Name: elementary steps in reaction mechanisms
Uploaded: 2020-09-24T15:06:50.000Z
Duration: 3 min 6 s
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North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.5.1

Reaction Mechanisms

Uma reação química é frequentemente representada por uma equação química global equilibrada que indica os reagentes e os produtos. Contudo, a reação atual é frequentemente mais complexa e revela múltiplas etapas. Por exemplo, esta reação do óxido nítrico com o hidrogénio que forma gás de nitrogénio e água ocorre em três etapas distintas e sucessivas.Estas etapas são chamadas de mecanismo de reação. Cada etapa no mecanismo de reação é chamada de reação elementar e representa a interação, como a quebra ou formação de ligações, entre as espécies reagentes. Moléculas específicas, como o dióxido de dinitrogênio e o óxido nitroso, são formados durante uma etapa elementar e consumidos durante outra.Essas espécies são chamadas de intermediários de reação. Os intermediários de reação são produtos de baixo consumo energético de uma reação elementar. São frequentemente de curta duração, o que explica a sua ausência na mistura do produto.Intermediários de reação não é o mesmo que complexos ativados. Os complexos ativados são estados de transição de elevado consumo energético existentes apenas durante a transformação dos reagentes em produtos. Combinando as etapas elementares dá origem à equação para o conjunto da reação química.Aqui, os intermediários de reação são eliminados e, portanto, não aparecem no conjunto da equação química. As diferentes reações elementares podem progredir a diferentes velocidades. O passo elementar mais lento determina a taxa de reação global.Aqui, a reação do dióxido de dinitrogênio com o gás de hidrogénio é a etapa limitadora da taxa. As reações elementares podem ser comummente caracterizadas de três tipos, dependendo do número de moléculas reagentes ou molecularidade. Numa reação unimolecular, uma única molécula reagente transforma-se em um ou mais produtos.Numa reação bimolecular, duas moléculas distintas reagem. Uma reação termolecular, embora muito rara, envolve três moléculas individuais que reagem a produtos ou intermediários de rendimento. Ao contrário da lei de taxas para uma reação química global, que é determinada de forma experimental, as leis de taxas para reações elementares podem ser previstas a partir dos coeficientes estequiométricos dos seus reagentes.Em suma, a molecularidade de uma reação elementar corresponde à ordem da reação global da etapa elementar. Assim, as reações unimoleculares são frequentemente reações de primeira ordem, as reações bimoleculares são de segunda ordem, e as reações termoleculares são de terceira ordem. Uma compreensão dos mecanismos de reação e cinética ajuda os químicos a identificar e otimizar reações químicas.

mecanismos de reação

As reações químicas ocorrem frequentemente de forma gradual, envolvendo duas ou mais reações distintas que ocorrem em sequência. Uma equação equilibrada indica a espécie reagente e a espécie do produto, mas não revela detalhes sobre como a reação ocorre a nível molecular. O mecanismo de reação (ou percurso de reação) fornece detalhes sobre o processo preciso passo a passo pelo qual ocorre uma reação.
Por exemplo, a decomposição do ozono parece seguir um mecanismo com dois passos:
<img alt="Eq1" src="/files/ftp_upload/11381/11381_Equation_1.png" style="text-align: center; height: 70px;" />
Cada um dos passos em um mecanismo de reação é chamado de reação elementar. Estas reações elementares ocorrem em sequência, tal como representadas nas equações de passo, e somam-se para produzir a equação química equilibrada que descreve a reação global:
<img alt="Eq2" src="/files/ftp_upload/11381/11381_Equation_2.png" style="text-align: center; height: 50px;" />
Note que o átomo de oxigénio produzido no primeiro passo é consumido durante o segundo e não aparece como um produto na reação geral. Tais espécies que são produzidas em uma etapa e consumidas em uma posterior são chamadas intermediárias da reação.
Enquanto que a equação geral da reação indica que duas moléculas de ozono reagem para dar três moléculas de oxigénio, o mecanismo de reação real não envolve a colisão direta e reação entre duas moléculas de ozono. Em vez disso, um O3 decompõe-se para produzir O2 e um átomo de oxigénio, e uma segunda molécula O3 reage subsequentemente com o átomo de oxigénio para produzir duas moléculas de O2 adicionais.
Ao contrário das equações equilibradas que representam uma reação geral, as equações para reações elementares são representações explícitas da alteração química. Uma equação de reação elementar retrata os reagentes reais no processo de quebra/criação de ligações, e os produtos formados. Assim, a lei de velocidade para uma reação elementar pode ser derivada diretamente da sua equação química equilibrada. No entanto, esse não é o caso para reações químicas típicas, para as quais as leis de velocidade podem ser determinadas de forma fiável apenas através da experimentação.
Reações Elementares Unimoleculares
A molecularidade de uma reação elementar é o número de espécies de reagentes (átomos, moléculas, ou iões). Por exemplo, uma reação unimolecular envolve a reação de um único reagente para produzir uma ou mais moléculas de produto:
<img alt="Eq3" src="/files/ftp_upload/11381/11381_Equation_3.png" style="text-align: center; height: 50px;" />
A lei da velocidade para uma reação unimolecular é de primeira ordem; velocidade = k [A].
Uma reação unimolecular pode ser uma de várias reações elementares em um mecanismo de reação complexo. Por exemplo, a reação (O3 (g) → O2 (g) + O) ilustra uma reação elementar unimolecular que ocorre como parte de um mecanismo de reação em duas etapas. No entanto, algumas reações unimoleculares podem ser o único passo de um mecanismo de reação de passo único. (Por outras palavras, uma reação “global” também pode ser uma reação elementar em alguns casos.) Por exemplo, a decomposição em fase gasosa do ciclobutano, C4H8, em etileno, C2H4, é representada pela equação química:
<img alt="Eq3A" src="/files/ftp_upload/11381/11381_Equation_3A.png" style="text-align: center; height: 50px;" />
Esta equação representa a reação geral, descrevendo um processo elementar unimolecular. A lei de velocidade prevista a partir desta equação, assumindo que é uma reação elementar, acaba por ser a mesma que a lei de velocidade derivada experimentalmente para a reação global, mostrando um comportamento de primeira ordem:
<img alt="Eq4" src="/files/ftp_upload/11381/11381_Equation_4.png" style="text-align: center; height: 70px;" />
Esse acordo entre a lei de velocidade observada e prevista indica que o processo unimolecular proposto, de passo único, é um mecanismo razoável para a reação do butadieno.
Reações Elementares Bimoleculares
Uma reação bimolecular envolve duas espécies de reagentes. Por exemplo:
<img alt="Eq5" src="/files/ftp_upload/11381/11381_Equation_5.png" style="text-align: center; height: 70px;" />
No primeiro tipo, onde as duas moléculas de reagentes são diferentes, a lei de velocidade é de primeira ordem em A e de primeira ordem em B (segunda ordem global)
<img alt="Eq6" src="/files/ftp_upload/11381/11381_Equation_6.png" style="text-align: center; height: 50px;" />
No segundo tipo, no qual duas moléculas idênticas colidem e reagem, a lei de velocidade é de segunda ordem em A:
<img alt="Eq7" src="/files/ftp_upload/11381/11381_Equation_7.png" style="text-align: center; height: 50px;" />
Algumas reações químicas ocorrem por mecanismos que consistem em uma única reação elementar bimolecular. Um exemplo é a reação do dióxido de azoto com monóxido de carbono:
<img alt="Eq8" src="/files/ftp_upload/11381/11381_Equation_8.png" style="text-align: center; height: 50px;" />
Reações elementares bimoleculares também podem estar envolvidas como passos em um mecanismo de reação de vários passos. A reação do oxigénio atómico com ozono é o segundo passo de um mecanismo de decomposição do ozono de dois passos:
<img alt="Eq9" src="/files/ftp_upload/11381/11381_Equation_9.png" style="text-align: center; height: 50px;" />
Reações Elementares Termoleculares
Uma reação elementar termolecular envolve a colisão simultânea de três átomos, moléculas, ou iões. Reações elementares termoleculares são incomuns porque a probabilidade de três partículas colidirem simultaneamente é muito rara. Existem, no entanto, algumas reações elementares termoleculares estabelecidas. A reação do óxido nítrico com oxigénio parece envolver passos termoleculares:
<img alt="Eq10" src="/files/ftp_upload/11381/11381_Equation_10.png" style="text-align: center; height: 70px;" />
Da mesma forma, a reação do óxido nítrico com cloro parece envolver passos termoleculares:
<img alt="Eq11" src="/files/ftp_upload/11381/11381_Equation_11.png" style="text-align: center; height: 70px;" />
Muitas vezes, um dos passos elementares em um mecanismo de reação com vários passos é significativamente mais lento do que os outros. Uma vez que uma reação não pode prosseguir mais depressa do que o seu passo mais lento, este passo irá limitar a velocidade à qual a reação global ocorre. O passo elementar mais lento é, portanto, chamado de passo de limitação de velocidade (ou passo de determinação de velocidade) da reação.
Este texto é adaptado de <a href="https://openstax.org/books/chemistry-2e/pages/12-6-reaction-mechanisms">Openstax, Chemistry 2e, 12.6: Reaction Mechanisms.</a>

Mecanismos de Reação

As reações químicas geralmente ocorrem de forma gradual, envolvendo duas ou mais reações distintas ocorrendo em uma sequência. Uma equação balanceada indica ...

Education

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elementary steps in reaction mechanisms

Chemical reactions can happen in steps, and a balanced equation does not show the full path at the molecular level. A reaction mechanism, also called a reaction path, gives the step-by-step process that explains how reactants become products. The decomposition of ozone is one example of a reaction that can be described by a sequence of steps.

Each step in a mechanism is an elementary reaction. These elementary reactions happen in order, and when they are added together, they give the balanced equation for the overall reaction. In the ozone example, an oxygen atom is made in the first step and used up in the second step, so it does not appear in the final equation. A species made in one step and used up in a later step is called a reaction intermediate.

The overall equation may show two ozone molecules forming three oxygen molecules, but the mechanism shows that the two ozone molecules do not react directly with each other. Instead, one O3 molecule breaks down to form O2 and an oxygen atom. Then a second O3 molecule reacts with that oxygen atom to make two more O2 molecules. This is why elementary reaction equations give a more direct picture of bond breaking and bond making than the overall equation does.

For an elementary reaction, the rate law can often be written directly from the balanced equation for that step. For most chemical reactions, the rate law must be found by experiment. That is why elementary reactions are useful when chemists build a mechanism that matches the observed behavior of a reaction.

Elementary reactions are described by molecularity, which is the number of reactant species in the step. A unimolecular reaction has one reactant species and forms one or more products. Its rate law is first order, written as rate = k[A]. A unimolecular step may be part of a larger mechanism, such as O3(g) → O2(g) + O in ozone decomposition. Some unimolecular reactions are also the whole reaction, not just one step. The gas-phase decomposition of cyclobutane, C4H8, to ethylene, C2H4, is one example, and its first-order experimental rate law agrees with the rate law predicted for an elementary single-step process.

A bimolecular reaction involves two reactant species. If the two reactants are different, the rate law is first order in A and first order in B, for a second-order overall rate. If the two reacting molecules are the same, the rate law is second order in A. Some reactions occur by a single bimolecular elementary reaction, such as the reaction of nitrogen dioxide with carbon monoxide. Bimolecular steps can also appear in multistep mechanisms, including the reaction of atomic oxygen with ozone in ozone decomposition.

A termolecular reaction involves the simultaneous collision of three atoms, molecules, or ions. These reactions are uncommon because three particles colliding at the same time is very unlikely. Still, a few termolecular elementary reactions are known. The reaction of nitric oxide with oxygen and the reaction of nitric oxide with chlorine both appear to involve termolecular steps.

In many multistep mechanisms, one elementary step is much slower than the others. A reaction cannot proceed faster than its slowest step, so that step controls the overall rate. This slowest step is called the rate-limiting step, or rate-determining step.

Elementary Steps in Reaction Mechanisms

reaction rate

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Reaction order from concentration shows how reactant amounts affect reaction rate. Rate laws, also called differential rate laws or rate equations, are mathematical expressions that connect reaction rate with the concentrations of reactants. For a generic reaction a A + b B → products, the rate law can be written using the reactant concentrations [A] and [B] and a rate constant k. In a rate law, k is the rate constant for one specific reaction at one specific temperature. The exponents m and n...

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reaction mechanisms

Chemical reactions can happen in steps, and a balanced equation does not show the full path at the molecular level. A reaction mechanism, also called a reaction path, gives the step-by-step process that explains how reactants become products. The decomposition of ozone is one example of a reaction that can be described by a sequence of steps. Each step in a mechanism is an elementary reaction. These elementary reactions happen in order, and when they are added together, they give the balanced...

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Explore how chemical reactions happen in steps, including elementary reactions, reaction intermediates, molecularity, rate laws, and the rate-limiting step.

Elementary Steps in Reaction Mechanisms | JoVE K12

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JoVE Core - Química dá vida a conceitos gerais de química usando vídeos de animação fáceis de entender. Além disso, nos vídeos Cientistas em ação, demonstramos experimentos de pesquisa clássicos e originais realizados atualmente por cientistas em laboratórios em todo o mundo.

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Elementary Steps in Reaction Mechanisms

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