Name: reaction order from concentration
Uploaded: 2020-09-24T15:06:50.000Z
Duration: 3 min 3 s
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IPC.7

IPC.7.F

Concentration and Rate Law

La velocità di una reazione dipende spesso dalle concentrazioni dei reagenti. Per qualsiasi reazione, la relazione fra la velocità di reazione e le concentrazioni dei reagenti può essere espressa matematicamente, usando la legge della velocità o l'equazione della velocità. In una legge della velocità, k è la costante di proporzionalità, o la costante di velocità, e n è l'ordine di reazione rispetto ad un singolo reagente, il cui valore è spesso un numero intero.

Nelle leggi di velocità per le reazioni multi-reagente, l'ordine di reazione complessivo è la somma di tutti gli ordini di reagenti. Per ogni reagente, la velocità di reazione, la costante di velocità, la concentrazione e l'ordine di reazione sono tutti determinati sperimentalmente. La legge della velocità esprime la relazione fra tutti questi parametri.

Gli ordini dei singoli reagenti assumono comunemente i valori 0, 1 o 2 e, in base all'ordine di reazione complessivo, le reazioni chimiche possono essere classificate come reazioni di ordine zero, di primo o di secondo ordine. Una reazione chimica a reagente singolo, o unimolecolare, per la quale la velocità di reazione rimane costante per tutta la sua durata, è una reazione di ordine zero. L'ordine dei reagenti in una reazione di ordine zero è zero e, secondo la legge della velocità, la concentrazione dei reagenti è elevata alla potenza zero.

Poiché il valore di qualsiasi numero elevato alla potenza zero è uno, la velocità di reazione di una reazione di ordine zero è uguale alla costante di velocità e, quindi, indipendente dalla concentrazione del reagente. Pertanto, in una reazione di ordine zero, anche se la concentrazione del reagente diminuisce, la velocità di reazione non rallenta. Una reazione chimica unimolecolare in cui la velocità di reazione è direttamente proporzionale alla concentrazione del reagente è una reazione del primo ordine.

L'ordine dei reagenti per una reazione del primo ordine è uno e, secondo la legge della velocità, la concentrazione del reagente viene elevata alla potenza di uno. Poiché il valore di qualsiasi numero elevato alla potenza di uno rimane lo stesso, la velocità di reazione di una reazione del primo ordine dipende direttamente dalla concentrazione del reagente. Quando la concentrazione del reagente diminuisce, la velocità di reazione diminuisce proporzionalmente in modo lineare.

Una reazione chimica unimolecolare, in cui la velocità di reazione dipende dal quadrato della concentrazione del reagente, è una reazione di secondo ordine. L'ordine dei reagenti è due e la concentrazione dei reagenti è elevata alla potenza di due. Pertanto, la velocità di reazione in una reazione del secondo ordine dipende direttamente dal quadrato della concentrazione del reagente.

Quando la concentrazione del reagente diminuisce, la velocità di reazione diminuisce esponenzialmente in modo quadratico.

legge sulla concentrazione e sulla velocit

La velocit di una reazione influenzata dalla concentrazione dei reagenti. Le leggi di velocit (leggi di velocit differenziale) o equazioni di velocit sono espressioni matematiche che descrivono la relazione tra la velocit di una reazione chimica e la concentrazione dei suoi reagenti.

Ad esempio, in una reazione generica aA + bB ? prodotti, dove aeb sono coefficienti stechiometrici, la legge cinetica pu essere scritta come:

 <img alt="Eq1" src="/files/ftp_upload/11375/11375_Equation_1.png" style="height: 50px;" />

[A] e [B] rappresentano le concentrazioni molari dei reagenti e k la costante di velocit, che specifica per una particolare reazione a una temperatura specifica.

Gli esponenti m e n sono gli ordini di reazione e sono tipicamente numeri interi positivi, sebbene possano essere frazioni, negativi o zero.

La costante di velocit k e gli ordini di reazione m e n vengono determinati sperimentalmente osservando come cambia la velocit di una reazione al variare della concentrazione dei reagenti. La costante di velocit k indipendente dalle concentrazioni dei reagenti, ma varia con la temperatura.

Gli ordini di reazione in una legge sulla velocit descrivono la dipendenza matematica della velocit dalle concentrazioni dei reagenti. Facendo riferimento alla generica legge di velocit (velocit = k[A]m[B]n), la reazione di ordine m rispetto ad A ed ordine n rispetto a B. Ad esempio, se m = 1 e n = 2, la la reazione del primo ordine in A e del secondo ordine in B. L'ordine complessivo della reazione semplicemente la somma degli ordini per ciascun reagente. Per la legge tariffaria di esempio qui, la reazione complessivamente del terzo ordine (1 + 2 = 3).

Un approccio sperimentale comune alla determinazione dellequazione cinetica il metodo delle equazioni iniziali. Questo metodo prevede la misurazione delle velocit di reazione per pi prove sperimentali effettuate utilizzando diverse concentrazioni iniziali dei reagenti. Confrontando le velocit misurate per queste prove possibile determinare gli ordini di reazione e, successivamente, la costante di velocit, che insieme vengono utilizzati per formulare una legge sulla velocit.

Le leggi sulla velocit possono mostrare ordini frazionari per alcuni reagenti e talvolta si osservano ordini di reazione negativi quando un aumento della concentrazione di un reagente provoca una diminuzione della velocit di reazione. importante notare che lequazione cinetica sono determinate solo mediante esperimenti e non sono previste in modo affidabile dalla stechiometria della reazione.

L'ordine di reazione determina la relazione tra la velocit di reazione e la concentrazione di reagenti o prodotti.

<ol style="list-style-type: disc;">
	<li>In una reazione di ordine zero, la concentrazione dei reagenti non ha alcun effetto sulla velocit della reazione, che rimane costante durante tutta la reazione.</li>
	<li>In una reazione del primo ordine, la velocit di reazione direttamente e linearmente proporzionale alla variazione della concentrazione del reagente. Man mano che la concentrazione del reagente diminuisce, anche la velocit di reazione diminuisce proporzionalmente.</li>
	<li>Nelle reazioni di secondo ordine o di ordine superiore, la velocit di reazione proporzionale al valore esponenziale dei reagenti. Pertanto, man mano che la reazione procede e la concentrazione dei reagenti diminuisce, la velocit di reazione diminuisce in modo esponenziale.</li>
</ol>

Questo testo adattato da <a href="https://openstax.org/books/chemistry-2e/pages/12-3-rate-laws#57033">Openstax, Chemistry 2e, Section 12.3: Rate Laws</a>

Legge sulla concentrazione e sulla velocit

La velocità di una reazione è influenzata dalle concentrazioni dei reagenti. Le leggi di velocità (leggi di velocità differenziali) o equazioni di velocità sono espressioni matematiche ...

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reaction order from concentration

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 are the reaction orders. They are usually positive integers, but they can also be fractions, negative numbers, or zero. The values of k, m, and n are found experimentally by measuring how the rate changes when reactant concentrations change.

The rate constant does not depend on the reactant concentrations, but it can change with temperature. The reaction orders show the mathematical link between concentration and rate. In the expression rate = k [A]m [B]n, the reaction is m order with respect to A and n order with respect to B. For example, if m = 1 and n = 2, the reaction is first order in A and second order in B.

The overall reaction order is the sum of the orders for each reactant. In the example above, the reaction is third order overall because 1 + 2 = 3. A common way to find a rate law is the method of initial rates. In this method, reaction rates are measured in several trials that start with different initial reactant concentrations. Comparing those rates makes it possible to find the reaction orders and then the rate constant.

Rate laws can also include fractional orders. In some cases, a reactant may have a negative order. That means increasing its concentration can lower the reaction rate. Rate laws are determined by experiment only, and they cannot be reliably predicted from reaction stoichiometry. The reaction order also describes how the rate changes as reactant or product concentration changes.

A zero-order reaction has a rate that does not change with reactant concentration. The rate stays constant throughout the reaction. In a first-order reaction, the rate is directly proportional to the change in reactant concentration. As the reactant concentration drops, the rate drops by the same proportion. In second-order or higher-order reactions, the rate depends on the reactants raised to an exponent. As the reaction continues and reactant concentration decreases, the rate falls exponentially.

Reaction Order from Concentration

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concentration and rate law

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 Order from Concentration | JoVE K12

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Reaction Order from Concentration

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