Name: catalyst action in reaction rates
Uploaded: 2020-09-24T15:06:50.000Z
Duration: 2 min 50 s
Channel: JoVE

K12 Standards

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North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.5.1

Catalysis

Lo studio dei meccanismi di reazione e della cinetica aiuta i ricercatori a ottimizzare la velocità di una reazione. Un incremento della temperatura o della concentrazione accelera la velocità, ma può anche portare alla decomposizione o prodotti secondari, se aumentata troppo. In alternativa, è possibile aggiungere alla miscela di reazione una sostanza chimica che accelera la velocità di reazione, senza essere alterata o consumata in modo permanente.

Queste sostanze sono dette catalizzatori"i quali influenzano la costante di velocità, modificando l'energia di attivazione. Ricordate che maggiore è l'energia di attivazione, più lenta è la velocità di reazione. Pertanto, molte reazioni chimiche, come la decomposizione del perossido di idrogeno acquoso, progrediscono lentamente.

L'aggiunta di un catalizzatore abbassa l'energia di attivazione fornendo un meccanismo di reazione alternativo, senza influenzare gli stati energetici dei reagenti e dei prodotti. Esistono due tipi di catalizzatori omogeneo ed eterogeneo. Un catalizzatore omogeneo esiste nella stessa fase dei reagenti.

Per esempio, il bromuro di sodio è aggiunto al perossido di idrogeno acquoso per accelerarne la decomposizione in ossigeno gassoso e acqua. Innanzitutto, gli ioni bromuro reagiscono con il perossido di idrogeno in un mezzo acido per formare una soluzione arancione di bromo acquoso e acqua. In seguito, il bromo acquoso e il perossido di idrogeno reagiscono e rilasciano ioni di bromuro indicati da una soluzione incolore.

Caratteristica di un catalizzatore, gli ioni bromuro accelerano la reazione ma rimangono non consumati e non compaiono nell'equazione bilanciata netta. Esiste un catalizzatore eterogeneo in uno stato fisico differente rispetto ai reagenti. La catalisi eterogenea è caratterizzata da quattro passaggi sottostanti:assorbimento, diffusione, reazione e desorbimento.

Per esempio, la reazione di idrogenazione del gas etenico insaturo, all'etano, viene accelerata dal palladio finemente disperso sul carbone della legna. Qui, il catalizzatore esiste nel suo stato solido, mentre i reagenti sono nelle loro forme gassose. Durante questa catalisi eterogenea, le molecole di etene e di idrogeno si adsorbono sulla superficie del catalizzatore.

Il legame idrogeno-idrogeno si infrange consentendo agli atomi di idrogeno di diffondersi sulla superficie del catalizzatore. Quando gli atomi di idrogeno diffuso incontrano molecole di etene adsorbite, reagiscono per formare etano saturo. Successivamente, i prodotti si desorbono e si allontanano dalla superficie metallica, lasciando dietro di sé il catalizzatore intatto.

0:02:44, 430 e combinando le costanti di velocità in una costante di velocità complessiva, si genera un'espressione consistente con la legge di velocità sperimentale. Combinando i passaggi elementari si ottiene l'equazione bilanciata complessiva, che soddisfa il secondo requisito per un meccanismo di reazione. Pertanto, il meccanismo di reazione a due fasi proposto per la formazione del bromuro di nitrosile è valido.

catalisi

La presenza di un catalizzatore influenza la velocità di una reazione chimica. Un catalizzatore è una sostanza che può aumentare la velocità di reazione senza essere consumata durante il processo. Una comprensione di base del ruolo dei catalizzatori durante le reazioni chimiche può essere contenuta nel concetto di meccanismi di reazione e diagrammi energetici.

<img alt="Image1" src="/files/ftp_upload/11383/11383_Image_1.png" style="text-align: center; height: 350px;" />

L'immagine illustrata rappresenta i diagrammi di reazione di un processo chimico endotermico che procede in assenza (curva rossa) e presenza curva blu) di un catalizzatore.

Entrambe le curve rappresentano la stessa reazione complessiva: iniziano e finiscono alle stesse energie. (In questo caso, i prodotti hanno più energia dei reagenti, quindi la reazione è endotermica). Tuttavia, i loro meccanismi di reazione sono diversi. La reazione non catalizzata procede attraverso un meccanismo ad una fase (con un solo stato di transizione osservato). Al contrario, la reazione catalizzata segue un meccanismo a due fasi (si osservano due stati di transizione) con un'energia di attivazione notevolmente inferiore. Questa differenza nei percorsi di reazione illustra il ruolo del catalizzatore nel fornire un meccanismo di reazione alternativo con un’energia di attivazione inferiore, accelerando così le reazioni.

Non è necessario che il meccanismo di reazione catalizzata coinvolga un numero maggiore di passaggi elementari rispetto al meccanismo non catalizzato. Tuttavia, deve fornire un percorso di reazione alternativo il cui passaggio che determina la velocità è più veloce (con un E_a o energia di attivazione inferiore).

Una reazione catalitica può essere classificata come omogenea o eterogenea, in base agli stati fisici in cui esistono catalizzatori e reagenti durante il processo catalitico.

Catalisi omogenea

Nella catalisi omogenea, il catalizzatore è presente nella stessa fase dei reagenti: solido, liquido o gassoso. Durante il processo, il catalizzatore interagisce con il reagente per formare una sostanza intermedia, che poi si decompone o reagisce con un altro reagente in uno o più passaggi per rigenerare il catalizzatore originale e formare il prodotto finale.

Un esempio di catalisi omogenea è il processo chimico che comporta la decomposizione dell’ozono che avviene nell’atmosfera superiore della terra. L’ozono è una molecola relativamente instabile che si decompone per produrre ossigeno biatomico. Questa reazione di decomposizione è coerente con il seguente meccanismo in due fasi:

<img alt="Eq1" src="/files/ftp_upload/11383/11383_Equation_1.png" style="text-align: center; height: 75px;" />

Molte sostanze possono catalizzare la decomposizione dell'ozono. Ad esempio, si ritiene che la decomposizione dell’ozono catalizzata dall’ossido nitrico avvenga attraverso il seguente meccanismo in tre fasi:

<img alt="Eq2" src="/files/ftp_upload/11383/11383_Equation_2.png" style="text-align: center; height: 115px;" />

Tuttavia, la reazione complessiva è la stessa sia per il meccanismo non catalizzato in due fasi che per il meccanismo catalizzato da NO in tre fasi:

<img alt="Eq3" src="/files/ftp_upload/11383/11383_Equation_3.png" style="text-align: center; height: 50px;" />

Nella reazione catalizzata, si noti che l'NO è un reagente nella prima fase del meccanismo e un prodotto nell'ultima fase. Questa è un'altra caratteristica di un catalizzatore: sebbene partecipi alla reazione chimica, non viene consumato dalla reazione. Inoltre, in questa catalisi omogenea, sia il reagente che il catalizzatore esistono in una fase gassosa.

Catalisi eterogenea

Nella catalisi eterogenea, il catalizzatore è presente in una fase diversa (solitamente solida) rispetto ai reagenti. Tali catalizzatori generalmente funzionano fornendo una superficie attiva su cui può avvenire una reazione. Le reazioni in fase gassosa e liquida catalizzate da catalizzatori eterogenei si verificano sulla superficie del catalizzatore anziché all'interno della fase gassosa o liquida.

La catalisi eterogenea coinvolge tipicamente i seguenti processi:

<ol>
	<li>Adsorbimento del/i reagente/i sulla superficie del catalizzatore</li>
	<li>Attivazione del(i) reagente(i) adsorbito(i)</li>
	<li>La reazione del(i) reagente(i) adsorbito(i)</li>
	<li>Desorbimento di prodotti dalla superficie del catalizzatore</li>
</ol>

<img alt="Image2" src="/files/ftp_upload/11383/11383_Image_2.png" style="text-align: center; height: 250px;" />

L'immagine illustrata rappresenta il meccanismo di reazione che coinvolge la catalisi eterogenea di etene e idrogeno gassoso su una superficie solida di nichel, formando gas etano (C_2H_4 + H_2 ⟶ C_2H_6):

(a) L'idrogeno viene adsorbito sulla superficie del nichel. Durante il processo, i legami idrogeno-idrogeno vengono rotti per formare legami nichel-idrogeno.

(b) L'etene si adsorbe anche sulla superficie del nichel rompendo il legame pi carbonio-carbonio e formando legami nichel-carbonio.

(c) Gli atomi di idrogeno si diffondono attraverso la superficie e formano nuovi legami carbonio-idrogeno quando si scontrano per formare etano (C_2H_6).

(d) Le molecole di etano si desorbono dalla superficie del nichel.

La catalisi eterogenea viene utilizzata per la produzione industriale di prodotti chimici come ammoniaca, acido nitrico, acido solforico e metanolo. Catalizzatori eterogenei vengono utilizzati anche nei convertitori catalitici presenti sulla maggior parte delle automobili a benzina.

Questo testo è adattato da <a href="https://openstax.org/books/chemistry-2e/pages/12-7-catalysis">Openstax, Chemistry 2e, Section 12.7: Catalysis</a>

Catalisi

La presenza di un catalizzatore influisce sulla velocità di una reazione chimica. Un catalizzatore è una sostanza in grado di aumentare la velocità di reazione senza essere consumata ...

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KEY TERMS AND CONCEPTS

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catalyst action in reaction rates

Catalysts change the rate of a chemical reaction by giving it a faster path. A catalyst increases reaction rate without being consumed during the process. Its effect is often explained with reaction mechanisms and energy diagrams.

For an endothermic reaction, the reactants and products start and end at the same energies in both cases. The uncatalyzed path is a one-step mechanism with one transition state. The catalyzed path follows a different mechanism with two transition states and a lower activation energy, or Ea. The lower rate-determining barrier is what makes the reaction faster.

Catalytic reactions are often grouped as homogeneous or heterogeneous. In homogeneous catalysis, the catalyst and reactants are in the same phase, which may be solid, liquid, or gas. The catalyst reacts with the reactant to form an intermediate, and that intermediate then breaks down or reacts again to form the product and regenerate the catalyst.

A good example of homogeneous catalysis is ozone decomposition in the upper atmosphere. Ozone breaks down to form diatomic oxygen, and nitric oxide can catalyze that process through a three-step mechanism. In this reaction, NO appears as a reactant in the first step and a product in the last step, which shows that it participates in the reaction but is not used up. Here, both the reactant and the catalyst are gases.

In heterogeneous catalysis, the catalyst is in a different phase from the reactants, usually a solid. These reactions happen on the catalyst surface. They typically include adsorption of reactants, activation on the surface, reaction of the adsorbed molecules, and desorption of the products.

The nickel-catalyzed reaction of ethene and hydrogen to form ethane shows this surface process. Hydrogen first adsorbs on nickel, and the H-H bond breaks to form nickel-hydrogen bonds. Ethene also adsorbs as its carbon-carbon pi bond breaks and nickel-carbon bonds form. Hydrogen atoms then move across the surface and make new carbon-hydrogen bonds, and the ethane product desorbs from the nickel surface.

Heterogeneous catalysts are important in industry. They help make ammonia, nitric acid, sulfuric acid, and methanol. They are also used in catalytic converters in most gasoline-powered cars.

Catalyst Action in Reaction Rates

reaction rate

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catalysis

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key terms and concepts

Explore how catalysts speed up reactions by lowering activation energy, with clear examples of homogeneous and heterogeneous catalysis for high school chemistry.

Catalyst Action in Reaction Rates | JoVE K12

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Catalysts in Chemical Reactions

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Catalyst Action in Reaction Rates

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