Name: reaction order and rate constants
Uploaded: 2020-03-26T07:22:26.000Z
Duration: 4 min 37 s
Channel: JoVE

K12 Standards

Minnesota Academic Standards in Science

9-12

Chemistry

3.2.1 9C.3.1.1.2

Concentration Dependence

La misura della velocità con cui procede una reazione è chiamata velocità di reazione. Per una reazione a passo singolo, la velocità è uguale alla variazione di concentrazione di ciascun reagente o prodotto nel tempo moltiplicata per l'inverso del corrispondente coefficiente stechiometrico. Si può pensare alla variazione della concentrazione nel tempo come la concentrazione al tempo t meno la concentrazione iniziale divisa per t. Le velocità di reazione sono sempre positive, quindi le espressioni dei reagenti hanno segni negativi. Questi tassi potrebbero non essere uguali nelle reazioni a più fasi, ma possiamo comunque utilizzare questa relazione come stima della reazione complessiva.
La legge della velocità, o equazione della velocità, descrive la relazione tra la velocità della reazione e le concentrazioni dei reagenti. In questa equazione, k è la costante di velocità, A e B sono i due reagenti e m e n sono i rispettivi ordini di reazione. L'ordine di reazione descrive la relazione tra la concentrazione di un reagente e la velocità e non è correlato alla stechiometria. È fondamentale ricordare che l'ordine di reazione non è lo stesso del coefficiente del reagente nell'equazione bilanciata.
Quando una reazione coinvolge due o più reagenti, dobbiamo considerare l'ordine di reazione per ciascun reagente. L'ordine di reazione complessivo è uguale alla somma degli ordini di reazione dei reagenti. Ad esempio, se il reagente A è del primo ordine e il reagente B è di ordine zero, l'ordine di reazione complessivo è uno. Gli ordini di reazione più comuni nelle reazioni semplici sono l'ordine zero, il primo ordine e il secondo ordine. Esaminiamoli usando una reazione unimolecolare come esempio.
Se la reazione è di ordine zero, la concentrazione del reagente non ha alcun effetto sulla velocità di reazione. Quindi, la velocità di reazione è uguale a k, la costante di velocità. Un grafico della velocità di reazione rispetto alla concentrazione è una linea orizzontale.
Nelle relazioni del primo ordine, la concentrazione del reagente è linearmente correlata alla velocità di reazione. Pertanto, la velocità è uguale alla costante di velocità moltiplicata per la concentrazione del reagente. Un grafico della velocità di reazione rispetto alla concentrazione sarà lineare, con k come pendenza.
Se la reazione è del secondo ordine, esiste una relazione quadratica tra la concentrazione e la velocità. Pertanto, il tasso è uguale alla costante di tasso moltiplicata per la concentrazione al quadrato. Un grafico della velocità rispetto alla concentrazione sarà parabolico, con k come pendenza.
La costante di velocità, k, è un valore dipendente dalla temperatura che mette in relazione l'energia di attivazione della reazione con la velocità di reazione. Esplorerai questo aspetto nel prossimo esperimento di laboratorio.
Le velocità di reazione sono tipicamente espresse in moli per litro al secondo. La velocità ha le stesse unità, indipendentemente dall'ordine di reazione. Pertanto, la costante di velocità ha unità diverse, a seconda dell'ordine di reazione. Ad esempio, se A è il primo ordine e B è l'ordine zero, allora abbiamo un'istanza di moli per litro nell'equazione del tasso. Pertanto, la costante di velocità deve essere espressa in secondi inversi.
Quindi, come determiniamo l'ordine di reazione? L'ordine di reazione deve essere determinato sperimentalmente utilizzando una serie di test. Se si dispone di due reagenti, un metodo consiste nel mantenere costante la concentrazione di un reagente, variare l'altro e calcolare il tempo necessario per produrre una certa quantità di prodotto. Lo stesso processo viene ripetuto per il secondo reagente. È quindi possibile stimare l'ordine di ciascun reagente tracciando la velocità rispetto alla concentrazione variabile dei reagenti e verificando se assomiglia a un grafico a zero, al primo o al secondo ordine. Abbinando i tuoi dati all'equazione di velocità corrispondente potrai anche calcolare k.
In questo laboratorio, determinerai gli ordini di reazione di due reagenti variando le loro concentrazioni e cronometrando il tempo necessario alla reazione per progredire fino a rendere opaca la soluzione.

dipendenza dalla concentrazione

Cinetica chimica e legge della velocità di reazione
La cinetica chimica si riferisce alla velocità o alla velocità di una reazione chimica. La velocità dipende dal meccanismo, dalla complessità e dal numero di reagenti nella reazione. Anche la concentrazione dei reagenti gioca un ruolo significativo nella velocità di una reazione.
La legge tariffaria quantifica questa relazione attraverso la sperimentazione. Ogni reagente contribuisce alla velocità della reazione con un fattore noto come ordine di reazione. Questo fattore può variare da zero a due e dipende dalla relazione di quel reagente con la velocità della reazione.
r = k[A]m[B]n per aA + bB + ... → cC
In questa equazione, r è la velocità di reazione, k è la costante di velocità di reazione, [A] e [B] sono le concentrazioni dei reagenti A e B, e m e n sono gli ordini di reazione dei reagenti A e B, rispettivamente. La velocità di reazione, r, è definita come la variazione della concentrazione del prodotto nel tempo e ha unità di moli per litro al secondo.
<h4>Ordine di reazione</h4>
L'ordine della reazione descrive la dipendenza di potenza che la concentrazione di reagenti ha sulla velocità di reazione. L'ordine di reazione complessivo è la somma degli ordini di reazione per ciascuno dei reagenti. È importante ricordare che l'ordine di reazione NON è correlato ai fattori stechiometrici dell'equazione chimica bilanciata. In altre parole, la stechiometria non può essere utilizzata per determinare gli ordini di reazione per una reazione chimica. Gli ordini di reazione devono essere determinati sperimentalmente
Per determinare l'ordine di reazione, la concentrazione del secondo reagente viene mantenuta costante, mentre la concentrazione del primo reagente viene variata. Il tempo necessario affinché si verifichi la reazione è misurato in secondi; Tuttavia, questo tempo non corrisponde al tempo necessario alla reazione per andare a compimento. Invece, questo è il tempo necessario per l'inizio della reazione. I tempi di reazione per le diverse concentrazioni di reagenti vengono quindi confrontati per determinare l'ordine. La stessa serie di reazioni viene eseguita per determinare l'ordine del secondo reagente, dove la concentrazione del primo reagente viene mantenuta costante e la concentrazione del secondo reagente viene variata.
Confrontando i tempi di reazione, è possibile determinare l'ordine di reazione. Ad esempio, se il tempo di reazione rimane costante nonostante le variazioni della concentrazione del reagente, allora il reagente è di ordine zero. Ciò significa che la velocità di reazione è uguale al coefficiente di velocità di reazione, k, che deve avere unità di M/s. Se il tempo di reazione cambia linearmente con le variazioni della concentrazione (cioè, il raddoppio della concentrazione influisce sul tempo di reazione di un fattore 2), allora il reagente è del primo ordine. Ciò significa che la velocità di reazione è uguale al prodotto del coefficiente della velocità di reazione e della concentrazione del reagente. In questo caso, k ha unità di 1/s. Infine, se il tempo di reazione è influenzato da un fattore 4 quando la concentrazione di un reagente è raddoppiata, il reagente è del secondo ordine. Ciò significa che la velocità della reazione è uguale alla costante di velocità moltiplicata per il quadrato della concentrazione del reagente, formando una relazione quadratica. In questo caso, le unità di k devono essere 1/M⋅s.
L'ordine complessivo della reazione è la somma dei singoli ordini di reazione. Ad esempio, se la reazione è del primo ordine rispetto ad A, allora m = 1. E se la reazione è di ordine zero rispetto a B, allora n = 0. L'ordine generale della reazione è del primo ordine perché m + n = 1.
<h4>Costante di velocità di reazione</h4>
La costante di velocità, k, è specifica per la reazione e la temperatura in cui viene eseguita la reazione. La costante di velocità è determinata dalla serie di esperimenti. Utilizzando la velocità misurata e le concentrazioni iniziali dei reagenti, l'equazione della velocità viene utilizzata per risolvere la costante di velocità. La costante di velocità dipende dalla temperatura ed è definita dall'equazione di Arrhenius. Questa equazione descrive la relazione tra k e l'energia di attivazione, Ea, la temperatura, T, e la costante di gas ideale, R. La costante A è una costante di proporzionalità e non deve essere confusa con il reagente A.
<img src="/CDNSource/lm/labs/50/50_Concepts_2.jpg">
<h4>referenze</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). Chimica e reattività chimica. Belmont, CA: Brooks/Cole, Cengage Learning.</li>
	<li style="font-size:17px">Silderberg, M.S. (2009). Chimica: la natura molecolare della materia e il cambiamento. Boston, MA: McGraw Hill.</li>
</ol>

Dipendenza dalla concentrazione

Cinetica chimica e legge della velocità di reazione
La cinetica chimica si riferisce alla velocità o alla velocità di una reazione chimica. La tariffa dipende dal meccanismo, ...

Education

Didattica

JoVE Lab Manual

Chimica

reaction order and rate constants

Chemical kinetics describes the speed of a chemical reaction. The rate depends on the reaction mechanism, the complexity of the reaction, and the number of reactants involved. Reactant concentration also affects how fast a reaction happens. The rate law is the experimental relationship that connects these factors to reaction rate.

In the rate law, each reactant contributes through a reaction order. The general form is r = k[A]^m[B]^n for aA + bB + ... → cC. Here, r is the reaction rate, k is the rate constant, and [A] and [B] are the reactant concentrations. The exponents m and n show the reaction order for each reactant.

The reaction rate is the change in product concentration over time. Its units are moles per liter per second. The overall reaction order is the sum of the individual orders. Reaction order is not the same as the numbers in the balanced chemical equation, so stoichiometric coefficients cannot be used to find it. Reaction orders must be measured experimentally.

To find a reaction order, one reactant concentration is changed while the other is held constant. The reaction time is measured in seconds, but this is the time for the reaction to start, not the time to completion. The times from different concentrations are compared to determine the order. The same process is repeated for the second reactant by switching which concentration is varied.

If the reaction time stays the same even when concentration changes, the reactant is zero-order. In that case, the rate equals k, and k has units of M/s. If the time changes linearly with concentration, the reactant is first order. Then the rate equals k times the reactant concentration, and k has units of 1/s. If doubling the concentration changes the time by a factor of 4, the reactant is second order. Then the rate equals k times the square of the concentration, and k has units of 1/M·s.

The overall order is the sum of the individual orders. For example, if A is first order and B is zero-order, the total order is first order because m + n = 1. The rate constant, k, is specific to the reaction and the temperature used. It is found from the experimental data by using the measured rate and initial concentrations in the rate equation.

The rate constant also depends on temperature through the Arrhenius equation. This equation relates k to activation energy, Ea, temperature, T, and the ideal gas constant, R. The constant A in the equation is a proportionality constant. It should not be confused with reactant A.

Reaction Order and Rate Constants

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Video completi e incentrati sul curriculum sia per gli istruttori che per gli studenti nei corsi di laboratorio di Chimica introduttiva.

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Learn how concentration affects reaction rate, how to determine zero-, first-, and second-order reactions, and how rate constants depend on temperature.

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New York State Science Learning Standards

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Molecular Orbital (MO) Theory

Source: Tamara M. Powers, Department of Chemistry, Texas A&#38;M University
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Effect of Temperature Change on Reaction Rate

The Arrhenius equation,

relates the activation energy and the rate constant, k, for many chemical reactions.
In this equation, R is the ideal gas ...

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Chemical Kinetics 
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Reaction Rate

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The formation of a solution is an example of a spontaneous process, which is a process that occurs under specified conditions without energy from some ...

General Properties of Solutions

Many common substances around us exist as a solution, such as ocean water, air, and gasoline. All solutions are mixtures of substances that are composed ...

Solutions and Concentrations

Source: Laboratory of Dr. Michael Evans &#8212; Georgia Institute of Technology
A solution is a homogeneous mixture containing some components in small ...

Solubility

Solubility
Solubility describes how much of a solute can dissolve in a given volume of a specific solvent. Solubility is usually reported in terms of ...

Solution, Solubility, and Solubility Equilibrium
A solution is a homogeneous mixture&#160;composed of a solvent, the major component, and a solute, the ...

Solution Equilibrium and Saturation

Imagine adding a small amount of sugar to a glass of water, stirring until all the sugar has dissolved, and then adding a bit more. You can repeat this ...

Intermolecular Forces in Solutions

The formation of a solution is an example of a spontaneous process, a process that occurs under specified conditions without energy from some external ...

Matter and Its Interactions: Develop models to illustrate the changes in the composition of the nucleus of the atom and the energy released during the processes of fission, fusion, and radioactive decay. (P: 2, CC: 5, CI: PS1)

9C.3.1.1.3

Chemical and Solubility Equilibria

The free energy change associated with dissolving a solute in a liter of solvent is called the free energy of a solution, &#916;Gsolution. The overall ...

Matter and Its Interactions: Apply scientific principles and evidence to provide an explanation about the effects of changing the surface area, agitation, temperature, and concentration of the reacting particles on the rate at which the reaction occurs. (P: 6, CC: 1, CI: PS1)

9C.3.2.1.2

3 Developing possible explanations of phenomena or designing solutions to engineering problems
 3.2 Constructing explanations and designing solutions 
  3.2.1 Students will be able to apply scientific principles and empirical evidence (primary or secondary) to explain the causes of phenomena or identify weaknesses in explanations developed by the students or others.

Reaction Order and Rate Constants

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