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

Para qualquer reação, a relação entre a taxa de reação e as concentrações do reagente pode ser expressa matematicamente usando uma lei de taxa ou a equação de taxa. 
Em uma lei de taxa, 'k' é a constante de proporcionalidade, ou a constante de taxa, e 'n' é a ordem de reação em relação a um único reagente, cujo valor geralmente é um número inteiro. Nas leis de taxa para reações multirreagentes, a ordem geral da reação é a soma de todas as ordens de reagentes.
Para cada reagente, a taxa de reação, a constante de taxa, a concentração e a ordem da reação são determinadas experimentalmente.
As ordens individuais de reagentes geralmente assumem os valores 0, 1 ou 2 e, com base na ordem geral da reação, as reações químicas podem ser categorizadas como reações de ordem zero, primeira ordem ou segunda ordem.
Uma reação química de reagente único, ou unimolecular, para a qual a taxa de reação permanece constante ao longo de sua duração é uma reação de ordem zero. A ordem do reagente em uma reação de ordem zero é zero e a concentração do reagente é elevada à potência zero.
Como o valor de qualquer número elevado à potência zero é um, a taxa de reação de uma reação de ordem zero é igual à constante de taxa e, portanto, independente da concentração do reagente.
Uma reação química unimolecular em que a taxa de reação é diretamente proporcional à concentração do reagente é uma reação de primeira ordem. A ordem do reagente para uma reação de primeira ordem é uma e, de acordo com a lei de taxa, a concentração do reagente é elevada à primeira potência.
Como o valor de qualquer número elevado à potência de um permanece o mesmo, a taxa de reação de uma reação de primeira ordem depende diretamente da concentração do reagente. À medida que a concentração do reagente diminui, a taxa de reação diminui proporcionalmente de maneira linear.
Uma reação química unimolecular em que a taxa de reação depende do quadrado da concentração do reagente é uma reação de segunda ordem. À medida que a concentração do reagente diminui, a taxa de reação diminui exponencialmente de maneira quadrática.

lei da velocidade e ordem de reação

A taxa de uma reação é afetada pelas concentrações dos reagentes. Leis de velocidade (leis de velocidade diferenciais) ou equações de velocidade são expressões matemáticas que descrevem a relação entre a taxa de uma reação química e a concentração de seus reagentes.

Por exemplo, em uma reação genérica aA + bB ⟶ produtos, onde a e b são coeficientes estequiométricos, a lei de velocidade pode ser escrita como:

velocidade = k[A]^ᵐ[B]^ⁿ

 [A] e [B] representam as concentrações molares dos reagentes, e k é a constante de velocidade, que é específica para uma dada reação a uma temperatura determinada.

Os expoentes m e n são as ordens de reação e geralmente são números inteiros positivos, embora possam ser frações, valores negativos ou zero.

A constante de velocidade k e as ordens de reação m e n são determinadas experimentalmente, observando como a taxa de uma reação muda à medida que as concentrações dos reagentes mudam. A constante de velocidade k é independente das concentrações dos reagentes, mas varia com a temperatura.

As ordens de reação em uma lei de velocidade descrevem a dependência matemática da velocidade nas concentrações dos reagentes. Referindo-se à lei de velocidade genérica (velocidade = k[A]m[B]n), a reação é de m-ésima ordem em relação a A e de n-ésima ordem em relação a B. Por exemplo, se m = 1 e n = 2, a reação é de primeira ordem em A e de segunda ordem em B. A ordem geral da reação é simplesmente a soma das ordens de cada reagente. Para o exemplo da lei de velocidade, a reação é de terceira ordem (1 + 2 = 3).

Uma abordagem experimental comum para a determinação das leis de velocidade é o método das velocidades iniciais. Este método envolve medir as velocidades de reação para vários ensaios experimentais realizados utilizando diferentes concentrações iniciais de reagentes. A comparação das velocidades medidas para esses ensaios permite determinar as ordens de reação e, subsequentemente, a constante de velocidade, que juntas são usadas para formular uma lei de velocidade.

As leis de velocidade podem apresentar ordens fracionárias para alguns reagentes, e ordens de reação negativas são às vezes observadas quando um aumento na concentração de um reagente causa uma diminuição na taxa de reação. É importante notar que as leis de velocidade são determinadas apenas por experimentos e não são previsíveis de forma confiável pela estequiometria da reação.

A ordem da reação determina a relação entre a taxa de reação e a concentração dos reagentes ou produtos.

<ul style="list-style-type: disc; display: flex; flex-direction: column; gap: 10px; padding: 0 0 0 40px;">
<li>Em uma reação de ordem zero, a concentração dos reagentes não tem nenhum efeito sobre a velocidade da reação, que permanece constante durante todo o processo.</li>

<li>Em uma reação de primeira ordem, a velocidade de reação é direta e linearmente proporcional à mudança na concentração do reagente. À medida que a concentração do reagente diminui, a velocidade de reação também diminui proporcionalmente.</li>

<li>Em reações de segunda ordem ou ordem superior, a velocidade de reação é proporcional ao valor exponencial dos reagentes. Portanto, à medida que a reação avança e a concentração dos reagentes diminui, a velocidade de reação diminui exponencialmente.</li>
</ul>

Este texto foi adaptado do<a href="https://openstax.org/books/chemistry-2e/pages/12-3-rate-laws#57033">Openstax, Chemistry 2e, Section 12.3: Rate Laws</a>.

Lei da velocidade e ordem de reação

A taxa de uma reação é afetada pelas concentrações de reagentes. Leis de taxa (leis de taxa diferencial) ou equações de taxa são expressões matemáticas ...

Education

JoVE Core

Organic Chemistry

Química Orgânica

Unit 1

Unidade 1

Thermodynamics and Chemical Kinetics

Termodinâmica e Cinética Química

KEY TERMS AND CONCEPTS

TERMOS-CHAVE E CONCEITOS

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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rate law and reaction order

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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key terms and concepts

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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