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

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Catalysis

Изучение механизмов и кинетики реакции помогает исследователям оптимизировать скорость реакции. Повышение температуры или концентрации увеличит скорость, но также может привести к разложению или образованию побочных продуктов при слишком большом увеличении. В качестве альтернативы в реакционную смесь может быть добавлено химическое вещество, которое увеличивает скорость реакции без постоянного изменения или расходования.Эти вещества называются катализаторами и влияют на константу скорости, изменяя энергию активации. Напомним, что чем выше энергия активации, тем медленнее скорость реакции. Следовательно, многие химические реакции, такие как разложение водной перекиси водорода, протекают медленно.Добавление катализатора снижает энергию активации, обеспечивая альтернативный механизм реакции, не влияя на энергетические состояния исходных веществ и продуктов. Катализаторы бывают двух типов гомогенные и гетерогенные. Гомогенный катализатор существует в той же фазе, что и исходные вещества.Например, бромид натрия добавляют к водному раствору перекиси водорода, чтобы ускорить его разложение на газообразный кислород и воду. Во-первых, ионы брома реагируют с перекисью водорода в кислой среде с образованием оранжевого раствора водного брома и воды. Затем водный бром и перекись водорода вступают в реакцию с высвобождением ионов брома, на которые указывает бесцветный раствор.Ионы брома, характерные для катализатора, ускоряют реакцию, но остаются неизрасходованными и не присутствуют в чистом сбалансированном уравнении. Гетерогенный катализатор существует в отличном от исходных веществ агрегатном состоянии. Гетерогенный катализ характеризуется четырьмя основными этапами адсорбцией, диффузией, реакцией и десорбцией.Например, реакция гидрирования ненасыщенного газообразного этена в этан ускоряется мелкодисперсным палладием на угле. Здесь катализатор находится в твердом состоянии, а исходные вещества в газообразной форме. Во время этого гетерогенного катализа молекулы этена и водорода адсорбируются на поверхности катализатора.Связь водород-водород разрывается, позволяя атомам водорода диффундировать по поверхности катализатора. Когда диффундирующие атомы водорода сталкиваются с адсорбированными молекулами этена, они реагируют с образованием насыщенного этана. Впоследствии продукты десорбируются и удаляются с поверхности металла, оставляя неизрасходованный катализатор.

Катализ

Наличие катализатора влияет на скорость химической реакции. Катализатор – это вещество, которое может увеличить скорость реакции без потребления во время процесса. Базовое понимание роли катализаторов во время химических реакций можно понять из концепции механизмов реакции и энергетических схем. 
 <img alt=""Image1"" src=""/files/ftp_upload/11383/11383_Image_1.png"" style=""text-align: center; height: 350px;"" />
 На иллюстрации представлены диаграммы реакции для эндотермического химического процесса, прогрессирующего в отсутствие (красная кривая) и присутствии (синяя кривая) катализатора. 
 Обе кривые представляют собой одну и ту же общую реакцию – они начинаются и заканчиваются при одних и тех же энергиях. (В этом случае, продукция имеет больше энергии, чем реагенты. Таким образом, реакция является эндотермической). Однако их механизмы реагирования отличаются. Некатализированная реакция протекает через одношаговый механизм (с одним наблюдаемым переходным состоянием). В отличие от этого, катализированная реакция следует двухэтапному механизму (наблюдается два переходных состояния) с заметно меньшей энергией активации. Эта разница в путях реакции иллюстрирует роль катализатора в предоставлении альтернативного механизма реакции с меньшей энергией активации, тем самым ускоряя реакции. 
 Механизм катализированной реакции не должен включать в себя большее число элементарных шагов, чем некатализированный механизм. Тем не менее, он должен обеспечить альтернативный путь реакции, шаг определения скорости которого быстрее (с меньшим EA или энергией активации). 
 Каталитическую реакцию можно классифицировать как однородную или разнородную, исходя из физических состояний, в которых катализаторы и реагенты существуют во время каталитического процесса. 
 Однородный катализ 
 При однородном катализе катализатор присутствует в той же фазе, что и реагенты – твердый, жидкий или газовый. Во время процесса катализатор взаимодействует с реагент, образуя промежуточное вещество, которое затем разлагается или реагирует с другим реагент в одном или нескольких шагах для регенерации исходного катализатора и формирования конечного продукта. 
 Примером гомогенного катализа является химический процесс, связанный с разложением озона, происходящего в верхней атмосфере Земли. Озон представляет собой относительно нестабильную молекулу, которая разлагается для получения диатомного кислорода. Эта реакция разложения соответствует следующему двухэтапному механизму: 
 <img alt=""Eq1"" src=""/files/ftp_upload/11383/11383_Equation_1.png"" style=""text-align: center; height: 75px;"" />
 Многие вещества могут катализировать разложение озона. Например, считается, что окислно-катализованный распад озона происходит через следующий трехступенчатый механизм: 
 <img alt=""Eq2"" src=""/files/ftp_upload/11383/11383_Equation_2.png"" style=""text-align: center; height: 115px;"" />
 Однако общая реакция одинакова как для двухступенчатого некатализированного механизма, так и для трехступенчатого НЕКАТАЛИЗИРОВАННОГО механизма: 
 <img alt=""Eq3"" src=""/files/ftp_upload/11383/11383_Equation_3.png"" style=""text-align: center; height: 50px;"" />
 В катализированной реакции обратите внимание на то, что НА первом этапе работы механизма нет реагент и продукт на последнем этапе. Это еще одна характерная черту катализатора: Хотя он участвует в химической реакции, он не потребляется реакцией. Кроме того, при однородном катализе реагент и катализатор находятся в газообразной фазе. 
 Гетерогенный катализ 
 В гетерогенном катализе катализатор присутствует на другой фазе (обычно твердой), чем реагенты. Такие катализаторы, как правило, функционируют путем создания активной поверхности, на которой может возникнуть реакция. Газовые и жидкие фазовые реакции, катализируемые разнородными катализаторами, происходят на поверхности катализатора, а не в газовой или жидкой фазе. 
 Гетерогенный катализ обычно включает в себя следующие процессы: 
 <ol>
 <li>Адсорбция восстановителя(ей) на поверхности катализатора </li>
 <li>Активация восстановителя(ей) с адсорбентом </li>
 <li>Реакция восстановителя(ей) с адсорбентом </li>
 <li>Десорбция продукта(ей) с поверхности катализатора </li>
 </ol>
 <img alt=""Image2"" src=""/files/ftp_upload/11383/11383_Image_2.png"" style=""text-align: center; height: 250px;"" />
 На рисунке показан механизм реакции, включающий гетерогенный катализ этилена и газообразного водорода на твердой никелевой поверхности, образующий этановый газ (C2H4 + H2 ⟶ C2H6): 
 А) водород абсорбируется на никелиевой поверхности. Во время этого процесса водородные связи разрываются, образуя никелево-водородные связи. 
 (b) Этен также адсорбируется на никелевой поверхности, разрывая углеродно-углеродную связь и образуя никелево-углеродные связи. 
 C) атомы водорода рассеивают по поверхности и образуют новые углерод-водородные связи, когда они сталкиваются с образованием этана (C2H6). 
 D) молекулы этана десорбция с поверхности никеля. 
 Гетерогенный катализ используется для промышленного производства таких химических продуктов, как аммиак, азотная кислота, серная кислота и метанол. Гетерогенные катализаторы также используются в каталитических нейтрализаторах, используемых в большинстве автомобилей с бензиновым двигателем. 
 Этот текст адаптирован из<a href="https://openstax.org/books/chemistry-2e/pages/12-7-catalysis"> Openstax, Химия 2е изд., раздел 12.7: Катализ</a>.

Наличие катализатора влияет на скорость химической реакции. Катализатор – это вещество, которое может увеличивать скорость реакции, не расходуя ...

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ОСНОВНЫЕ ТЕРМИНЫ И ПОНЯТИЯ

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

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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...

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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

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Atomic Radii and Effective Nuclear Charge

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Ionization Energy

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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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Lewis Symbols and the Octet Rule

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Ionic Bonding and Electron Transfer

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The Born-Haber Cycle

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Covalent Bonding and Lewis Structures

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Electronegativity values of ...

bond-polarity-dipole-moment-and-percent-ionic-character

Bond Polarity, Dipole Moment, and Percent Ionic Character

Bond Polarity

The absolute value of the difference in electronegativity (&Delta;EN) of two bonded atoms provides a rough measure of the polarity to be ...

lewis-structures-of-molecular-compounds-and-polyatomic-ions

Lewis Structures of Molecular Compounds and Polyatomic Ions

To draw Lewis structures for complicated molecules and molecular ions, it is helpful to follow a step-by-step procedure as outlined:


	Determine the ...

Resonance

The Lewis structure of a nitrite anion (NO2&minus;) may actually be drawn in two different ways, distinguished by the locations of the N-O and N=O ...

Formal Charges

In some cases, there are seemingly more than one valid Lewis structures for molecules and polyatomic ions. The concept of formal charges can be used to ...

Exceptions to the Octet Rule

Many covalent molecules have central atoms that do not have eight electrons in their Lewis structures. These molecules fall into three categories:


	...

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

molecular-comparison-of-gases-liquids-and-solids

Molecular Comparison of Gases, Liquids, and Solids

Particles in a solid are tightly packed together (fixed shape) and often arranged in a regular pattern; in a liquid, they are close together with no ...

Intermolecular vs Intramolecular Forces

Intermolecular forces (IMF) are electrostatic attractions arising from charge-charge interactions between molecules. The strength of the intermolecular ...

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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