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

Химическая реакция часто представляется в виде общего сбалансированного химического уравнения с указанием реагентов и продуктов.
Однако фактическая реакция часто бывает более сложной и протекает в несколько этапов. Например, эта реакция оксида азота с образованием водорода газообразного азота и воды происходит в три отдельных последовательных этапа. Эти этапы называются механизмом реакции.
Каждый шаг в механизме реакции называется элементарной реакцией и представляет собой взаимодействие, такое как разрыв или образование связи, между реагирующими веществами.
Специфические молекулы, такие как диоксид азота и закись азота, образуются на одном элементарном этапе и потребляются на другом. Такие виды называются промежуточными продуктами реакции.
Промежуточные продукты реакции – это не то же самое, что переходные состояния, которые существуют только при превращении реагентов в продукты.
Механизмы реакций выдвигаются на основе их сбалансированных химических уравнений и экспериментально определяемых законов скорости каждого элементарного шага.
Каждый шаг имеет определенную скорость реакции, постоянную скорость и энергию активации. Самый медленный шаг называется шагом, определяющим скорость, и влияет на чистую скорость реакции. Он может быть использован для проверки закона скорости для общей реакции и для валидации предложенного механизма реакции.
Рассмотрим разложение закиси азота до азота и кислорода. Экспериментально установленный закон скорости не соответствует скоростному выражению одностадийной реакции, что подтверждается наблюдаемым присутствием атомов кислорода — промежуточного продукта реакции.
Следовательно, предложен механизм реакции, в котором все этапы накапливаются, чтобы получить общую реакцию.
Во-первых, константы скорости указывают на то, что первый шаг является шагом ограничения скорости. Он самый медленный, и тем самым влияет на общую скорость реакции. Закон тарифов, предложенный на этом шаге, может быть установлен равным общему закону тарифов.
Этот предполагаемый закон скорости, непосредственно выведенный из молекулярной концентрации элементарного реагента, согласуется с экспериментальным законом скорости и проверяет предсказанный механизм реакции.
Этот текст адаптирован из <a href="https://openstax.org/books/chemistry-2e/pages/12-6-reaction-mechanisms">Openstax, Chemistry 2e, 12.6: Reaction Mechanism</a>.

Многоэтапные реакции

Химические реакции часто протекают ступенчато, включая две или более различных реакции, протекающие последовательно. Сбалансированное уравнение указывает реагирующие виды и виды продукта, но не раскрывает подробностей о том, как реакция происходит на молекулярном уровне. Механизм реакции (или путь реакции) предоставляет подробную информацию о точном, пошаговом процессе, посредством которого происходит реакция. Каждая из стадий механизма реакции называется элементарной реакцией. Эти элементарные реакции происходят последовательно, как представлено в пошаговых уравнениях, и их сумма дает сбалансированное химическое уравнение, описывающее общую реакцию. В многоступенчатом механизме реакции одна из элементарных стадий протекает медленнее других, иногда значительно медленнее. Этот самый медленный этап называется этапом ограничения скорости (или этапом определения скорости). Реакция не может протекать быстрее, чем ее самая медленная стадия, и, следовательно, стадия, определяющая скорость, ограничивает общую скорость реакции.

В отличие от сбалансированных уравнений, представляющих общую реакцию, уравнения элементарных реакций являются явным представлением химических изменений. Уравнение элементарной реакции описывает фактические реагенты, подвергающиеся разрыву/образованию связей, и образующиеся продукты. Законы скорости могут быть выведены непосредственно из сбалансированных химических уравнений элементарных реакций. Однако это не относится к большинству химических реакций, где сбалансированные уравнения часто представляют собой общее изменение в химической системе, возникающее в результате многоступенчатых механизмов реакции. Поэтому закон скорости необходимо определять из экспериментальных данных, а затем из закона скорости выводить механизм реакции.

Например, рассмотрим реакцию NO_2 и CO:

<img alt="Figure1" src="/files/ftp_upload/11699/11699_Equation1.svg" style="text-align: center; height: 22px;">

Экспериментальный закон скорости этой реакции при температуре выше 225 °C:

<img alt="Figure2" src="/files/ftp_upload/11699/11699_Equation2.svg" style="text-align: center; height: 19px;">

По закону скорости реакция имеет первый порядок по NO_2 и первый порядок по CO. Однако при температурах ниже 225 °С реакция описывается другим законом скорости,– имеющим второй порядок по NO_2:

<img alt="Figure3" src="/files/ftp_upload/11699/11699_Equation3.svg" style="text-align: center; height: 21px;">

Этот закон скорости не согласуется с одноступенчатым механизмом, но согласуется со следующим двухступенчатым механизмом:

<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;">

Определяющий скорость (более медленный) этап дает закон скорости, показывающий зависимость второго порядка от концентрации NO_2, а сумма двух элементарных уравнений дает результирующую общую реакцию.

В общем, когда определяющая скорость (более медленная) стадия является первой стадией механизма реакции, закон скорости всей реакции такой же, как закон скорости для этой стадии. Однако, когда этапу, определяющему скорость, предшествует элементарный этап, включающий быстро обратимую реакцию, вывод закона скорости всей реакции может оказаться более трудным, часто из-за присутствия промежуточных продуктов реакции.

В таких случаях можно использовать концепцию, согласно которой обратимая реакция находится в равновесии, когда скорости прямого и обратного процессов равны.

Химические реакции часто протекают ступенчато, включая две или более различных реакции, происходящих в одной последовательности. Сбалансированное уравнение указывает на ...

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Organic Chemistry

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Unit 1

Раздел 1

Thermodynamics and Chemical Kinetics

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KEY TERMS AND CONCEPTS

ОСНОВНЫЕ ТЕРМИНЫ И ПОНЯТИЯ

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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multi-step reactions

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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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Reaction Mechanisms and Rate-Determining Steps

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