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

K12 Standards

north-carolina-k-12-science-standard-course-of-study

North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.5.1

Catalysis

对反应机理和动力学的研究 有助于研究人员优化反应速度。温度或浓度的升高 会加快反应速度，但如果 增加过多，也会导致 分解或副产物。或者，可以向反应混合物中加入一种 能加速反应速度而不被永久改变或消耗 的化学物质。这些物质被称为催化剂，通过改变活化能来影响速率常数。回想一下，活化能越高，反应速度越慢。因此，许多化学反应，如过氧化氢 水溶液的分解，进展缓慢。催化剂的加入通过提供一种 替代反应机制，降低了活化能，而并不影响反应物和产物的 能量状态。催化剂有两种类型：均相催化剂 和非均相催化剂。均相催化剂与反应物 同相存在。例如，将溴化钠添加到过氧化氢 水溶液中，以加速它分解为 气态氧和水的反应。首先，溴离子在酸性介质中与 过氧化氢反应，形成溴水和水的 橙色溶液。接下来，溴水和过氧化氢发生 反应，释放出以无色溶液表示的 溴离子。作为催化剂的特征，溴离子 加速了反应，但未被消耗，也未出现在净平衡方程中。与反应物相比，多相催化剂以不同的 物理状态存在。多相催化的特点是四个 基本步骤—吸附、扩散、反应 和解吸。例如，不饱和乙烯气 加氢制乙烷的反应 是由在木炭上精细分散的钯 来加速的。在这里，催化剂以其固态形式存在，而反应物以气态形式存在。在这个多相催化过程中，乙烯和氢分子吸附在催化剂表面。氢键断裂 使氢原子在催化剂表面 扩散。当扩散的氢原子遇到被吸附的乙烯 分子时，它们发生反应形成饱和乙烷。随后，产物脱附并从金属表面 离开，留下完好无损的催化剂。

催化

催化剂的存在会影响化学反应的率值。 催化剂是一种物质，可以在过程中不消耗而增加反应率值。 从反应机理和能量图表的概念可以理解催化剂在化学反应过程中的作用。 
 <img alt="Image1" src="/files/ftp_upload/11383/11383_Image_1.png" style="text-align: center; height: 350px;" />
 图中所示的图像代表了在催化剂缺失 (红色曲线) 和存在 (蓝色曲线) 的情况下进行的吸热化学过程的反应图。 
 两条曲线代表相同的整体反应–它们以相同的能量开始和结束。 (在这种情况下，生成物的能量多于反应物。 因此，反应是吸热)。 但是，他们的反应机理有所不同。 未催化的反应通过一步式机理进行 (仅观察到一个过渡状态)。 相比之下，催化反应遵循两步机理 (观察到两个过渡状态) ，激活能量明显较少。 反应途径的这一差异说明了催化剂在提供具有较低启动能量的替代反应机理方面的作用，从而加速反应。 
 催化反应机理不需要涉及比未催化机理更多的基本步骤。 但是，它必须提供一个替代反应路径，其速率确定步骤更快 (EA或激活能量更低)。 
 根据催化过程中存在催化剂和反应物的物理状态，催化反应可以归类为均匀或异构。 
 均相催化 
 在均相催化中，催化剂与反应物处于相同的相位 - 固体，液体或气体。 在此过程中，催化剂与反应物相互作用，形成一种中间物质，然后通过一个或多个步骤分解另一个反应物或与之作出反应，以再生原催化剂并形成最终的生成物。 
 均相催化的一个例子是涉及臭氧分解的化学过程，该过程发生在地球的上层大气中。 臭氧是一种相对不稳定的分子，分解成产率二原子氧。 这种分解反应与以下两步式机理一致： 
 <img alt="Eq1" src="/files/ftp_upload/11383/11383_Equation_1.png" style="text-align: center; height: 75px;" />
 许多物质可以催化臭氧的分解。 例如，据认为，通过以下三步式机理，会发生一氧化氮催化的臭氧分解： 
 <img alt="Eq2" src="/files/ftp_upload/11383/11383_Equation_2.png" style="text-align: center; height: 115px;" />
 但是，两步未催化机理和三步无催化机理的整体反应相同： 
 <img alt="Eq3" src="/files/ftp_upload/11383/11383_Equation_3.png" style="text-align: center; height: 50px;" />
 在催化反应中，请注意，在机理的第一步中没有反应物，在最后一步中是生成物。 这是催化剂的另一个特征：尽管它参与化学反应，但它不会被反应所消耗。 此外，在该均相催化中，反应物和催化剂都存在于气态阶段。 
 异构分析 
 在异构催化中，催化剂的存在阶段与反应物不同 (通常为固体)。 此类催化剂通常通过提供一个可发生反应的活性表面来发挥作用。 混合催化剂催化的气体和液体相反应发生在催化剂表面，而不是气体或液体相内。 
 异构催化通常涉及以下过程： 
 <ol>
 <li>催化剂表面的反应剂吸附 </li>
 <li>激活吸附剂 </li>
 <li>吸附剂的反应 </li>
 <li>从催化剂表面对产品进行解吸 </li>
 </ol>
 <img alt="Image2" src="/files/ftp_upload/11383/11383_Image_2.png" style="text-align: center; height: 250px;" />
 图中所示的图像代表了反应机理，它涉及在固体镍表面上形成乙烷气体 (C2H4 + H2 ⟶ C2H6) 的乙醇和氢气混合催化： 
 (a) 氢气吸附在镍表面上。 在此过程中，氢键是破损，形成镍氢键。 
 乙烯还通过分解碳 - 碳 Pi-键和形成镍 - 碳键在镍表面上吸附。 
 (c) 氢原子在撞击形成乙烷时会扩散到地表并形成新的碳氢键 (C2H6)。 
 (d) 镍表面的甲烷分子脱粒。 
 异构催化作用用于工业生产化学生成物，如氨，硝酸，硫酸和甲醇。 在大多数汽油驱动汽车上的催化转化器中也使用了混合型催化剂。 
 本文改编自<a href="https://openstax.org/books/chemistry-2e/pages/12-7-catalysis"> Openstax, 化学 2e, 第12.7节：催化</a>。

催化剂的存在会影响化学反应的速率。催化剂是一种能够加快反应速率而自身在反应中不被消耗的物质……

Education

教育

JoVE Core

化学

Chemical Kinetics

化学动力学

KEY TERMS AND CONCEPTS

关键术语与概念

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 化学通过简单明了且通俗易懂的动画视频课程，将普通化学概念带入到现实生活当中。此外，在科学家行动视频中，我们演示了在实验室中进行的经典和原始研究实验。

Pennsylvania State Academic Standards

Physical Science

Chemical Reactions

3.2.9-12.D

3.2.9-12.E

ohios-learning-standards-and-model-curriculum

Ohio’s Learning Standards and Model Curriculum

Interactions Of Matter

Chemical reactions

Virginia  Standards of Learning

HIgh School

CH.7

CH.7.f

Arizona Science Standards

High School

HS.P1

HS.P1U1.3

Next Generation Science Standards

Matter and Its Interactions

HS-PS1-4

HS-PS1-5

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.

Georgia Standards of Excellence

SC4.

Catalysts in Chemical Reactions

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Radioactive Decay and Radiometric Dating

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Nuclear Fission

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Nuclear Power

Controlled nuclear fission reactions are used to generate electricity. Any nuclear reactor that produces power via the fission of uranium or plutonium by ...

Nuclear Fusion

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PS.Chm.1.2

Atomic Orbitals

An atomic orbital represents the three-dimensional regions in an atom where an electron has the highest probability to reside. The radial distribution ...

The Pauli Exclusion Principle

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The Energies of Atomic Orbitals

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The Aufbau Principle and Hund's Rule

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electron-configuration-of-multielectron-atoms

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PS.Chm.1.3

Periodic Classification of the Elements

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The Periodic Table

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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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Ionic Radii

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Ionization Energy

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Electron Affinity

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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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	...

Bond Energies and Bond Lengths

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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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Catalyst Action in Reaction Rates

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