Name: how reactant concentration sets reaction rate
Uploaded: 2025-05-22T09:05:33.000Z
Duration: 2 min 33 s
Channel: JoVE

K12 Standards

Florida Department of Education

High School

Physical Science

SC.912.P.12

SC.912.P.12.12

Rate Law and Reaction Order

Per qualsiasi reazione, la relazione tra la velocità di reazione e le concentrazioni dei reagenti può essere espressa matematicamente utilizzando una legge di velocità o l'equazione della velocità. 
In una legge di velocità, 'k' è la costante di proporzionalità, o costante di velocità, e 'n' è l'ordine di reazione rispetto a un singolo reagente, il cui valore è spesso un intero. Nelle leggi di velocità per le reazioni multi-reagenti, l'ordine di reazione complessivo è la somma di tutti gli ordini dei reagenti.
Per ogni reagente, la velocità di reazione, la costante di velocità, la concentrazione e l'ordine di reazione sono tutti determinati sperimentalmente.
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 singolo reagente, 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 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.
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 del tasso, la concentrazione del reagente viene elevata alla prima potenza.
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. Al diminuire della concentrazione del reagente, 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 del secondo ordine. Al diminuire della concentrazione del reagente, la velocità di reazione diminuisce esponenzialmente in modo quadratico.

equazione di velocità e ordine di reazione

La velocità di una reazione è influenzata dalla concentrazione dei reagenti. Le leggi di velocità (leggi di velocità differenziale) o equazioni di velocità sono espressioni matematiche che descrivono la relazione tra la velocità di una reazione chimica e la concentrazione dei suoi reagenti.

Ad esempio, in una reazione generica aA + bB ⟶ prodotti, dove a e b sono coefficienti stechiometrici, la legge cinetica può essere scritta come:

velocità = k[A]^m[B]^n

[A] e [B] rappresentano le concentrazioni molari dei reagenti e k è la costante di velocità, che è specifica per una particolare reazione ad una temperatura specifica.

Gli esponenti m e n sono gli ordini di reazione e sono tipicamente numeri interi positivi, sebbene possano essere frazioni, valori negativi o zero.

La costante di velocità k e gli ordini di reazione m e n vengono determinati sperimentalmente osservando come cambia la velocità di una reazione al variare della concentrazione dei reagenti. La costante di velocità k è indipendente dalla concentrazione dei reagenti ma varia con la temperatura.

Gli ordini di reazione in una equazione di velocità descrivono la dipendenza matematica della velocità dalle concentrazioni dei reagenti. Facendo riferimento alla generica legge di velocità (velocità = k[A]^m[B]^n), la reazione è di ordine mth rispetto ad A ed ordine nth rispetto a B. Ad esempio, se m = 1 e n = 2, la reazione è del primo ordine in A e del secondo ordine in B. L'ordine complessivo della reazione è semplicemente la somma degli ordini per ciascun reagente. Per la legge di velocità dell’esempio, la reazione è complessivamente del terzo ordine (1 + 2 = 3).

Un approccio sperimentale comune alla determinazione delle equazioni di velocità è il metodo delle velocità iniziali. Questo metodo prevede la misurazione delle velocità di reazione per più prove sperimentali effettuate utilizzando diverse concentrazioni iniziali dei reagenti. Confrontando le velocità misurate durante queste prove è possibile determinare gli ordini di reazione e, successivamente, la costante di velocità, che, insieme, vengono utilizzati per formulare una legge sulla velocità.

Le leggi di velocità 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 velocità di reazione. È importante notare che le leggi di velocità 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 velocità di reazione e la concentrazione di reagenti o prodotti.

<ul style="list-style-type: disc; padding-left: 35px; margin: 0;">
	<li>In una reazione di ordine zero, la concentrazione dei reagenti non ha alcun effetto sulla velocità della reazione, che rimane costante per tutta la reazione.</li>
	<li>In una reazione del primo ordine, la velocità di reazione è direttamente e linearmente proporzionale alla variazione della concentrazione del reagente. Man mano che la concentrazione del reagente diminuisce, anche la velocità di reazione diminuisce proporzionalmente.</li>
	<li>Nelle reazioni di secondo ordine o di ordine superiore, la velocità di reazione è proporzionale al valore esponenziale dei reagenti. Pertanto, man mano che la reazione procede e la concentrazione dei reagenti diminuisce, la velocità di reazione diminuisce in modo esponenziale.</li>
</ul>

Questo testo è 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>.

Equazione di velocità e ordine di reazione

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 ...

Education

Didattica

JoVE Core

Organic Chemistry

Chimica Organica

Unit 1

Unità 1

Thermodynamics and Chemical Kinetics

Termodinamica e Cinetica Chimica

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

how reactant concentration sets reaction rate

Reaction rate depends on reactant concentration, and rate laws show that relationship with a mathematical equation. A rate law, also called a differential rate law or rate equation, links the rate of a chemical reaction to the concentrations of its reactants.

For a generic reaction, a A + b B  products, the rate law can be written as rate = k [A]^m [B]^n. Here, [A] and [B] are the molar concentrations of the reactants. The constant k is the rate constant, and it is specific to one reaction at a specific temperature.

The exponents m and n are the reaction orders. They are usually positive integers, but they can also be fractions, negative values, or zero. 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 separate orders, so this example is third order overall.

Rate law values are found by experiment, not by stoichiometry. Scientists change the initial reactant concentrations in several trials and measure the reaction rate. This method, called the method of initial rates, helps determine the reaction orders and then the rate constant.

Some reactions show zero-order behavior. In a zero-order reaction, changes in reactant concentration do not change the rate, so the rate stays constant. In a first-order reaction, the rate changes in direct proportion to the reactant concentration. In second-order and higher-order reactions, the rate depends on the concentration raised to an exponent, so the rate drops more sharply as reactants are used up.

Rate laws can also show fractional orders, and some can even show negative orders. A negative order means that increasing one reactant can decrease the reaction rate. These patterns are useful for describing how reaction rate changes in real experiments. They are not reliably predicted from the reaction equation alone, so the rate law must be determined experimentally.

How Reactant Concentration Sets Reaction Rate

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Reaction rate depends on reactant concentration, and rate laws show that relationship with a mathematical equation. A rate law, also called a differential rate law or rate equation, links the rate of a chemical reaction to the concentrations of its reactants. For a generic reaction, a A + b B  products, the rate law can be written as rate = k [A]^m [B]^n. Here, [A] and [B] are the molar concentrations of the reactants. The constant k is the rate constant, and it is specific to one reaction at...

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Learn how rate laws connect reactant concentration to reaction speed, how reaction orders work, and how the method of initial rates is used to find them.

How Reactant Concentration Sets Reaction Rate | JoVE K12

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How Reactant Concentration Sets Reaction Rate

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