Name: catalyst action in reaction rates
Uploaded: 2020-09-24T15:06:50.000Z
Duration: 2 min 50 s
Channel: JoVE

K12 Standards

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North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.5.1

Catalysis

O estudo do mecanismo de reação e da cinética ajuda os investigadores a otimizar a rapidez de uma reação. Um aumento na temperatura ou na concentração irá acelerar a taxa, mas também pode levar à decomposição ou a produtos secundários quando aumentadas em demasia. Em alternativa, um produto químico que acelera a taxa de reação sem ser permanentemente alterado ou consumido pode ser acrescentado à mistura de reação.Estas substâncias são chamadas de catalisadores e afetam a taxa constante ao alterar a energia de ativação. Recorde-se que quanto mais elevada a energia de ativação, mais lenta a taxa de reação. Daí, muitas reações químicas, como a decomposição do peróxido hidrogénio aquoso, progridem lentamente.A adição de um catalisador reduz a energia de ativação fornecendo um mecanismo de reação alternativo sem afetar os estados energéticos dos reagentes e produtos. Existem dois tipos de catalisadores-homogéneos e heterogéneos. Um catalisador homogéneo existe na mesma fase que os reagentes.Por exemplo, o brometo de sódio é adicionado ao peróxido de hidrogénio aquoso para acelerar a sua decomposição em oxigénio gasoso e água. Primeiro, os íons de brometo reagem com o peróxido de hidrogénio num meio ácido para formar uma solução laranja de bromo aquoso e água. A seguir, o bromo aquoso e o peróxido de hidrogénio reagem e libertam íons de brometo indicados por uma solução sem cor.Característica de um catalisador, os íons de brometo aceleram a reação mas continuam por consumir e não aparecem na equação líquida equilibrada. Existe um catalisador heterogéneo num estado físico diferente em comparação com os reagentes. Uma catálise heterogénea é caracterizada por quatro etapas subjacentes adsorção, difusão, reação, e dessorção.Por exemplo, a reação de hidrogenação do gás eteno insaturado com o etano é acelerada por um paládio disperso finamente sobre carvão vegetal. Aqui, o catalisador existe no seu estado sólido, enquanto os reagentes estão nas suas formas gasosas. Durante esta catálise heterogénea, as moléculas de eteno e hidrogénio adsorvem na superfície do catalisador.A ligação de hidrogénio Hidrogénio quebra-se permitindo aos átomos de hidrogénio difundirem-se sobre a superfície do catalisador. Ao difundirem-se os átomos de hidrogénio encontram as moléculas de eteno absorvido, elas reagem para formar etano saturado. Posteriormente, os produtos desforçam-se e mudam-se a partir da superfície metálica, deixando o catalisador intacto para trás.

catálise

A presença de um catalisador afeta a velocidade de uma reação química. Um catalisador é uma substância que pode aumentar a velocidade de reação sem ser consumida durante o processo. A compreensão básica do papel dos catalisadores durante as reações químicas pode ser entendida a partir do conceito de mecanismos de reação e diagramas de energia.
<img alt="Image1" src="/files/ftp_upload/11383/11383_Image_1.png" style="text-align: center; height: 350px;" />
A imagem ilustrada representa os diagramas de reação para um processo químico endotérmico progredindo na ausência (curva vermelha) e presença (curva azul) de um catalisador.
Ambas as curvas representam a mesma reação geral – começam e terminam com as mesmas energias. (Neste caso, os produtos têm mais energia do que reagentes. Portanto, a reação é endotérmica). No entanto, os seus mecanismos de reação são diferentes. A reação não catalisada prossegue através de um mecanismo de um passo (com apenas um estado de transição observado). Em contraste, a reação catalisada segue um mecanismo de dois passos (dois estados de transição são observados) com energia de ativação notavelmente menor. Esta diferença nas vias de reação ilustra o papel do catalisador no fornecimento de um mecanismo de reação alternativo com menor energia de ativação, acelerando assim as reações.
O mecanismo de reação catalisada não precisa envolver um maior número de etapas elementares do que o mecanismo não catalisado. No entanto, deve fornecer um caminho de reação alternativo cujo passo determinante de velocidade seja mais rápido (com uma menor Ea ou energia de ativação).
Uma reação catalítica pode ser categorizada como homogénea ou heterogénea, com base nos estados físicos em que existem os catalisadores e reagentes durante o processo catalítico.
Catálise Homogénea
Em uma catálise homogénea, o catalisador está presente na mesma fase que os reagentes – sólido, líquido, ou gás. Durante o processo, o catalisador interage com o reagente para formar uma substância intermédia que, em seguida, decompõe ou reage com outro reagente em um ou mais passos para regenerar o catalisador original e formar o produto final.
Um exemplo de catálise homogénea é o processo químico que envolve a decomposição do ozono existente na atmosfera superior da Terra. O ozono é uma molécula relativamente instável que se decompõe para produzir oxigénio diatómico. Esta reação de decomposição é consistente com o seguinte mecanismo de dois passos:
<img alt="Eq1" src="/files/ftp_upload/11383/11383_Equation_1.png" style="text-align: center; height: 75px;" />
Muitas substâncias podem catalisar a decomposição do ozono. Por exemplo, acredita-se que a decomposição catalisada do ozono pelo óxido nítrico ocorra através do seguinte mecanismo de três passos:
<img alt="Eq2" src="/files/ftp_upload/11383/11383_Equation_2.png" style="text-align: center; height: 115px;" />
No entanto, a reação global é a mesma tanto para o mecanismo não catalisado de dois passos como para o mecanismo de três passos catalisado por NO:
<img alt="Eq3" src="/files/ftp_upload/11383/11383_Equation_3.png" style="text-align: center; height: 50px;" />
Na reação catalisada, observe que NO é um reagente na primeira etapa do mecanismo e um produto na última etapa. Esta é outra característica particular de um catalisador: Embora participe da reação química, não é consumido pela reação. Além disso, nesta catálise homogénea, tanto o reagente como o catalisador existem em fase gasosa.
Catálise Heterogénea
Em uma catálise heterogénea, o catalisador está presente em uma fase diferente (geralmente sólida) da dos reagentes. Tais catalisadores funcionam geralmente fornecendo uma superfície ativa em cima da qual uma reação pode ocorrer. Reações de fase gasosa e líquida catalisadas por catalisadores heterogéneos ocorrem na superfície do catalisador, em vez de dentro da fase de gás ou líquido.
Uma catálise heterogénea normalmente envolve os seguintes processos:
<ol>
<li>Adsorção do(s) reagente(s) na superfície do catalisador</li>
<li>Ativação do(s) reagente(s) adsorvido(s)</li>
<li>A reação do(s) reagente(s) adsorvido(s)</li>
<li>Dessorção do(s) produto(s) da superfície do catalisador</li>
</ol>
<img alt="Image2" src="/files/ftp_upload/11383/11383_Image_2.png" style="text-align: center; height: 250px;" />
A imagem ilustrada representa o mecanismo de reação envolvendo a catálise heterogénea de eteno e hidrogénio gasosos em uma superfície sólida de níquel, formando etano gasoso (C2H4 + H2 ⟶ C2H6):
(a) O hidrogénio é adsorvido na superfície de níquel. Durante o processo, as ligações hidrogénio-hidrogénio são quebradas para formar ligações níquel-hidrogénio. 
(b) O eteno é também adsorvido na superfície do níquel quebrando a ligação pi carbono-carbono e formando ligações níquel–carbono. 
(c) Os átomos de hidrogénio difundem-se pela superfície e formam novas ligações carbono-hidrogénio quando colidem para formar etano (C2H6). 
(d) As moléculas de etano sofrem dessorção da superfície do níquel.
A catálise heterogénea é usada para fabricar industrialmente produtos químicos como amoníaco, ácido nítrico, ácido sulfúrico, e metanol. Catalisadores heterogéneos também são usados nos conversores catalíticos encontrados na maioria dos automóveis movidos a gasolina.
Este texto é adaptado de <a href="https://openstax.org/books/chemistry-2e/pages/12-7-catalysis">Openstax, Chemistry 2e, Section 12.7: Catalysis</a>.

Catálise

A presença de um catalisador afeta a taxa de uma reação química. Um catalisador é uma substância que pode aumentar a taxa de reação sem ser consumida ...

Education

JoVE Core

Química

Chemical Kinetics

Cinética Química

KEY TERMS AND CONCEPTS

TERMOS-CHAVE E CONCEITOS

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 activation energy, or Ea. The lower rate-determining barrier is what makes the reaction faster.

Catalytic reactions are often grouped as homogeneous or heterogeneous. In homogeneous catalysis, the catalyst and reactants are in the same phase, which may be solid, liquid, or gas. The catalyst reacts with the reactant to form an intermediate, and that intermediate then breaks down or reacts again to form the product and regenerate the catalyst.

A good example of homogeneous catalysis is ozone decomposition in the upper atmosphere. Ozone breaks down to form diatomic oxygen, and nitric oxide can catalyze that process through a three-step mechanism. In this reaction, NO appears as a reactant in the first step and a product in the last step, which shows that it participates in the reaction but is not used up. Here, both the reactant and the catalyst are gases.

In heterogeneous catalysis, the catalyst is in a different phase from the reactants, usually a solid. These reactions happen on the catalyst surface. They typically include adsorption of reactants, activation on the surface, reaction of the adsorbed molecules, and desorption of the products.

The nickel-catalyzed reaction of ethene and hydrogen to form ethane shows this surface process. Hydrogen first adsorbs on nickel, and the H-H bond breaks to form nickel-hydrogen bonds. Ethene also adsorbs as its carbon-carbon pi bond breaks and nickel-carbon bonds form. Hydrogen atoms then move across the surface and make new carbon-hydrogen bonds, and the ethane product desorbs from the nickel surface.

Heterogeneous catalysts are important in industry. They help make ammonia, nitric acid, sulfuric acid, and methanol. They are also used in catalytic converters in most gasoline-powered cars.

Catalyst Action in Reaction Rates

reaction rate

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...

measuring reaction rates

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

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

Explore how catalysts speed up reactions by lowering activation energy, with clear examples of homogeneous and heterogeneous catalysis for high school chemistry.

Catalyst Action in 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.

Pennsylvania State Academic Standards

Physical Science

Chemical Reactions

3.2.9-12.D

3.2.9-12.E

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Ohio’s Learning Standards and Model Curriculum

Interactions Of Matter

Chemical reactions

Virginia  Standards of Learning

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CH.7

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HS.P1

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New York State Science Learning Standards

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Catalysts in Chemical Reactions

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Radioactive Decay and Radiometric Dating

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Use models to compare nuclear reactions including alpha decay, beta decay and gamma decay; nuclear fusion and nuclear fission.

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The Pauli Exclusion Principle

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The Aufbau Principle and Hund's Rule

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Use models to explain how electrons are distributed in atoms.

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Atomic Mass

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Molar Mass

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Use the Periodic Table as a model to predict the relative properties of elements based on the pattern of valence electrons in the outermost energy levels of atoms.

PS.Chm.2.1

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Atomic Radii and Effective Nuclear Charge

The elements in groups of the periodic table exhibit similar chemical behavior. This similarity occurs because the members of a group have the same number ...

Ionic Radii

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Ionization Energy

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Electron Affinity

The electron affinity (EA) is the energy change for adding an electron to a gaseous atom to form an anion (negative ion).



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Construct an explanation to infer the atomic size, reactivity, electronegativity, and ionization energy of an element based on its position in the Periodic Table.

PS.Chm.2.2

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Resonance

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Formal Charges

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Exceptions to the Octet Rule

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	...

Bond Energies and Bond Lengths

Stable molecules exist because covalent bonds hold the atoms together. The strength of a covalent bond is measured by the energy required to break it, ...

Analyze and interpret data to explain the mechanisms and properties of the two main types of intramolecular (ionic and covalent) bonds.

PS.Chm.3.1

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Molecular Comparison of Gases, Liquids, and Solids

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Intermolecular vs Intramolecular Forces

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Construct an explanation to summarize the influences intermolecular forces have on the properties of chemical compounds.

PS.Chm.3.2

Analyze the structure of atoms and isotopes.

Catalyst Action in Reaction Rates

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