazione. Ciascuno dei passaggi di un meccanismo di reazione è chiamato reazione elementare. Queste reazioni elementari si verificano in sequenza, come rappresentato nelle equazioni dei passaggi, e si sommano per produrre l'equazione chimica bilanciata che descrive la reazione complessiva. In un meccanismo di reazione multifase, uno dei passi elementari procede più lentamente degli altri, a volte molto più lentamente. Questa fase più lenta è chiamata fase di limitazione della velocità (o fase di determinazione della velocità). Una reazione non può procedere più velocemente della fase più lenta e, quindi, la fase che determina la velocità limita la velocità complessiva della reazione.

A differenza delle equazioni bilanciate che rappresentano una reazione complessiva, le equazioni per le reazioni elementari sono rappresentazioni esplicite del cambiamento chimico. Un'equazione di reazione elementare descrive il(i) reagente(i) effettivo(i) sottoposto(i) a rottura/formazione del legame e il(i) prodotto(i) formato(i). Le equazioni di velocità possono essere derivate direttamente dalle equazioni chimiche bilanciate per le reazioni elementari. Tuttavia, questo non è il caso della maggior parte delle reazioni chimiche, dove le equazioni bilanciate spesso rappresentano il cambiamento complessivo nel sistema chimico risultante da meccanismi di reazione a più fasi. Pertanto, la legge cinetica deve essere determinata dai dati sperimentali e da questa deve essere successivamente dedotto il meccanismo di reazione.

Ad esempio, consideriamo la reazione di NO_2 e CO:

<img alt="Figure1" src="/files/ftp_upload/11699/11699_Equation1.svg" style="text-align: center; height: 22px;">

La legge sperimentale sulla velocità per questa reazione a temperature superiori di 225 °C è:

<img alt="Figure2" src="/files/ftp_upload/11699/11699_Equation2.svg" style="text-align: center; height: 19px;">

Secondo la legge cinetica la reazione è del primo ordine rispetto a NO_2 e del primo ordine rispetto al CO. Tuttavia, a temperature inferiori di 225 °C, la reazione è descritta da una diversa legge cinetica che è del secondo ordine rispetto a NO_2:

<img alt="Figure3" src="/files/ftp_upload/11699/11699_Equation3.svg" style="text-align: center; height: 21px;">

Questa equazione di velocità non è coerente con il meccanismo a fase singola, ma è coerente con il seguente meccanismo a due fasi:

<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;">

La fase di determinazione della velocità (più lenta) fornisce una legge cinetica che mostra una dipendenza di secondo ordine dalla concentrazione di NO_2, e la somma delle due equazioni elementari dà la reazione complessiva netta.

In generale, quando la fase che determina la velocità (più lenta) è la prima fase del meccanismo di reazione, la legge cinetica per la reazione complessiva è la stessa della legge cinetica per questa fase. Tuttavia, quando la fase che determina la velocità è preceduta da una fase elementare che comporta una reazione rapidamente reversibile, la legge cinetica per la reazione complessiva può essere più difficile da ricavare, spesso a causa della presenza di intermedi di reazione.

In tali casi, è possibile utilizzare il concetto che una reazione reversibile è all'equilibrio quando le velocità dei processi diretti e inversi sono uguali.

Reazioni a più fasi

Le reazioni chimiche avvengono spesso in modo graduale coinvolgendo due o più reazioni distinte che avvengono in una sequenza. Un'equazione bilanciata indica ...

Education

Didattica

JoVE Core

Organic Chemistry

Chimica Organica

Unit 1

Unità 1

Thermodynamics and Chemical Kinetics

Termodinamica e Cinetica Chimica

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

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

Combustion is a burning reaction that releases energy as heat, light, or sound. It happens when a substance reacts with an oxidizing agent, usually molecular oxygen. The heat of combustion is also called the enthalpy of combustion. The energy released when one mole of a substance undergoes complete combustion at constant pressure is the molar heat of combustion. Combustion reactions are exothermic, so they give off energy. For that reason, the sign of ΔH is negative. In 1772, Antoine Lavoisier...

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

gibbs-free-energy-and-thermodynamic-favorability

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

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

effect-of-temperature-change-on-reaction-rate

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

Bond strength and activation energy help explain why organic reactions happen. Bond energy is the energy needed to break a bond homolytically, which means each atom takes one of the shared electrons. When a bond value is given for one specific bond, it is called bond dissociation energy. When it is given for a bond type across many compounds, it is called average bond energy. Bond dissociation energy usually follows a clear pattern. A single bond is weaker than a double bond, and a double bond...

energy diagrams, transition states, and intermediates

energy-diagrams-transition-states-and-intermediates

Reaction Coordinate Diagrams in Chemical Reactions

Reaction coordinate diagrams show how energy changes during a chemical reaction. These free-energy graphs use the horizontal axis to show reaction progress. Points near the reactants look like the reactants, while points near the products look like the products. The peaks on the diagram represent stable structures with measurable lifetimes. Other points on the curve are unstable and cannot be isolated. The high-energy, unstable peak is called the transition state, or activated complex. At the...

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 spiega i concetti di base della stereochimica, delle reazioni e dei meccanismi usando video-lezioni concise e facili da capire. Inoltre, i video scientist-in-action dimostrano l'applicazione di tali concetti in esperimenti di ricerca classici e originali eseguiti al giorno d'oggi nei laboratori in tutto il mondo.

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

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-and-organismal-elements

The Periodic Table And Organismal Elements

Atomic Structure

The Greek philosopher Democritus proposed that everything on Earth is made up of tiny particles called atomos, Greek for &#34;indivisible,&#34; from which ...

Electron Configurations

Electron configurations and orbital diagrams can be determined by applying the Aufbau principle (each added electron occupies the subshell of lowest ...

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

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 ...

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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 ...

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Sources and Properties of Electric Charge

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Electric Fields

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An electric field is generated ...

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

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Temperature Dependence on Reaction Rate

The Collision Theory
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stoichiometry-product-yield-and-limiting-reactants

Stoichiometry, Product Yield, and Limiting Reactants

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	Preparation of 25% v/v Ethylenediamine in Deionized WaterExpand
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physical-and-chemical-properties-of-matter

Physical and Chemical Properties of Matter

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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

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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

Cellular processes such as building and breaking down complex molecules occur through stepwise chemical reactions. Some of these chemical reactions are ...

Energy Diagrams, Transition States, and Intermediates

Free-energy diagrams, or reaction coordinate diagrams, are graphs showing the energy changes that occur during a chemical reaction. The reaction ...

What is Metabolism?

Overview

Metabolism represents all of the chemical activity in a cell, including reactions that build molecules (anabolism) and those that break ...

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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 ...

factors-influencing-the-rate-of-chemical-reactions

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

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Temperature Dependence

Chemical Kinetics 
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Molecular Kinetic Energy

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Reaction Rate

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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.

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Reaction Mechanisms and Rate-Determining Steps

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