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

Uma reação química envolve a transformação química de reagentes em produtos. À medida que a reação avança, a concentração de reagentes diminui enquanto a concentração de produtos aumenta. Esta variação nas concentrações de reagentes e produtos pode ser traçada num gráfico em função do tempo.A velocidade a que uma reação progride é chamada a taxa de reação. Mede a taxa do desaparecimento dos reagentes ou a taxa de aparecimento dos produtos e é expressa em unidades de molaridade por segundo. A taxa de reação média pode ser calculada da mudança de concentrações de reagentes ou produtos durante um período de tempo específico.O valor da concentração molar é assinalado entre parênteses, t representa o período de tempo, e o símbolo do delta representa mudança em segundos. Uma vez que os reagentes estão esgotados durante uma reação química, o valor da concentração de mudança de reagentes é sempre negativo. Portanto, uma taxa calculada da reação em termos de reagentes é complementada por um sinal negativo, para fazer o valor global positivo.A taxa de reação não é uniforme ao longo de uma reação. A taxa no início da reação, em tempo-zero'chama-se a taxa de reação inicial. Como as concentrações de reagentes reduzem, a taxa de reação diminui, ou a reação abranda.A taxa de reação num dado ponto temporal, ou a taxa de reação instantanea, é medida através de cálculos do declive de um desenho tangente à curva de reação, para quer reagentes, quer produtos, no momento de interesse. O valor do declive ou a taxa instantânea num ponto de tempo específico é igual para todos os reagentes e produtos. A taxa de reação de uma reação química também reflete os reais coeficientes estequiométricos de reagentes e produtos.Assim, para qualquer equilíbrio da reação, em que um mol de A reage com mols de B, produzindo mols c de C e d moles de D, a taxa de reação pode ser expressa e calculada utilizando a fórmula genérica:menos 1/a vezes a mudança em concentração de reagente A com o tempo, igual a menos 1/b de mudança em concentração de reagente B com o tempo. Estes valores são os mesmos que mais 1/c vezes, mudança na concentração do produto C com o tempo, que é o mesmo que mais 1/d vezes mudança na concentração do produto D com o tempo. Determinação das taxas de reação é fundamental para o estudo cinética química, o que ajuda a compreender como a rapidez de um fármaco, catalisador, ou uma reação sintética funciona de modo a que pode ser melhor controlado e otimizado na sua função.

velocidade de reação

A velocidade de reação é a alteração na quantidade de um reagente ou produto por unidade de tempo. As velocidades de reação são, portanto, determinadas medindo a dependência temporal de alguma propriedade que possa estar relacionada com quantidades de reagentes ou de produtos. As velocidades de reações que consomem ou produzem substâncias gasosas, por exemplo, são convenientemente determinadas através da medição de alterações no volume ou na pressão.
A representação matemática da mudança na concentração de reagentes e produtos, ao longo do tempo, é a expressão da velocidade de reação. Os parênteses indicam concentrações molares e o símbolo delta (Δ) indica “alteração”.
Por exemplo, a concentração de peróxido de hidrogénio, H2O2, em uma solução aquosa, muda lentamente ao longo do tempo, à medida que se decompõe de acordo com a equação:
 <img alt="Eq1" src="/files/ftp_upload/11373/11373_Equation_1.png" style="height: 50px;" />
A velocidade de decomposição do peróxido de hidrogénio pode ser expressa em termos da velocidade de variação da sua concentração:
 <img alt="Eq2" src="/files/ftp_upload/11373/11373_Equation_2.png" style="height: 70px;" />
Assim, [H2O2]t1 representa a concentração molar de peróxido de hidrogénio no tempo t1; da mesma forma, [H2O2]t2 representa a concentração molar de peróxido de hidrogénio em um tempo posterior t2; e Δ[H2O2] representa a alteração na concentração molar de peróxido de hidrogénio durante o intervalo de tempo Δt (ou seja, t2 − t1). Uma vez que a concentração de reagentes diminui à medida que a reação prossegue, Δ[H2O2 é uma quantidade negativa. As velocidades de reação são, por convenção, quantidades positivas, pelo que esta alteração negativa da concentração é multiplicada por −1.
Velocidade de reação média e velocidade de reação instantânea
As velocidades de reação variam com o tempo e diminuem à medida que a reação avança. Pode ser calculada uma velocidade de reação média ao longo de um intervalo de tempo, utilizando as concentrações no início e no fim deste período, durante as quais a velocidade da reação está a mudar. A qualquer momento específico, a velocidade à qual uma reação está a ser realizada é conhecida como a sua velocidade instantânea. A velocidade instantânea de uma reação no “tempo zero,”, quando a reação começa, é sua velocidade inicial. 
A velocidade instantânea de uma reação pode ser determinada por uma de duas formas. Se as condições experimentais permitirem a medição de alterações de concentração em intervalos de tempo curtos, o cálculo das velocidades médias proporciona aproximações razoavelmente boas das velocidades instantâneas. Em alternativa, pode ser utilizado um procedimento gráfico. Por exemplo, no exemplo da decomposição do peróxido de hidrogénio, ao traçar a concentração de peróxido de hidrogénio em relação ao tempo, a velocidade instantânea de decomposição do H2O2 pode ser calculada a qualquer tempo ‘t’ a partir do declive de uma tangente traçada à curva nesse tempo.
 <img alt="Image1" src="/files/ftp_upload/11373/11373_Image_1.png" style="height: 300px;" />
Este gráfico mostra uma representação de concentração versus tempo para uma solução de H2O2 de 1,000 M. A valocidade em qualquer momento é igual ao negativo do declive de uma linha tangente à curva nesse momento. As tangentes são mostradas em t = 0 h (“velocidade inicial”) e em t = 12 h (“velocidade instantânea” às 12 h).
Velocidades de Reação Relativas
A velocidade de reação pode ser expressa como a alteração da concentração de qualquer reagente ou produto. Para qualquer reação, essas expressões de velocidade estão todas relacionadas entre si de acordo com a estequiometria da reação. A velocidade da reação geral aA ⟶ bB pode ser expressa em termos da diminuição da concentração de A ou do aumento da concentração de B. Estas duas expressões de velocidade estão relacionadas com a estequiometria da reação, onde:
 <img alt="Eq3" src="/files/ftp_upload/11373/11373_Equation_3.png" style="height: 70px;" />
Note que foi incluído um sinal negativo como um factor para ter em conta os sinais opostos das duas variações de quantidade (a quantidade de reagentes está a diminuir enquanto que a quantidade de produtos está a aumentar).
Este texto é adaptado de <a href="https://openstax.org/books/chemistry-2e/pages/12-introduction">Openstax, Chemistry 2e, Chapter 12 Introduction</a> e <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>

Velocidade de Reação

A taxa de reação é a mudança na quantidade de um reagente ou produto por unidade de tempo. As taxas de reação são, portanto, determinadas medindo o tempo ...

Education

JoVE Core

Chemistry

Química

Chemical Kinetics

Cinética Química

KEY TERMS AND CONCEPTS

TERMOS-CHAVE E CONCEITOS

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

Collision theory explains why reaction speed depends on how particles collide. Atoms, molecules, or ions must come close together before they can react. The theory says the reaction rate depends on the collision rate, the orientation of the colliding species, and whether the collision has enough energy for bonds to change. A gas-phase reaction between carbon monoxide and oxygen shows why orientation matters. The molecules can collide in more than one way. If the oxygen side of carbon monoxide...

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

chemistry

JoVE Core - Química dá vida a conceitos gerais de química usando vídeos de animação fáceis de entender. Além disso, nos vídeos Cientistas em ação, demonstramos experimentos de pesquisa clássicos e originais realizados atualmente por cientistas em laboratórios em todo o mundo.

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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

Physical Science

Chemical Reactions

3.2.9-12.D

3.2.9-12.E

Louisiana Student Standards for Science

Matter And Its Interactions

HS-PS1-4

HS-PS1-6

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Ohio’s Learning Standards and Model Curriculum

Interactions Of Matter

Chemical reactions

Virginia  Standards of Learning

HIgh School

CH.7

CH.7.e

Minnesota Academic Standards in Science

3.2.1 9C.3.1.1.2

Arizona Science Standards

HS.P1

HS.P1U1.3

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New Jersey Student Learning Standards for Science

Matter and Its Interactions

HS-PS1-5

Alabama Course of Study Science

Matter and Its Interactions: Energy

9-12.C.3.3.d

Next Generation Science Standards

Florida Department of Education

SC.912.P.12

SC.912.P.12.12

new-york-state-science-learning-standards

New York State Science Learning Standards

HS. Chemical Reactions

HS-PS1-5.

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IPC.7.B

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IPC.7.D

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IPC.7.E

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Measuring Reaction Rates

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