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

Reakcja chemiczna jest często reprezentowana przez ogólnie zrównoważone równanie chemiczne wskazujące reagenty i produkty.
Jednak rzeczywista reakcja jest często bardziej złożona i przebiega w wielu etapach. Na przykład ta reakcja tlenku azotu z wodorem, w wyniku której powstaje gazowy azot i woda, zachodzi w trzech odrębnych, następujących po sobie etapach. Te kroki nazywane są mechanizmem reakcji.
Każdy etap w mechanizmie reakcji nazywany jest reakcją elementarną i reprezentuje interakcję, taką jak zerwanie lub utworzenie wiązania, między reagującymi gatunkami.
Określone cząsteczki, takie jak dwutlenek diazotu i podtlenek azotu, powstają podczas jednego etapu podstawowego i są zużywane podczas innego. Takie gatunki nazywane są półproduktami reakcji.
Półprodukty reakcji to nie to samo, co stany przejściowe, które występują tylko podczas przemiany reagentów w produkty.
Mechanizmy reakcji są stawiane na podstawie ich zrównoważonych równań chemicznych i eksperymentalnie wyznaczonych praw szybkości każdego etapu elementarnego.
Każdy krok ma określoną szybkość reakcji, stałą szybkości i energię aktywacji. Najwolniejszy krok nazywany jest krokiem określającym szybkość i wpływa na szybkość reakcji netto. Może być używany do weryfikacji prawa szybkości dla całej reakcji i do walidacji proponowanego mechanizmu reakcji.
Rozważ rozkład podtlenku azotu na azot i tlen. Eksperymentalnie wyznaczone prawo szybkości nie odpowiada wyrażeniu szybkości reakcji jednoetapowej, co potwierdza obserwowana obecność atomów tlenu - reakcji pośredniej.
W związku z tym zaproponowano mechanizm reakcji, w którym wszystkie kroki kumulują się, aby uzyskać ogólną reakcję.
Po pierwsze, stałe szybkości wskazują, że pierwszym krokiem jest krok ograniczający szybkość. Jest najwolniejszy, a tym samym wpływa na ogólną szybkość reakcji. Ustawa o stawkach zaproponowana na tym etapie może zostać ustalona na równi z ogólną ustawą o stawkach.
To proponowane prawo szybkości, wyprowadzone bezpośrednio ze stężenia molekularnego elementarnego reagenta, jest zgodne z eksperymentalnym prawem szybkości i weryfikuje przewidywany mechanizm reakcji.
Ten tekst jest adaptacją z <a href="https://openstax.org/books/chemistry-2e/pages/12-6-reaction-mechanisms">Openstax, Chemistry 2e, 12.6: Reaction Mechanisms</a>.

reakcje wieloetapowe

Reakcje chemiczne często zachodzą etapami i obejmują dwie lub więcej odrębnych reakcji zachodzących w sekwencji.Mechanizm reakcji (lub ścieżka reakcji) zawiera szczegóły dotyczące dokładnego procesu krok po kroku, w którym zachodzi reakcja. Każdy z etapów mechanizmu reakcji nazywany jest reakcją elementarną. Każdy etap mechanizmu reakcji nazywany jest reakcją elementarną. Te elementarne reakcje zachodzą sekwencyjnie, jak przedstawiono w równaniach etapów, i sumują się, tworząc zrównoważone równanie chemiczne opisujące ogólną reakcję. W wieloetapowym mechanizmie reakcji jeden z podstawowych etapów przebiega wolniej niż pozostałe – czasami znacznie wolniej. Ten najwolniejszy krok nazywany jest krokiem ograniczającym szybkość (lub krokiem określającym szybkość). Reakcja nie może przebiegać szybciej niż jej najwolniejszy etap, dlatego też etap określający szybkość ogranicza ogólną szybkość reakcji.

W przeciwieństwie do zrównoważonych równań przedstawiających reakcję ogólną, równania reakcji elementarnych są wyraźnym przedstawieniem przemian chemicznych. Elementarne równanie reakcji przedstawia rzeczywisty(e) reagent(y) podlegający zerwaniu/tworzeniu wiązania oraz powstały(e) produkt(y). Prawa szybkości można wyprowadzić bezpośrednio ze zrównoważonych równań chemicznych dla reakcji elementarnych. Nie dotyczy to jednak większości reakcji chemicznych, gdzie zrównoważone równania często przedstawiają ogólną zmianę w układzie chemicznym wynikającą z wieloetapowych mechanizmów reakcji. Dlatego prawo szybkości musi zostać określone na podstawie danych eksperymentalnych, a mechanizm reakcji należy następnie wywnioskować z prawa szybkości.

Rozważmy na przykład reakcję NO_2 i CO:

<img alt="Figure1" src="/files/ftp_upload/11699/11699_Equation1.svg" style="text-align: center; height: 22px;">

Eksperymentalne prawo szybkości tej reakcji w temperaturach powyżej 225 °C jest następujące:

<img alt="Figure2" src="/files/ftp_upload/11699/11699_Equation2.svg" style="text-align: center; height: 19px;">

 Zgodnie z prawem szybkości reakcja jest pierwszego rzędu w odniesieniu do NO_2 i pierwszego rzędu w odniesieniu do CO. Jednakże w temperaturach poniżej 225 °C reakcję opisuje inne prawo szybkości, drugiego rzędu w odniesieniu do do NO_2:

<img alt="Figure3" src="/files/ftp_upload/11699/11699_Equation3.svg" style="text-align: center; height: 21px;">

To prawo stopowe nie jest spójne z mechanizmem jednoetapowym, ale jest zgodne z następującym mechanizmem dwuetapowym:

<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;">

 Etap określający szybkość (wolniejszy) daje prawo szybkości pokazujące zależność drugiego rzędu od stężenia NO_2, a suma dwóch równań elementarnych daje całkowitą reakcję netto.

Ogólnie rzecz biorąc, gdy etap określający szybkość (wolniejszy) jest pierwszym krokiem w mechanizmie reakcji, prawo szybkości dla całej reakcji jest takie samo, jak prawo szybkości dla tego etapu. Jednakże, gdy etap określający szybkość jest poprzedzony elementarnym etapem obejmującym szybko odwracalną reakcję, określenie prawa szybkości dla całej reakcji może być trudniejsze, często ze względu na obecność półproduktów reakcji.

W takich przypadkach można zastosować koncepcję, że reakcja odwracalna jest w równowadze, gdy szybkości procesów do przodu i do tyłu są równe.

Reakcje wieloetapowe

Reakcje chemiczne często zachodzą w sposób krokowy, obejmujący dwie lub więcej odrębnych reakcji zachodzących w sekwencji. Zrównoważone równanie wskazuje, że ...

Education

JoVE Core

Organic Chemistry

Chemia organiczna

Unit 1

Jednostka 1

Thermodynamics and Chemical Kinetics

Termodynamika i kinetyka chemiczna

KEY TERMS AND CONCEPTS

Kluczowe terminy i pojęcia

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.

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Next Generation Science Standards

Physical Science

Matter and Its Interactions

HS-PS1-2

HS-PS1-7

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South Carolina College and Career Ready Science Standards

C-PS1-4

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HS-PS1-4

Reaction Mechanisms and Rate-Determining Steps

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