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

תגובה כימית מיוצגת לעתים קרובות על ידי משוואה כימית מאוזנת הכוללת המציינת את המגיבים והתוצרים.
עם זאת, התגובה בפועל היא לעתים קרובות מורכבת יותר ומתרחשת במספר שלבים. לדוגמה, תגובה זו של תחמוצת החנקן עם מימן היוצר גז חנקן ומים מתרחשת בשלושה שלבים נפרדים ועוקבים. שלבים אלה נקראים מנגנון התגובה.
כל שלב במנגנון התגובה נקרא תגובה יסודית ומייצג את האינטראקציה, כגון שבירת קשר או היווצרותו, בין המינים המגיבים.
מולקולות ספציפיות, כמו דו-תחמוצת הדיניטרוגן ותחמוצת החנקן, נוצרות במהלך שלב יסודי אחד ונאכלות במהלך שלב אחר. מינים כאלה נקראים מתווכי תגובה.
מתווכי תגובה אינם זהים למצבי מעבר, הקיימים רק במהלך טרנספורמציה של מגיבים למוצרים.
מנגנוני התגובה משערים בהתבסס על המשוואות הכימיות המאוזנות שלהם, וחוקי קצב שנקבעו בניסוי של כל צעד יסודי.
לכל שלב יש קצב תגובה מסוים, קבוע קצב ואנרגיית שפעול. השלב האיטי ביותר נקרא שלב קביעת הקצב והוא משפיע על קצב התגובה נטו. ניתן להשתמש בו כדי לאמת את חוק התעריף עבור התגובה הכוללת וכדי לאמת מנגנון תגובה מוצע.
שקול את הפירוק של תחמוצת החנקן לחנקן וחמצן. חוק הקצב שנקבע בניסוי אינו תואם את ביטוי הקצב של תגובה חד-שלבית, אשר מאוששת על ידי נוכחות נצפית של אטומי חמצן – תגובת ביניים.
לפיכך, מוצע מנגנון תגובה שבו כל השלבים מצטברים כדי לתת את התגובה הכוללת.
ראשית, קבועי הקצב מצביעים על כך שהצעד הראשון הוא שלב הגבלת הקצב. הוא האיטי ביותר, ולכן משפיע על קצב התגובה הכולל. חוק התעריפים המוצע מצעד זה יכול להיקבע כשווה לחוק התעריפים הכולל.
חוק קצב מוצע זה, הנגזר ישירות מהריכוז המולקולרי של המגיב האלמנטרי, תואם את חוק קצב הניסוי ומאמת את מנגנון התגובה החזוי.
טקסט זה עובד מתוך <a href="https://openstax.org/books/chemistry-2e/pages/12-6-reaction-mechanisms">Openstax, Chemistry 2e, 12.6: Reaction Mechanisms</a>.

﻿תגובות מרובות שלבים

<div style="direction:rtl"> תגובות כימיות מתרחשות לעתים קרובות באופן שלבי הכוללות שתי תגובות שונות או יותר המתרחשות ברצף. משוואה מאוזנת מציינת את הצורונים המגיבים ואת הצורון התוצר, אך היא לא חושפת פרטים על אופן התגובה ברמה המולקולרית. מנגנון התגובה (או נתיב התגובה) מספק פרטים לגבי התהליך המדויק, שלב אחר שלב, שבו מתרחשת תגובה. כל אחד מהשלבים במנגנון תגובה נקרא תגובה אלמנטרית. התגובות האלמנטריות הללו מתרחשות ברצף, כפי שמוצגות במשוואות השלבים, והן מסתכמות כדי להניב את המשוואה הכימית המאוזנת המתארת ​​את התגובה הכוללת. במנגנון תגובה רב-שלבי, אחד השלבים האלמנטרים מתקדם לאט יותר מהאחרים - לפעמים לאט יותר באופן משמעותי. השלב האיטי ביותר הזה נקרא השלב המגביל קצב (או שלב קובע הקצב). תגובה לא יכולה להתקדם מהר יותר מהשלב האיטי ביותר שלה, ומכאן, השלב הקובע את הקצב מגביל את קצב התגובה הכולל. בניגוד למשוואות מאוזנות המייצגות תגובה כוללת, המשוואות לתגובות אלמנטריות הן ייצוגים מפורשים של השינוי הכימי. משוואת תגובה אלמנטרית מתארת ​​את המגיב/ים בפועל שעוברים שבירה/יצירת קשר ואת התוצר/ים שנוצרו. חוקי קצב עשויים להיגזר ישירות מהמשוואות הכימיות המאוזנות לתגובות אלמנטריות. עם זאת, זה לא המקרה עבור רוב התגובות הכימיות, שבהן משוואות מאוזנות מייצגות לעתים קרובות את השינוי הכולל במערכת הכימית הנובעת ממנגנוני תגובה רב-שלבים. לפיכך, יש לקבוע את חוק השיעור מנתוני ניסוי, ולאחר מכן להסיק את מנגנון התגובה מחוק הקצב. לדוגמה, חשבו על התגובה של NO_2 ו-CO: <img alt="Figure1" src="/files/ftp_upload/11699/11699_Equation1.svg" style="text-align: center; height: 22px;"> חוק הקצב הניסויי עבור תגובה זו בטמפרטורות מעל 225 מעלות צלזיוס הוא: <img alt="Figure2" src="/files/ftp_upload/11699/11699_Equation2.svg" style="text-align: center; height: 19px;"> על פי חוק הקצב, התגובה היא מסדר ראשון ביחס ל-NO_2 ומסדר ראשון ביחס ל-CO. עם זאת, בטמפרטורות מתחת ל-225 מעלות צלזיוס, התגובה מתוארת על ידי חוק קצב שונה שהוא מסדר שני ביחס ל-NO_2: <img alt="Figure3" src="/files/ftp_upload/11699/11699_Equation3.svg" style="text-align: center; height: 21px;"> חוק קצב זה אינו תואם את המנגנון החד-שלבי, אך הוא תואם את המנגנון הדו-שלבי הבא: <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;"> באופן כללי, כאשר השלב הקובע את הקצב (האיטי יותר) הוא השלב הראשון במנגנון התגובה, חוק הקצב עבור התגובה הכוללת זהה לחוק הקצב עבור שלב זה. עם זאת, כאשר קדם לשלב קביעת הקצב שלב אלמנטרי הכולל תגובה הפיכה בקצב, חוק הקצב עבור התגובה הכוללת עשוי להיות קשה יותר להפקה, לעתים קרובות בשל נוכחותם של תוצרי ביניים של תגובה. במקרים כאלה, ניתן להשתמש בתפיסה לפיה תגובה הפיכה נמצאת בשיווי משקל כאשר קצבי התהליכים קדימה ואחורה שווים. </div>

תגובות כימיות מתרחשות לעתים קרובות בצורה מדורגת הכוללת שתי תגובות נפרדות או יותר המתרחשות ברצף. משוואה מאוזנת מצביעה על ...

Education

JoVE Core

Organic Chemistry

כימיה אורגנית

Unit 1

יחידה 1

Thermodynamics and Chemical Kinetics

תרמודינמיקה וקינטיקה כימית

KEY TERMS AND CONCEPTS

מונחים ומושגים מרכזיים

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

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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

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

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

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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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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

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

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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

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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

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

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

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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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The Collision Theory
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Stoichiometry, Product Yield, and Limiting Reactants

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Dynamic Equilibrium

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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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Ionic Bonding and Electron Transfer

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Electric Dipoles and Dipole Moment

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HS-PS1-3

Bond Dissociation Energy and Activation Energy

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

Factors Influencing the Rate of Chemical 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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