Name: measuring reaction rates
Uploaded: 2020-09-24T15:06:50.000Z
Duration: 2 min 53 s
Channel: JoVE

K12 Standards

Texas Essential Knowledge and Skills

High School

Chemistry and Physics

IPC.7

IPC.7.F

Reaction Rate

Una reazione chimica comporta la trasformazione chimica dei reagenti in prodotti. Man mano che la reazione progredisce, la concentrazione dei reagenti diminuisce mentre la concentrazione dei prodotti aumenta. Questa variazione nelle concentrazioni di reagenti e prodotti può essere tracciata in un grafico in funzione del tempo.

La velocità con cui progredisce una reazione è detta velocità di reazione. Tale valore misura la velocità di scomparsa dei reagenti, o la velocità di comparsa dei prodotti, ed è espressa in unità di molarità al secondo. La velocità di reazione media può essere calcolata dalla variazione delle concentrazioni di reagenti o prodotti durante un periodo di tempo specifico.

Il valore della concentrazione molare è indicato tra parentesi, t rappresenta il periodo di tempo e il simbolo delta rappresenta cambiamento in. Poiché i reagenti si esauriscono durante una reazione chimica, il valore della variazione di concentrazione dei reagenti è sempre negativo. Pertanto, una velocità di reazione calcolata in termini di reagenti è supportata da un segno negativo, per rendere positivo il valore complessivo.

La velocità di reazione non è uniforme durante una reazione. La velocità all'inizio della reazione, al tempo zero"è detta velocità di reazione iniziale. Quando le concentrazioni dei reagenti si riducono, la velocità di reazione diminuisce e la reazione rallenta.

La velocità di reazione in un dato momento, o la velocità di reazione istantanea, viene misurata calcolando la pendenza di una tangente tracciata alla curva di reazione, per reagenti o prodotti, al momento di interesse. Il valore della pendenza o la velocità istantanea in un punto temporale specifico è uguale per tutti i reagenti e prodotti. La velocità di reazione di una reazione chimica riflette anche i coefficienti stechiometrici effettivi di reagenti e prodotti.

Quindi, per qualsiasi reazione bilanciata, dove una moli di A reagisce con b moli di B, producendo c moli di C e d moli di D, la velocità di reazione può essere espressa e calcolata usando la formula generica:meno 1/a volte cambia in concentrazione del reagente A nel tempo, pari a meno 1/b volte la variazione della concentrazione del reagente B nel tempo. Questi valori sono gli stessi di più 1/c volte, la variazione della concentrazione del prodotto C nel tempo, che è uguale a più 1/d volte la variazione della concentrazione del prodotto D nel tempo. Determinare le velocità di reazione è fondamentale per lo studio della cinetica chimica, che aiuta a capire quanto velocemente un farmaco, un catalizzatore, o una reazione sintetica funziona in modo che possa essere meglio controllato e ottimizzato nella sua funzione.

velocità di reazione

La velocità di reazione è la variazione della quantità di un reagente o di un prodotto nell'unità di tempo. Le velocità di reazione vengono quindi determinate misurando la dipendenza dal tempo di alcune proprietà che possono essere correlate alle quantità di reagenti o prodotti. La velocità delle reazioni che consumano o producono sostanze gassose, ad esempio, può essere convenientemente determinata misurando le variazioni di volume o di pressione.

La rappresentazione matematica della variazione della concentrazione di reagenti e prodotti, nel tempo, è l'espressione della velocità della reazione. Le parentesi indicano le concentrazioni molari e il simbolo delta (Δ) indica "cambiamento in".

Ad esempio, la concentrazione del perossido di idrogeno, H_2O_2, in una soluzione acquosa cambia lentamente nel tempo mentre si decompone secondo l'equazione:

 <img alt="Eq1" src="/files/ftp_upload/11373/11373_Equation_1.png" style="height: 50px;" />

La velocità con cui il perossido di idrogeno si decompone può essere espressa in termini di velocità di variazione della sua concentrazione:

 <img alt="Eq2" src="/files/ftp_upload/11373/11373_Equation_2.png" style="height: 70px;" />

Pertanto, [H_2O_2]_t1 rappresenta la concentrazione molare del perossido di idrogeno al tempo _t1; analogamente, [H_2O_2]_t2 rappresenta la concentrazione molare dell'acqua ossigenata in un momento successivo _t2; e Δ[H_2O_2] rappresenta la variazione della concentrazione molare del perossido di idrogeno durante l'intervallo di tempo Δt (cioè t_2 − t_1). Poiché la concentrazione del reagente diminuisce man mano che la reazione procede, Δ[H_2O_2] è una quantità negativa. Le velocità di reazione sono, per convenzione, quantità positive, quindi questa variazione negativa della concentrazione viene moltiplicata per −1.

Velocità di reazione media e velocità di reazione istantanea

Le velocità di reazione variano nel tempo e diminuiscono man mano che la reazione procede. È possibile calcolare una velocità di reazione media su un intervallo di tempo utilizzando le concentrazioni all'inizio e alla fine di questo periodo, durante il quale la velocità di reazione cambia. In qualsiasi momento specifico, la velocità con cui procede una reazione è nota come velocità istantanea. La velocità istantanea di una reazione al “tempo zero”, quando la reazione inizia, è la sua velocità iniziale.

La velocità istantanea di una reazione può essere determinata in due modi. Se le condizioni sperimentali consentono la misurazione delle variazioni di concentrazione su brevi intervalli di tempo, il calcolo delle velocità medie fornisce approssimazioni ragionevolmente buone delle velocità istantanee. In alternativa si può utilizzare una procedura grafica. Ad esempio, nell'esempio della decomposizione del perossido di idrogeno, tracciando la concentrazione del perossido di idrogeno rispetto al tempo, la velocità istantanea di decomposizione dell'H_2O_2 può essere calcolata in qualsiasi momento "t" dalla pendenza di una tangente tracciata sulla curva in quel momento.

 <img alt="Image1" src="/files/ftp_upload/11373/11373_Image_1.png" style="height: 300px;" />

Questo grafico mostra un grafico della concentrazione in funzione del tempo per una soluzione 1.000 M di H_2O_2. La velocità in ogni momento è uguale al negativo della pendenza di una linea tangente alla curva in quel momento. Le tangenti sono mostrate a t = 0 h (“velocità iniziale”) e a t = 12 h (“velocità istantanea” a 12 h).

Velocità relative di reazione

La velocità di una reazione può essere espressa come la variazione della concentrazione di qualsiasi reagente o prodotto. Per ogni data reazione, queste espressioni di velocità sono tutte correlate tra loro secondo la stechiometria della reazione. La velocità della reazione generale aA ⟶ bB può essere espressa in termini di diminuzione della concentrazione di A o aumento della concentrazione di B. Queste due espressioni di velocità sono correlate dalla stechiometria della reazione, dove:

 <img alt="Eq3" src="/files/ftp_upload/11373/11373_Equation_3.png" style="height: 70px;" />

Si noti che è stato incluso un segno negativo come fattore per tenere conto dei segni opposti delle due variazioni di quantità (la quantità di reagente diminuisce mentre la quantità di prodotto aumenta).

Questo testo è adattato da <a href="https://openstax.org/books/chemistry-2e/pages/12-introduction">Openstax, Chemistry 2e, Chapter 12 Introduction</a> and <a href="https://openstax.org/books/chemistry-2e/pages/12-1-chemical-reaction-rates#12163">Openstax, Chemistry 2e, Section 12.1: Chemical Reaction Rates.</a>

Velocità di reazione

La velocità di reazione è la variazione della quantità di un reagente o di un prodotto per unità di tempo. Le velocità di reazione sono quindi determinate misurando il tempo ...

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KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

measuring reaction rates

Measuring reaction rates shows how fast reactants are used up or products are formed over time. The rate of reaction is the change in the amount of a reactant or product per unit time. Because of that, scientists measure how a property changes as the reaction proceeds and connect that change to reactant or product amounts.

For reactions that make or use a gas, the rate can be followed by changes in volume or pressure. A rate expression is the mathematical way to write how concentration changes over time. In this expression, brackets mean molar concentration, and the delta symbol means change in amount.

Hydrogen peroxide provides a clear example. As H2O2 decomposes in water, its concentration slowly decreases with time. The reaction rate can be written as the rate of change of the H2O2 concentration. If the concentration at time t1 is compared with the concentration at a later time t2, the change in concentration over the time interval Δt is found.

Because the reactant concentration goes down, Δ[H2O2] is negative. Reaction rates are reported as positive values, so the negative sign is reversed by multiplying by -1. This keeps the reaction rate positive even when the reactant amount is falling.

Reaction rates are not constant. They usually change as the reaction continues, so an average reaction rate is calculated over a time interval using the starting and ending concentrations. The rate at one exact moment is the instantaneous rate, and the instantaneous rate at time zero is the initial rate.

Instantaneous rate can be estimated in two ways. If concentration changes can be measured over very short time intervals, the average rate gives a good approximation. A graph can also be used. When concentration is plotted against time, the instantaneous rate at a given time equals the negative slope of a tangent line to the curve. In the hydrogen peroxide example, tangents can be drawn at t = 0 h for the initial rate and at t = 12 h for the instantaneous rate at 12 h.

The rate of a reaction can be written using either a reactant or a product. For a general reaction aA → bB, the decrease in A and the increase in B are linked by the reaction stoichiometry. The two rate expressions are related by that stoichiometry, and a negative sign is included to account for the reactant decreasing while the product increases.

Measuring Reaction Rates

reaction rate

Measuring reaction rates shows how fast reactants are used up or products are formed over time. The rate of reaction is the change in the amount of a reactant or product per unit time. Because of that, scientists measure how a property changes as the reaction proceeds and connect that change to reactant or product amounts. For reactions that make or use a gas, the rate can be followed by changes in volume or pressure. A rate expression is the mathematical way to write how concentration changes...

Tracking Reaction Kinetics with Light and Pressure

Reaction kinetics can be tracked with polarimetry, spectrometry, and pressure measurements. These methods let scientists follow a reaction without disturbing the sample. They also help determine how reactant or product concentrations change over time. Polarimetry is used for optically active substances. These substances rotate the plane of polarized light, which is called optical rotation. The effect comes from molecular structure. Normal monochromatic light is unpolarized, but after it passes...

concentration and rate law

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...

determining order of reaction

Finding Rate Law from Initial Rates

Rate laws connect the speed of a chemical reaction to the concentration of its reactants. The rate constant, k, and the reaction orders are not taken from the chemical equation. They are found experimentally by watching how the rate changes when the reactant concentrations change. One common way to do this is the method of initial rates. In this method, several trials are run with different starting concentrations. The measured rates are then compared to determine the reaction orders and,...

the integrated rate law: the dependence of concentration on time

the-integrated-rate-law-the-dependence-of-concentration-on-time

Reading Reaction Progress with Integrated Laws

Integrated rate laws connect reaction concentration to time. They help show how much reactant or product is present after a certain period. They can also help estimate how long a reaction must run to reach a given extent. A common use is finding how long radioactive material must be stored before its radioactivity drops to a safe level. These equations come from the differential rate law after it is integrated with respect to time. For a first-order reaction, where rate = k[A], the integrated...

half-life of a reaction

Reaction Half-Life in Kinetics

Reaction half-life is the time needed for half of a reactant to be consumed. After each half-life, half of the remaining reactant is used up. A hydrogen peroxide decomposition example shows this pattern clearly. The concentration drops from 1.000 M to 0.500 M in the first 6.00 hours, then to 0.250 M in the next 6.00 hours, and then to 0.125 M in the third period. Each later 6.00-hour interval removes half of what is left. For first-order reactions, the half-life does not depend on the reactant...

temperature dependence on reaction rate

Collision Theory and Reaction Speed

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arrhenius plots

Determining Activation Energy from Temperature

The Arrhenius equation links activation energy and the rate constant, k, for chemical reactions. It is written as k = Ae−Ea/RT. In this equation, R is the ideal gas constant with a value of 8.314 J/mol·K, T is the temperature on the kelvin scale, Ea is the activation energy in J/mole, e is the constant 2.7183, and A is the frequency factor. The frequency factor depends on collision frequency and the orientation of reacting molecules. Experimental kinetic data can be used to find the activation...

reaction mechanisms

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...

rate-determining steps

Reaction Bottlenecks in Chemical Kinetics

Reaction bottlenecks help explain rate-determining steps in multistep reaction mechanisms. In a mechanism with several elementary steps, one step is often much slower than the others. That slowest step is called the rate-limiting step, or rate-determining step. A reaction cannot move faster than its slowest step, so this step controls the overall reaction rate. The idea is like traffic on a four-lane freeway with one lane closed. Cars can only move as fast as the bottleneck allows. In the same...

catalysis

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...

enzymes

Enzyme Action and Inhibition

Enzymes are biological catalysts that speed up many reactions in living organisms. They help control cellular metabolism by lowering the activation energy needed for biochemical reactions. When activation energy drops, reaction rates can increase. Enzyme problems can also lead to serious disease. A genetic abnormality that causes G6PD, or glucose-6-phosphate dehydrogenase, deficiency can disrupt the pathway that supplies NADPH to cells. In red blood cells, this can reduce glutathione and...

key terms and concepts

Learn how reaction rates are measured using concentration, volume, pressure, average rate, and instantaneous rate with the hydrogen peroxide example.

Measuring Reaction Rates | JoVE K12

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JoVE Core: Chimica illustra concetti chiave di chimica attraverso video animazioni concise e di immediata comprensione. Inoltre, i video della sezione "scienziati in azione" mostrano procedure metodologiche realizzate da ricercatori provenienti dai migliori laboratori internazionali.

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North Carolina K-12 Science Standard Course of Study

9-12

Matter and its Interactions

PS.Chm.5.1

Pennsylvania State Academic Standards

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Chemical Reactions

3.2.9-12.D

3.2.9-12.E

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HS-PS1-4

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Interactions Of Matter

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New Jersey Student Learning Standards for Science

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SC.912.P.12

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Measuring Reaction Rates

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