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

化学反応は、多くの場合、反応物と生成物を示す全体的にバランスの取れた化学方程式で表されます。
しかし、実際の反応はもっと複雑で、複数のステップで起こることが多いです。例えば、この一酸化窒素と水素形成窒素ガスおよび水との反応は、3つの異なる連続したステップで行われます。これらのステップを反応メカニズムと呼びます。
反応メカニズムの各ステップは基本反応と呼ばれ、反応する種間の結合の切断や形成などの相互作用を表します。
二酸化二窒素や亜酸化窒素などの特定の分子は、ある基本ステップで形成され、別のステップで消費されます。このような種は反応中間体と呼ばれます。
反応中間体は、反応物が生成物に変換される間のみ存在する遷移状態と同じではありません。
反応機構は、そのバランスの取れた化学方程式と、各基本ステップの速度法則を実験的に決定して仮定します。
各ステップには、特定の反応速度、速度定数、および活性化エネルギーがあります。最も遅いステップはレート決定ステップと呼ばれ、正味の反応速度に影響を与えます。これは、全体的な反応の速度則を検証し、提案された反応メカニズムを検証するために使用できます。
亜酸化窒素の窒素と酸素への分解を考えてみましょう。実験的に決定された速度則は、酸素原子(反応中間体)の観察された存在によって裏付けられるシングルステップ反応の速度発現に対応していません。
したがって、すべてのステップが累積されて全体的な反応を与える反応メカニズムが提案されます。
まず、レート定数は、最初のステップがレート制限ステップであることを示します。これは最も遅いため、全体的な反応速度に影響を与えます。このステップから提案されるレート法は、全体的なレート法と同等に設定できます。
この提案された速度則は、素反応物の分子濃度から直接導出され、実験速度法則と一致し、予測された反応メカニズムを検証します。
このテキストは、<a href="https://openstax.org/books/chemistry-2e/pages/12-6-reaction-mechanisms">Openstax, Chemistry 2e, 12.6: Reaction Mechanismsから引用しています</a>。

多段階反応

化学反応は多くの場合、2 つ以上の異なる反応が連続して起こる段階的な形で起こります。 平衡のとれた方程式は、反応物と生成物を表しますが、反応が分子レベルでどのように起こるかについての詳細は明らかにしません。 反応機構 (または反応経路) は、反応が起こる正確な段階的なプロセスに関する詳細を提供します。 反応機構の各段階を素反応といいます。 これらの基本反応は、ステップ方程式で表されるように順番に発生し、それらを合計すると、全体の反応を説明するバランスの取れた化学方程式が得られます。 多段階の反応メカニズムでは、基本段階の 1 つが他の段階よりも遅く進行し、場合によっては大幅に遅くなります。 この最も遅いステップは、律速ステップ (または律速ステップ) と呼ばれます。 反応は最も遅いステップよりも速く進行することはできないため、律速ステップが全体の反応速度を制限します。

全体的な反応を表す平衡方程式とは異なり、素反応の方程式は化学変化を明示的に表します。 基本反応方程式は、結合の切断/生成が起こる実際の反応物と形成される生成物を示します。 速度法則は、素反応の平衡化学方程式から直接導き出すことができます。 ただし、これはほとんどの化学反応には当てはまりません。多くの場合、バランスの取れた方程式は、多段階の反応メカニズムから生じる化学系の全体的な変化を表します。 したがって、速度則は実験データから決定する必要があり、その後反応機構は速度則から推定する必要があります。

たとえば、NO_2 と CO の反応を考えてみましょう。



<img alt="Figure1" src="/files/ftp_upload/11699/11699_Equation1.svg" style="text-align: center; height: 22px;">



225 °C 以上の温度でのこの反応の実験速度則は次のとおりです。



<img alt="Figure2" src="/files/ftp_upload/11699/11699_Equation2.svg" style="text-align: center; height: 19px;">



速度則によれば、反応は NO_2 に関しては 1 次であり、CO に関しては 1 次です。ただし、225 °C 未満の温度では、反応は NO2 に関しては 2 次である別の速度則で記述されます。 NO_2へ:



<img alt="Figure3" src="/files/ftp_upload/11699/11699_Equation3.svg" style="text-align: center; height: 21px;">



このレートの法則は、単一ステップのメカニズムとは一致しませんが、次の 2 ステップのメカニズムとは一致します。



<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;">



速度決定 (遅い) ステップでは、NO_2 濃度への 2 次依存性を示す速度則が得られ、2 つの基本方程式の合計が正味の全体的な反応を与えます。

一般に、律速を決定する (遅い) ステップが反応機構の最初のステップである場合、反応全体の速度則はこのステップの速度則と同じになります。 ただし、速度決定ステップの前に急速に可逆的な反応を伴う基本ステップがある場合、多くの場合、反応中間体の存在により、反応全体の速度則を導き出すことがより困難になる可能性があります。

このような場合には、順方向プロセスと逆方向プロセスの速度が等しいときに可逆反応が平衡にあるという概念を利用できます。

化学反応は、多くの場合、2つ以上の異なる反応が順番に起こるという段階的な方法で発生します。バランスの取れた方程式は...

Education

JoVE Core

Organic Chemistry

有機化学

Unit 1

ユニット1

Thermodynamics and Chemical Kinetics

熱力学と化学反応速度論

KEY TERMS AND CONCEPTS

主要用語と概念

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...

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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

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

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

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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

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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 有機化学では、立体化学、反応、メカニズムの基本概念を、簡潔でわかりやすいアニメーション ビデオ レッスンを使用して説明します。さらに、科学者の実際のビデオでは、世界中の今日の研究室で行われている古典的および独自の研究実験における関連概念の適用を示しています。

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

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Periodic Classification of the Elements

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The Periodic Table And Organismal Elements

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Electric Fields

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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

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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

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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 ...

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HS-PS1-3

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Bond Energies and Bond Lengths

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HS-PS1-4

Reaction Mechanisms and Rate-Determining Steps

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