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

反応メカニズムと反応速度論の研究は、研究者が反応の速度を最適化するのに役立ちます。温度や濃度が上昇すると速度が加速しますが、増加しすぎると分解や副産物につながる可能性もあります。
あるいは、恒久的に変化したり消費されたりすることなく反応速度を加速する化学物質を反応混合物に添加することができる。これらの物質は触媒と呼ばれ、活性化エネルギーを変化させることで速度定数に影響を与えます。
活性化エネルギーが高いほど、反応速度が遅くなることを思い出してください。したがって、過酸化水素水の分解のような多くの化学反応はゆっくりと進行します。
触媒の添加は、反応物や生成物のエネルギー状態に影響を与えることなく、代替の反応メカニズムを提供することにより、活性化エネルギーを低下させます。
触媒には、均質触媒と不均一触媒の2種類があります。
反応物と同じ相には均質な触媒が存在します。たとえば、臭化ナトリウムは、過酸化水素水溶液に添加して、ガス状の酸素と水への分解を促進します。
まず、臭化物イオンは酸性媒体中で過酸化水素と反応して、臭素水と水のオレンジ溶液を形成します。次に、臭素水と過酸化水素が反応し、無色の溶液で示される臭化物イオンを放出します。触媒の特徴として、臭化物イオンは反応を加速しますが、消費されず、正味の平衡方程式には現れません。
不均一系触媒は、反応物とは異なる物理的状態で存在します。不均一系触媒作用は、吸着、拡散、反応、脱着の4つの基本的なステップによって特徴付けられます。
例えば、不飽和エテンガスからエタンへの水素化反応は、炭上にパラジウムを細かく分散させることで加速されます。ここでは、触媒は固体状態で存在し、反応物は気体の形で存在します。
この不均一系触媒作用の間、エテン分子と水素分子が触媒表面に吸着します。水素-水素結合が切断され、水素原子が触媒表面に拡散します。
拡散した水素原子が吸着したエテン分子に出会うと、それらは反応して飽和エタンを形成します。その後、生成物は脱着して金属表面から離れ、無傷の触媒が残ります。

触媒作用

触媒の存在は化学反応の速度に影響を与えます。触媒は、プロセス中に消費されることなく反応速度を上げることができる物質です。触媒の化学反応中における役割は、反応メカニズムとエネルギー図の概念から理解できます。
 <img alt = "Image1" src = "/ files / ftp_upload / 11383 / 11383_Image_1.png" style = "text-align:center; height:350px;" />
 この図は、触媒の非存在下（赤い曲線）と存在下（青い曲線）で進行する吸熱化学プロセスの反応図を表しています。
 両方の曲線は同じ全体的な反応を表している–それらは同じエネルギーで始まり、同じエネルギーで終わります。 （この場合、生成物は反応物よりも多くのエネルギーを持っています。したがって、反応は吸熱反応である）。ただし、それらの反応メカニズムは異なります。触媒されていない反応は、ワンステップメカニズムを介して進行します（遷移状態が1つだけ観察される）。対照的に、触媒反応は、著しく低い活性化エネルギーで2段階のメカニズム（2つの遷移状態が観察される）に従う。反応経路のこの違いは、より低い活性化エネルギーで代替反応メカニズムを提供し、それによって反応を加速するという触媒の役割を示しています。
 どちらの曲線も同じ全体反応を表しており、開始と終了時のエネルギーはそれぞれ等しい(この場合、生成物は反応物よりも大きなエネルギーを持ちます。したがって、この反応は吸熱性である）。 しかし、これらの反応メカニズムは異なります。無触媒反応は、1段階のメカニズムで進行する（観察される遷移状態は1つだけである）。一方、触媒反応は2段階のメカニズム（2つの遷移状態が観察される）をたどり、活性化エネルギーは格段に小さくなります。このような反応経路の違いは、より低い活性化エネルギーを持つ代替の反応メカニズムを提供することで、反応を促進するという触媒の役割を示しています。
 触媒反応機構は、無触媒反応機構よりも素過程の数が多い必要はないが、律速段階がより速くなる（Eaまたは活性化エネルギーが低くなる）ような代替反応経路が新たに形成される必要があります。
 触媒反応は、触媒プロセス中に触媒と反応物が存在する物理的な状態に基づいて、均一系と不均一系に分類されます。
 均一系触媒
 均一系触媒では、触媒は反応物と同じ相（固体、液体、気体）に存在します。プロセス中、触媒は反応物と相互作用して反応中間体を形成し、その中間体が1つ以上のステップで分解または別の反応物と反応して元の触媒を再生し、最終的な生成物をつくます。
 均一系触媒反応の例として、地球上の高層大気中で発生するオゾンの分解を伴う化学プロセスが挙げられます。オゾンは比較的不安定な分子であり、分解すると二原子酸素が発生します。この分解反応は、次のような2段階のメカニズムから成ます。
 <img alt="Eq1" src="/files/ftp_upload/11383/11383_Equation_1.png" style="text-align: center; height: 75px;" />
 多くの物質がオゾンの分解を触媒します。例えば、一酸化窒素を触媒とするオゾンの分解は、次のような3段階のメカニズムで起こると考えられています。
 <img alt="Eq2" src="/files/ftp_upload/11383/11383_Equation_2.png" style="text-align: center; height: 115px;" />
 しかし、2段階の無触媒メカニズムでも3段階のNO触媒メカニズムでも、全体の反応は同じです。
 <img alt="Eq3" src="/files/ftp_upload/11383/11383_Equation_3.png" style="text-align: center; height: 50px;" />
 触媒反応では、NOがメカニズムの最初のステップで反応物となり、最後のステップで生成物となっていることに注目しましょう。これが触媒の特徴の1つであり、化学反応には参加するものの、反応全体で消費されることはありません。また、この均一系触媒反応では、反応物と触媒の両方が気体の状態で存在しています。
 不均一系触媒
 不均一系触媒反応では、触媒は反応物とは異なる相（通常は固体）に存在します。このような触媒は一般的に、反応を起こすための活性表面を提供することで機能します。不均一系触媒による気相・液相反応は、気相・液相内ではなく、触媒の表面で起こります。
 不均一系触媒反応には、一般的に次のようなプロセスからなります。
 <ol>
 <li>触媒の表面への反応物の吸着</li>
 <li>吸着した反応物の活性化</li>
 <li>吸着した反応物の反応</li>
 <li>触媒表面からの生成物の脱離</li>
 </ol>
 <img alt="Image2" src="/files/ftp_upload/11383/11383_Image_2.png" style="text-align: center; height: 250px;" />
 図は、固体ニッケル表面上でエテンと水素ガスが不均一触媒反応を起こし、エタンガスが生成される反応メカニズム（C2H4+ H2⟶ C2H6）を示したものです。
 (a) ニッケルの表面に水素が吸着します。その際、水素と水素の結合が切れて、ニッケルと水素の結合が生成します。 
 (b) また、エチレンは、炭素-炭素のπ結合を切断してニッケル–炭素結合を形成することで、ニッケル表面に吸着します。 
 (c) 水素原子は表面を拡散し、衝突して新たな炭素-水素結合を形成することで、エタン（C2H6）が生成します。 
 (d) エタン分子がニッケル表面から脱離します。
 不均一系触媒は、アンモニア、硝酸、硫酸、メタノールなどの化学製品を工業的に製造するために用いられます。また、ガソリン車に搭載されている触媒コンバーターにも不均一系触媒が使用されています。
 上記の文章は以下から引用しました。 <a href="https://openstax.org/books/chemistry-2e/pages/12-7-catalysis">Openstax, Chemistry 2e, Section 12.7: Catalysis</a>.

触媒の存在は化学反応の速度に影響を与えます。触媒とは、消費されることなく反応速度を上げることができる物質のことであり......

Education

JoVE Core

化学

Chemical Kinetics

化学反応速度論

KEY TERMS AND CONCEPTS

主要用語と概念

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 化学は、簡潔で理解しやすいアニメーションビデオレッスンを使用して、一般的な化学の概念を生き生きとしたものにします。さらに、活動する科学者のビデオでは、現在世界中の実験室で行われている古典的そして独創的な研究・実験をお見せします。

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

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

Scientific Laws and Theories

Scientific Laws

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The Scientific Method

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Classifying Matter by State

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Classifying Matter by Composition

Matter: Pure Substances and Mixtures
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The Atomic Theory of Matter

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Subatomic Particles

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Radioactivity and Nuclear Equations

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Types of Radioactivity

The most common types of radioactivity are &#945; decay, &#946; decay, &#947; decay, neutron emission, and electron capture.
Alpha (&#945;) decay is the ...

Nuclear Stability

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Nuclear Binding Energy

The difference between the calculated and experimentally measured masses is known as the mass defect of the atom. In the case of helium-4, the mass defect ...

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

Many heavier elements with smaller binding energies per nucleon can decompose into more stable elements that have intermediate mass numbers and larger ...

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

The process of converting very light nuclei into heavier nuclei is also accompanied by the conversion of mass into large amounts of energy, a process ...

Nuclear Transmutation

Nuclear transmutation is the conversion of one nuclide into another. It can occur by the radioactive decay of a nucleus, or the reaction of a nucleus with ...

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

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

The arrangement of electrons in the orbitals of an atom is called its electron configuration. We describe an electron configuration with a symbol that ...

The Energies of Atomic Orbitals

In an atom, the negatively charged electrons are attracted to the positively charged nucleus. In a multielectron atom, electron-electron repulsions are ...

The Aufbau Principle and Hund's Rule

To determine the electron configuration for any particular atom, we can build the structures in the order of atomic numbers. Beginning with hydrogen, and ...

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Electron Configuration of Multielectron Atoms

The alkali metal sodium (atomic number 11) has one more electron than the neon atom. This electron must go into the lowest-energy subshell available, the ...

Use models to explain how electrons are distributed in atoms.

PS.Chm.1.3

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 ...

The Periodic Table

As early chemists discovered more elements, they realized that various elements could be grouped by their similar chemical behaviors. One such grouping ...

Atomic Mass

Atoms &mdash; and the protons, neutrons, and electrons that compose them &mdash; are extremely small. For example, a carbon atom weighs less than 2 ...

Molar Mass

The identity of a substance is defined not only by the types of atoms or ions it contains but by the quantity of each type of atom or ion. For example, ...

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

Ionic radius is the measure used to describe the size of an ion. A cation always has fewer electrons and the same number of protons as the parent atom; it ...

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).



This process can be either ...

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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Lewis Symbols and the Octet Rule

Chemical bonds are complex interactions between two or more atoms or ions, which reduce the potential energy of the molecule. Gilbert N. Lewis developed a ...

Ionic Bonding and Electron Transfer

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The Born-Haber Cycle

Lattice Energy&#xA0;

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Covalent Bonding and Lewis Structures

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Electronegativity

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Bond Polarity

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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:


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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

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 ...

Intermolecular Forces

Atoms and molecules interact through bonds (or forces): intramolecular and intermolecular. The forces are electrostatic as they arise from interactions ...

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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