Name: reaction order from concentration
Uploaded: 2020-09-24T15:06:50.000Z
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K12 Standards

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Chemistry and Physics

IPC.7

IPC.7.F

Concentration and Rate Law

A taxa de uma reação depende frequentemente das concentrações de reagentes. Para qualquer reação, a relação entre a taxa de reação e as concentrações do reagente podem ser expressas matematicamente utilizando uma lei de taxa ou uma equação. Numa lei de tarifas, k é a constante de proporcionalidade, ou a taxa constante, e n é a ordem de reação com respeito a um único reagente, cujo valor é frequentemente um número inteiro.Em lei de taxas para reações multi-reagentes, a ordem de reação global é a soma de todas as ordens reagentes. Para cada reagente, a taxa de reação, taxa constante, concentração, e a ordem de reação estão todas determinadas a título experimental. A lei das taxas expressa a relação entre todos estes parâmetros.Ordens individuais de reagente comumente tomando os valores 0, 1, ou 2, e com base na ordem global da reação reações químicas podem ser categorizadas como ordem zero, primeira ordem, ou reações de segunda ordem. Um único reagente, ou unimolecular, reação química para a qual a taxa de reação permanece constante durante toda a sua duração é uma reação de ordem zero. A ordem do reagente numa reação de ordem zero é zero, e de acordo com a lei de taxas, a concentração do reagente é elevada à potência zero.Uma vez que 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 da taxa e, portanto, independente da concentração do reagente. Portanto, numa reação de ordem zero, mesmo quando o reagente na concentração decresce, a taxa de reação não diminui. Uma reação química unimolecular em que a taxa de reação é diretamente proporcional a a concentração do reagente é uma reação de primeira ordem.A ordem do reagente para uma reação de primeira ordem é um, e, de acordo com a lei de tarifas, a concentração do reagente é elevada à primeira potência. Uma vez que o valor de qualquer número elevado a primeira potência permanece a mesma, a taxa de uma reação de primeira ordem depende diretamente da concentração do reagente. Como o reagente na concentração decresce, a taxa de reação decresce proporcionalmente de uma forma linear.Uma reação química unimolecular em que a taxa de reação está dependente do quadrado da concentração do reagente é uma reação de segunda ordem. A ordem dos reagentes é dois, e a concentração do reagente é elevado para a segunda potência. Consequentemente, a taxa de reação numa reação de segunda ordem depende diretamente da taxa da concentração do reagente.Como o reagente na concentração decresce, a taxa de reação decresce exponencialmente de uma forma quadrática.

concentração e lei da velocidade

A velocidade de uma reação é afetada pelas concentrações de reagentes. As leis de velocidade (leis de velocidade diferencial) ou equações de velocidade são expressões matemáticas que descrevem a relação entre a velocidade de uma reação química e a concentração dos seus reagentes.
Por exemplo, em produtos de uma reação genérica aA + bB ⟶, onde a e b são coeficientes estequiométricos, a lei da velocidade pode ser escrita como:
 <img alt="Eq1" src="/files/ftp_upload/11375/11375_Equation_1.png" style="height: 50px;" />
[A] e [B] representam as concentrações molares dos reagentes, e k é a constante de velocidade, que é específica para uma reação particular a uma temperatura específica.
Os expoentes m e n são as ordens de reação e são tipicamente inteiros positivos, embora possam ser frações, negativos, ou zero.
A constante de velocidade k e as ordens de reação m e n são determinadas experimentalmente observando como a velocidade de uma reação muda à medida que as concentrações dos reagentes são alteradas. 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 em concentrações de reagentes. No que se refere à lei de velocidade genérica (velocidade = k[A]m[B]n), a reacção é de ordem m no que se refere A e de ordem n no que se refere 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 de reação geral é simplesmente a soma das ordens para cada reagente. Para este exemplo de lei de velocidade, a reação é de terceira ordem no geral (1 + 2 = 3).
Uma abordagem experimental comum para a determinação das leis de velocidade é o método das velocidades iniciais. Este método envolve a medição das velocidades de reação para ensaios experimentais múltiplos efetuados com diferentes concentrações iniciais de reagentes. A comparação das velocidades medidas para estes ensaios permite determinar as ordens de reação e, posteriormente, a constante da velocidade, que em conjunto são utilizadas para formular uma lei de velocidade.
As leis de velocidade podem exibir 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 velocidade de reação. É importante notar que as leis de velocidade são determinadas apenas por experiência e não são previstas de forma confiável pela estequiometria da reação.
A ordem da reação determina a relação entre a velocidade de reação e a concentração de reagentes ou produtos.
• 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 tempo. 
• Em uma reação de primeira ordem, a velocidade de reação é diretamente e linearmente proporcional à mudança na concentração de reagentes. À medida que a concentração de reagentes diminui, a velocidade de reação também diminui proporcionalmente. 
• Em reações de segunda ordem ou superiores, a velocidade de reação é proporcional ao valor exponencial dos reagentes. Portanto, à medida que a reação progride e a concentração dos reagentes diminui, a velocidade de reação diminui exponencialmente.
Este texto é adaptado de <a href="https://openstax.org/books/chemistry-2e/pages/12-3-rate-laws#57033">Openstax, Chemistry 2e, Section 12.3: Rate Laws</a>.

Concentração e Lei da Velocidade

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 ...

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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 are the reaction orders. They are usually positive integers, but they can also be fractions, negative numbers, or zero. The values of k, m, and n are found experimentally by measuring how the rate changes when reactant concentrations change.

The rate constant does not depend on the reactant concentrations, but it can change with temperature. The reaction orders show the mathematical link between concentration and rate. In the expression rate = k [A]m [B]n, the reaction is m order with respect to A and n order with respect to B. For example, 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 orders for each reactant. In the example above, the reaction is third order overall because 1 + 2 = 3. A common way to find a rate law is the method of initial rates. In this method, reaction rates are measured in several trials that start with different initial reactant concentrations. Comparing those rates makes it possible to find the reaction orders and then the rate constant.

Rate laws can also include fractional orders. In some cases, a reactant may have a negative order. That means increasing its concentration can lower the reaction rate. Rate laws are determined by experiment only, and they cannot be reliably predicted from reaction stoichiometry. The reaction order also describes how the rate changes as reactant or product concentration changes.

A zero-order reaction has a rate that does not change with reactant concentration. The rate stays constant throughout the reaction. In a first-order reaction, the rate is directly proportional to the change in reactant concentration. As the reactant concentration drops, the rate drops by the same proportion. In second-order or higher-order reactions, the rate depends on the reactants raised to an exponent. As the reaction continues and reactant concentration decreases, the rate falls exponentially.

Reaction Order from Concentration

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concentration and rate law

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...

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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,...

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key terms and concepts

Learn how concentration affects reaction rate, how rate laws are written, and how initial rates reveal reaction order, the rate constant, and overall order.

Reaction Order from Concentration | JoVE K12

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Reaction Order from Concentration

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