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

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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

Die Untersuchung von Reaktionsmechanismen und Kinetik hilft Forschern, die Geschwindigkeit einer Reaktion zu optimieren. Ein Anstieg der Temperatur oder Konzentration beschleunigt die Geschwindigkeit, kann aber auch zu Zersetzung oder Nebenprodukten führen, wenn sie zu stark erhöht wird.
Alternativ kann dem Reaktionsgemisch eine Chemikalie zugesetzt werden, die die Reaktionsgeschwindigkeit beschleunigt, ohne dauerhaft verändert oder verbraucht zu werden. Diese Substanzen werden als Katalysatoren bezeichnet und beeinflussen die Geschwindigkeitskonstante, indem sie die Aktivierungsenergie verändern.
Denken Sie daran, dass die Reaktionsgeschwindigkeit umso langsamer ist, je höher die Aktivierungsenergie ist. Daher verlaufen viele chemische Reaktionen, wie z.B. die Zersetzung von wässrigem Wasserstoffperoxid, nur langsam ab.
Die Zugabe eines Katalysators senkt die Aktivierungsenergie, indem ein alternativer Reaktionsmechanismus bereitgestellt wird, ohne die Energiezustände von Reaktanten und Produkten zu beeinflussen.
Es gibt zwei Arten von Katalysatoren – homogene und heterogene.
Ein homogener Katalysator existiert in der gleichen Phase wie die Reaktanten. Zum Beispiel wird Natriumbromid zu wässrigem Wasserstoffperoxid hinzugefügt, um dessen Zersetzung in gasförmigen Sauerstoff und Wasser zu beschleunigen.
Zunächst reagieren die Bromid-Ionen mit Wasserstoffperoxid in einem sauren Medium zu einer orangefarbenen Lösung aus wässrigem Brom und Wasser. Als nächstes reagieren wässriges Brom und Wasserstoffperoxid und setzen Bromidionen frei, die durch eine farblose Lösung angezeigt werden. Charakteristisch für einen Katalysator sind Bromidionen, die die Reaktion beschleunigen, aber nicht verbraucht werden und nicht in der bilanzierten Nettogleichung erscheinen.
Ein heterogener Katalysator liegt in einem anderen Aggregatzustand vor als die Reaktanten. Die heterogene Katalyse ist durch vier zugrundeliegende Schritte gekennzeichnet: Adsorption, Diffusion, Reaktion und Desorption.
So wird beispielsweise die Hydrierungsreaktion von ungesättigtem Ethengas zu Ethan durch fein dispergiertes Palladium auf Holzkohle beschleunigt. Dabei liegt der Katalysator in seinem festen Zustand vor, während die Reaktanten in ihrer gasförmigen Form vorliegen.
Bei dieser heterogenen Katalyse adsorbieren Ethen- und Wasserstoffmoleküle an der Katalysatoroberfläche. Die Wasserstoff-Wasserstoff-Bindung bricht, so dass die Wasserstoffatome auf der Katalysatoroberfläche diffundieren können.
Wenn diffundierende Wasserstoffatome auf adsorbierte Ethenmoleküle treffen, reagieren sie zu gesättigtem Ethan. Anschließend desorbieren die Produkte und bewegen sich von der Metalloberfläche weg, wobei der intakte Katalysator zurückbleibt.

katalyse

Die Anwesenheit eines Katalysators beeinflusst die Geschwindigkeit einer chemischen Reaktion. Ein Katalysator ist eine Substanz, die die Reaktionsgeschwindigkeit erhöhen kann, ohne während des Prozesses verbraucht zu werden. Ein grundlegendes Verständnis der Rolle eines Katalysators bei chemischen Reaktionen lässt sich anhand des Konzepts von Reaktionsmechanismen und Energiediagrammen verstehen. <img alt="Image1" src="/files/ftp_upload/11383/11383_Image_1.png" style="text-align: center; height: 350px;"> Das dargestellte Bild stellt die Reaktionsdiagramme für einen endothermen chemischen Prozess dar, der in Abwesenheit (rote Kurve) und Anwesenheit (blaue Kurve) eines Katalysators abläuft. Beide Kurven stellen die gleiche Gesamtreaktion dar – sie beginnen und enden bei den gleichen Energien. (In diesem Fall haben Produkte mehr Energie als Reaktanten. Daher ist die Reaktion endotherm). Ihre Reaktionsmechanismen sind jedoch unterschiedlich. Die unkatalysierte Reaktion verläuft über einen einstufigen Mechanismus (mit nur einem beobachteten Übergangszustand). Im Gegensatz dazu folgt die katalysierte Reaktion einem zweistufigen Mechanismus (es werden zwei Übergangszustände beobachtet) mit deutlich geringerer Aktivierungsenergie. Dieser Unterschied in den Reaktionswegen verdeutlicht die Rolle des Katalysators bei der Bereitstellung eines alternativen Reaktionsmechanismus mit geringerer Aktivierungsenergie und damit der Beschleunigung von Reaktionen. Der katalysierte Reaktionsmechanismus muss nicht mehr Elementarschritte umfassen als der unkatalysierte Mechanismus. Es muss jedoch einen alternativen Reaktionsweg bieten, dessen geschwindigkeitsbestimmender Schritt schneller ist (mit einer niedrigeren E_a oder Aktivierungsenergie). Eine katalytische Reaktion kann basierend auf den physikalischen Zuständen, in denen Katalysatoren und Reaktanten während des katalytischen Prozesses vorliegen, als homogen oder heterogen kategorisiert werden. Homogene Katalyse Bei der homogenen Katalyse liegt der Katalysator in derselben Phase wie die Reaktanten vor – fest, flüssig oder gasförmig. Während des Prozesses interagiert der Katalysator mit dem Reaktanten und bildet eine Zwischensubstanz, die sich dann zersetzt oder in einem oder mehreren Schritten mit einem anderen Reaktanten reagiert, um den ursprünglichen Katalysator zu regenerieren und das Endprodukt zu bilden. Ein Beispiel für die homogene Katalyse ist der chemische Prozess der Zersetzung von Ozon in der oberen Erdatmosphäre. Ozon ist ein relativ instabiles Molekül, das sich zu zweiatomigem Sauerstoff zersetzt. Diese Zersetzungsreaktion steht im Einklang mit dem folgenden zweistufigen Mechanismus: <img alt="Eq1" src="/files/ftp_upload/11383/11383_Equation_1.png" style="text-align: center; height: 75px;"> Viele Stoffe können den Ozonabbau katalysieren. Es wird beispielsweise angenommen, dass die durch Stickoxid katalysierte Zersetzung von Ozon über den folgenden dreistufigen Mechanismus erfolgt: <img alt="Eq2" src="/files/ftp_upload/11383/11383_Equation_2.png" style="text-align: center; height: 115px;"> Die Gesamtreaktion ist jedoch sowohl für den zweistufigen unkatalysierten Mechanismus als auch für den dreistufigen NO-katalysierten Mechanismus gleich: <img alt="Eq3" src="/files/ftp_upload/11383/11383_Equation_3.png" style="text-align: center; height: 50px;"> Beachten Sie, dass NO bei der katalysierten Reaktion im ersten Schritt des Mechanismus ein Reaktant und im letzten Schritt ein Produkt ist. Dies ist ein weiteres charakteristisches Merkmal eines Katalysators: Obwohl er an der chemischen Reaktion beteiligt ist, wird er von der Reaktion nicht verbraucht. Darüber hinaus liegen bei dieser homogenen Katalyse sowohl der Reaktant als auch der Katalysator in einer Gasphase vor. Heterogene Katalyse Bei der heterogenen Katalyse liegt der Katalysator in einer anderen Phase (meist ein Feststoff) vor als die Reaktanten. Solche Katalysatoren funktionieren im Allgemeinen, indem sie eine aktive Oberfläche bereitstellen, auf der eine Reaktion stattfinden kann. Durch heterogene Katalysatoren katalysierte Gas- und Flüssigphasenreaktionen finden auf der Oberfläche des Katalysators und nicht innerhalb der Gas- oder Flüssigphase statt. Die heterogene Katalyse umfasst typischerweise die folgenden Prozesse: <ol> <li>Adsorption des/der Reaktanten auf der Oberfläche des Katalysators</li> <li>Aktivierung des/der adsorbierten Reaktanten</li> <li>Die Reaktion des/der adsorbierten Reaktanten</li> <li>Desorption von Produkten von der Oberfläche des Katalysators</li> </ol> <img alt="Image2" src="/files/ftp_upload/11383/11383_Image_2.png" style="text-align: center; height: 250px;"> Das abgebildete Bild stellt den Reaktionsmechanismus dar, der die heterogene Katalyse von Ethen und Wasserstoffgas auf einer festen Nickeloberfläche beinhaltet, wodurch Ethangas entsteht (C2H4 + H2 ⟶ C2H6): (a) Wasserstoff adsorbiert auf der Nickeloberfläche. Während des Prozesses werden Wasserstoff-Wasserstoff-Bindungen aufgebrochen, um Nickel-Wasserstoff-Bindungen zu bilden. (b) Ethen adsorbiert auch auf der Nickeloberfläche, indem es die Kohlenstoff-Kohlenstoff-Pi-Bindung aufbricht und Nickel-Kohlenstoff-Bindungen bildet. (c) Wasserstoffatome diffundieren über die Oberfläche und bilden neue Kohlenstoff-Wasserstoff-Bindungen, wenn sie unter Bildung von Ethan (C2H6) kollidieren. (d) Ethanmoleküle desorbieren von der Nickeloberfläche. Mithilfe der heterogenen Katalyse werden chemische Produkte wie Ammoniak, Salpetersäure, Schwefelsäure und Methanol industriell hergestellt. Heterogene Katalysatoren kommen auch in den Katalysatoren der meisten benzinbetriebenen Automobile zum Einsatz. Dieser Text ist adaptiert von <a href="https://openstax.org/books/chemistry-2e/pages/12-7-catalysis">Openstax, Chemistry 2e, Section 12.7: Catalysis</a>

Katalyse

Das Vorhandensein eines Katalysators beeinflusst die Geschwindigkeit einer chemischen Reaktion. Ein Katalysator ist eine Substanz, die die Reaktionsgeschwindigkeit erhöhen kann, ohne ...

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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...

arrhenius plots

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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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Catalysts in Chemical Reactions

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Nuclear Binding Energy

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Radioactive Decay and Radiometric Dating

Radioactivity is a spontaneous disintegration of an unstable nuclide and is a random process, as all the nuclei in the sample do not decay simultaneously. ...

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 ...

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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 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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Use models to explain how electrons are distributed in atoms.

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Periodic Classification of the Elements

The periodic table arranges atoms based on increasing atomic number so that elements with the same chemical properties recur periodically. When their ...

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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.

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Ionic Radii

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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.

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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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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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