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

A medida da rapidez com que uma reação ocorre é chamada de taxa de reação. Para uma reação de etapa única, a taxa é igual à mudança na concentração de cada reagente ou produto ao longo do tempo multiplicada pelo inverso do coeficiente estequiométrico correspondente. Você pode pensar na mudança na concentração ao longo do tempo como a concentração no tempo t menos a concentração inicial dividida por t. As taxas de reação são sempre positivas, então as expressões do reagente têm sinais negativos. Essas taxas podem não ser iguais em reações de várias etapas, mas ainda podemos usar essa relação como uma estimativa para a reação geral.
A lei de taxa, ou equação de taxa, descreve a relação entre a velocidade da reação e as concentrações do reagente. Nesta equação, k é a constante de taxa, A e B são os dois reagentes e m e n são suas respectivas ordens de reação. A ordem da reação descreve a relação entre a concentração de um reagente e a taxa e não está relacionada à estequiometria. É vital lembrar que a ordem da reação não é a mesma que o coeficiente do reagente na equação balanceada.
Quando uma reação envolve dois ou mais reagentes, devemos considerar a ordem de reação para cada reagente. A ordem geral da reação é igual à soma das ordens de reação dos reagentes. Por exemplo, se o reagente A é de primeira ordem e o reagente B é de ordem zero, a ordem geral da reação é um. As ordens de reação mais comuns em reações simples são ordem zero, primeira ordem e segunda ordem. Vamos examiná-los usando uma reação unimolecular como exemplo.
Se a reação for de ordem zero, a concentração do reagente não terá efeito na taxa de reação. Assim, a taxa de reação é igual a k, a constante de taxa. Um gráfico da taxa de reação em relação à concentração é uma linha horizontal.
Nas relações de primeira ordem, a concentração do reagente está linearmente relacionada à taxa de reação. Assim, a taxa é igual à constante de taxa vezes a concentração do reagente. Um gráfico da taxa de reação em relação à concentração será linear, com k como inclinação.
Se a reação for de segunda ordem, há uma relação quadrática entre a concentração e a taxa. Assim, a taxa é igual à constante de taxa vezes a concentração ao quadrado. Um gráfico da taxa em relação à concentração será parabólico, com k como inclinação.
A constante de taxa, k, é um valor dependente da temperatura que relaciona a energia de ativação da reação com a taxa de reação. Você explorará isso no próximo experimento de laboratório.
As taxas de reação são normalmente dadas em moles por litro por segundo. A taxa tem as mesmas unidades, independentemente da ordem de reação. Assim, a constante de taxa tem unidades diferentes, dependendo da ordem de reação. Por exemplo, se A é de primeira ordem e B é de ordem zero, então temos uma instância de moles por litro na equação de taxa. Portanto, a constante de taxa deve estar em segundos inversos.
Então, como determinamos a ordem de reação? A ordem de reação deve ser determinada experimentalmente por meio de uma série de testes. Se você tiver dois reagentes, um método é manter a concentração de um reagente constante, variar o outro e cronometrar quanto tempo leva para fazer uma certa quantidade de produto. O mesmo processo é repetido para o segundo reagente. Você pode então estimar a ordem de cada reagente plotando a taxa versus a concentração variável dos reagentes e vendo se ela se parece com um gráfico de zero, primeira ou segunda ordem. Combinar seus dados com a equação de taxa correspondente também permitirá que você calcule k.
Neste laboratório, você determinará as ordens de reação de dois reagentes variando suas concentrações e cronometrando quanto tempo leva para a reação progredir para tornar a solução opaca.

dependência de concentração

Cinética Química e a Lei da Taxa de Reação
A cinética química refere-se à taxa ou velocidade de uma reação química. A taxa depende do mecanismo, complexidade e número de reagentes na reação. A concentração do reagente também desempenha um papel significativo na taxa de uma reação.
A lei de taxa quantifica essa relação por meio da experimentação. Cada reagente contribui para a velocidade da reação por um fator conhecido como ordem de reação. Esse fator pode variar de zero a dois e depende da relação desse reagente com a taxa da reação.
r = k[A]m[B]n for aA + bB + ... → cC
Nesta equação, r é a taxa de reação, k é a constante de taxa de reação, [A] e [B] são as concentrações dos reagentes A e B, e m e n são as ordens de reação dos reagentes A e B, respectivamente. A taxa de reação, r, é definida como a mudança na concentração do produto com o tempo e tem unidades de moles por litro por segundo.
<h4>Ordem de uma reação</h4>
A ordem da reação descreve a dependência de potência que a concentração de reagentes tem sobre a taxa de reação. A ordem geral da reação é a soma das ordens de reação para cada um dos reagentes. É importante lembrar que a ordem da reação NÃO está relacionada aos fatores estequiométricos da equação química balanceada. Em outras palavras, a estequiometria não pode ser usada para determinar as ordens de reação de uma reação química. As ordens de reação devem ser determinadas experimentalmente
Para determinar a ordem de reação, a concentração do segundo reagente é mantida constante, enquanto a concentração do primeiro reagente é variada. O tempo que leva para a reação ocorrer é medido em segundos; No entanto, esse tempo não corresponde ao tempo que a reação leva para ser concluída. Em vez disso, este é o tempo que leva para a reação começar. Os tempos de reação para as diferentes concentrações de reagentes são então comparados para determinar a ordem. A mesma série de reações é realizada para determinar a ordem do segundo reagente, onde a concentração do primeiro reagente é mantida constante e a concentração do segundo reagente é variada.
Ao comparar os tempos de reação, a ordem de reação pode ser determinada. Por exemplo, se o tempo de reação permanece constante apesar das mudanças na concentração do reagente, então o reagente é de ordem zero. Isso significa que a taxa da reação é igual ao coeficiente de taxa de reação, k, que deve ter unidades de M / s. Se o tempo de reação muda linearmente com as mudanças na concentração (ou seja, dobrar a concentração afeta o tempo de reação por um fator de 2), então o reagente é de primeira ordem. Isso significa que a taxa da reação é igual ao produto do coeficiente de taxa de reação e à concentração do reagente. Nesse caso, k tem unidades de 1/s. Finalmente, se o tempo de reação é afetado por um fator de 4 quando a concentração de um reagente é dobrada, o reagente é de segunda ordem. Isso significa que a taxa da reação é igual à constante de taxa vezes o quadrado da concentração do reagente, formando uma relação quadrática. Neste caso, as unidades de k devem ser 1/M⋅s.
A ordem geral da reação é a soma das ordens de reação individuais. Por exemplo, se a reação for de primeira ordem em relação a A, então m = 1. E se a reação for de ordem zero em relação a B, então n = 0. A ordem geral da reação é de primeira ordem porque m + n = 1.
<h4>Constante de taxa de reação</h4>
A constante de taxa, k, é específica para a reação e a temperatura em que a reação é realizada. A constante de taxa é determinada a partir da série de experimentos. Usando a taxa medida e as concentrações iniciais do reagente, a equação da taxa é usada para resolver a constante de taxa. A constante de taxa depende da temperatura e é definida pela equação de Arrhenius. Esta equação descreve a relação entre k e a energia de ativação, Ea, a temperatura, T, e a constante do gás ideal, R. A constante A é uma constante de proporcionalidade e não deve ser confundida com o reagente A.
<img src="/CDNSource/lm/labs/50/50_Concepts_2.jpg">
<h4>Referências</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). Química e Reatividade Química. Belmont, CA: Brooks / Cole, Cengage Learning.</li>
	<li style="font-size:17px">Silderberg, M.S. (2009). Química: A Natureza Molecular da Matéria e da Mudança. Boston, MA: Colina McGraw.</li>
</ol>

Dependência de Concentração

Cinética Química e a Lei da Taxa de Reação
A cinética química refere-se à taxa ou velocidade de uma reação química. A taxa depende do mecanismo, ...

Education

JoVE Lab Manual

Química

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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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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