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

Badanie mechanizmów i kinetyki reakcji pomaga naukowcom zoptymalizować szybkość reakcji. Wzrost temperatury lub stężenia przyspieszy tempo, ale może również prowadzić do rozkładu lub produktów ubocznych, gdy jest zbyt duży.
Alternatywnie, do mieszaniny reakcyjnej można dodać substancję chemiczną, która przyspiesza szybkość reakcji bez trwałej zmiany lub zużycia. Substancje te nazywane są katalizatorami i wpływają na stałą szybkości poprzez zmianę energii aktywacji.
Przypomnijmy, że im wyższa energia aktywacji, tym wolniejsze tempo reakcji. W związku z tym wiele reakcji chemicznych, takich jak rozkład wodnego roztworu nadtlenku wodoru, przebiega powoli.
Dodanie katalizatora obniża energię aktywacji, zapewniając alternatywny mechanizm reakcji bez wpływu na stany energetyczne reagentów i produktów.
Istnieją dwa rodzaje katalizatorów – jednorodne i niejednorodne.
Jednorodny katalizator występuje w tej samej fazie co reagenty. Na przykład bromek sodu dodaje się do wodnego roztworu nadtlenku wodoru, aby przyspieszyć jego rozkład na gazowy tlen i wodę.
Po pierwsze, jony bromkowe reagują z nadtlenkiem wodoru w środowisku kwaśnym, tworząc pomarańczowy roztwór wodnego roztworu bromu i wody. Następnie wodny roztwór bromu i nadtlenku wodoru reagują i uwalniają jony bromkowe wskazane przez bezbarwny roztwór. Charakterystyczne dla katalizatora jest to, że jony bromkowe przyspieszają reakcję, ale pozostają niezużyte i nie pojawiają się w równaniu bilansu netto.
Niejednorodny katalizator występuje w innym stanie fizycznym w porównaniu z reagentami. Kataliza heterogeniczna charakteryzuje się czterema podstawowymi etapami - adsorpcją, dyfuzją, reakcją i desorpcją.
Na przykład reakcja uwodornienia nienasyconego gazu etenowego do etanu jest przyspieszana przez drobno zdyspergowany pallad na węglu drzewnym. Tutaj katalizator występuje w stanie stałym, podczas gdy reagenty są w postaci gazowej.
Podczas tej niejednorodnej katalizy cząsteczki etenu i wodoru adsorbują się na powierzchni katalizatora. Wiązanie wodór-wodór pęka, umożliwiając atomom wodoru dyfuzję na powierzchni katalizatora.
Kiedy dyfundujące atomy wodoru napotykają zaadsorbowane cząsteczki etenu, reagują, tworząc nasycony etan. Następnie produkty ulegają desorpcji i odsuwają się od powierzchni metalu, pozostawiając nienaruszony katalizator.

kataliza

Obecność katalizatora wpływa na szybkość reakcji chemicznej. Katalizator to substancja, która może zwiększyć szybkość reakcji, nie ulegając zużyciu w trakcie procesu. Podstawowe zrozumienie roli katalizatorów podczas reakcji chemicznych można zrozumieć na podstawie koncepcji mechanizmów reakcji i diagramów energetycznych.

<img alt="Image1" src="/files/ftp_upload/11383/11383_Image_1.png" style="text-align: center; height: 350px;" />

Zilustrowany obraz przedstawia diagramy reakcji endotermicznego procesu chemicznego przebiegającego pod nieobecność (czerwona krzywa) i obecność (niebieska krzywa) katalizatora.

Obie krzywe przedstawiają tę samą ogólną reakcję – zaczynają się i kończą na tych samych energiach. (W tym przypadku produkty mają więcej energii niż reagenty. Dlatego reakcja jest endotermiczna). Jednak mechanizmy ich reakcji są różne. Niekatalizowana reakcja przebiega poprzez mechanizm jednoetapowy (z tylko jednym obserwowanym stanem przejściowym). Natomiast reakcja katalizowana przebiega według dwuetapowego mechanizmu (obserwuje się dwa stany przejściowe) ze znacznie mniejszą energią aktywacji. Ta różnica w ścieżkach reakcji ilustruje rolę katalizatora w zapewnianiu alternatywnego mechanizmu reakcji o niższej energii aktywacji, przyspieszając w ten sposób reakcje.

Katalizowany mechanizm reakcji nie musi obejmować większej liczby elementarnych etapów niż mechanizm niekatalizowany. Musi jednak zapewniać alternatywną ścieżkę reakcji, której etap determinujący szybkość jest szybszy (przy niższym Ea lub energii aktywacji).

Reakcję katalityczną można sklasyfikować jako jednorodną lub niejednorodną, w oparciu o stany fizyczne, w których występują katalizatory i reagenty podczas procesu katalitycznego.

Kataliza homogeniczna

W katalizie jednorodnej katalizator występuje w tej samej fazie co reagenty – w postaci stałej, ciekłej lub gazowej. Podczas procesu katalizator oddziałuje z reagentem, tworząc substancję pośrednią, która następnie rozkłada się lub reaguje z innym reagentem w jednym lub kilku etapach, regenerując pierwotny katalizator i tworząc produkt końcowy.

Przykładem katalizy jednorodnej jest proces chemiczny polegający na rozkładzie ozonu zachodzący w górnych warstwach atmosfery ziemskiej. Ozon jest stosunkowo niestabilną cząsteczką, która rozkłada się, tworząc dwuatomowy tlen. Ta reakcja rozkładu jest zgodna z następującym dwuetapowym mechanizmem:

<img alt="Eq1" src="/files/ftp_upload/11383/11383_Equation_1.png" style="text-align: center; height: 75px;" />

Wiele substancji może katalizować rozkład ozonu. Uważa się, że rozkład ozonu katalizowany tlenkiem azotu zachodzi poprzez następujący trzyetapowy mechanizm:

<img alt="Eq2" src="/files/ftp_upload/11383/11383_Equation_2.png" style="text-align: center; height: 115px;" />

Ogólna reakcja jest taka sama zarówno w przypadku dwuetapowego mechanizmu niekatalizowanego, jak i trzyetapowego mechanizmu katalizowanego NO:

<img alt="Eq3" src="/files/ftp_upload/11383/11383_Equation_3.png" style="text-align: center; height: 50px;" />

W reakcji katalizowanej należy zauważyć, że NO jest reagentem w pierwszym etapie mechanizmu i produktem w ostatnim etapie. Jest to kolejna charakterystyczna cecha katalizatora: chociaż uczestniczy on w reakcji chemicznej, nie jest w niej zużywany. Dodatkowo w tej homogenicznej katalizie zarówno reagent, jak i katalizator występują w fazie gazowej.

Kataliza heterogeniczna

W katalizie heterogenicznej katalizator występuje w innej fazie (zwykle w postaci stałej) niż reagenty. Takie katalizatory na ogół działają poprzez zapewnienie aktywnej powierzchni, na której może zachodzić reakcja. Reakcje w fazie gazowej i ciekłej katalizowane przez katalizatory heterogeniczne zachodzą na powierzchni katalizatora, a nie w fazie gazowej lub ciekłej.

Kataliza heterogeniczna zazwyczaj obejmuje następujące procesy:

<ol>
	<li>Adsorpcja reagentów na powierzchni katalizatora</li>
	<li>Aktywacja zaadsorbowanych reagentów</li>
	<li>Reakcja zaadsorbowanych reagentów</li>
	<li>Desorpcja produktu(ów) z powierzchni katalizatora</li>
</ol>

<img alt="Image2" src="/files/ftp_upload/11383/11383_Image_2.png" style="text-align: center; height: 250px;" />

Zilustrowany obraz przedstawia mechanizm reakcji obejmującej heterogeniczną katalizę etenu i gazowego wodoru na stałej powierzchni niklu, tworząc gazowy etan (C2H4 + H2 ⟶ C2H6):

(a) Wodór adsorbuje się na powierzchni niklu. Podczas tego procesu wiązania wodorowo-wodorowe rozrywają się, tworząc wiązania niklowo-wodorowe.

(b) Eten również adsorbuje się na powierzchni niklu, rozrywając wiązanie pi węgiel-węgiel i tworząc wiązania nikiel-węgiel.

(c) Atomy wodoru dyfundują po powierzchni i tworzą nowe wiązania węgiel-wodór, gdy zderzają się, tworząc etan (C2H6).

(d) Cząsteczki etanu desorbują z powierzchni niklu.

Kataliza heterogeniczna jest stosowana do przemysłowego wytwarzania produktów chemicznych, takich jak amoniak, kwas azotowy, kwas siarkowy i metanol. Katalizatory heterogeniczne są również stosowane w katalizatorach stosowanych w większości samochodów napędzanych benzyną.

Kataliza

Obecność katalizatora wpływa na szybkość reakcji chemicznej. Katalizator to substancja, która może zwiększyć szybkość reakcji bez zużycia ...

Education

JoVE Core

Chemia

Jeden

Chemical Kinetics

Kinetyka chemiczna

KEY TERMS AND CONCEPTS

Kluczowe terminy i pojęcia

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

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JoVE Core Chemistry brings general chemistry concepts to life using concise and easy-to-understand animated video lessons. Plus, in the scientist-in-action videos, we demonstrate classical and original research experiments performed in today's laboratories worldwide.

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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Arizona Science Standards

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New York State Science Learning Standards

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Catalysts in Chemical Reactions

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PS.Chm.1.3

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Molar Mass

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PS.Chm.2.1

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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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	...

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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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