Name: tracking reaction kinetics with light and pressure
Uploaded: 2020-09-24T15:06:50.000Z
Duration: 3 min 9 s
Channel: JoVE

K12 Standards

Texas Essential Knowledge and Skills

High School

Chemistry and Physics

IPC.7

IPC.7.F

Measuring Reaction Rates

反応速度は、反応物または生成物の濃度の変化を時間の関数として決定することによって調べることができます。
濃度変化は、偏光測定、分光法、圧力測定などの実験技術で測定できます。
偏光法は、電場が1つの平面のみに沿って配向された平面偏光を使用します。これは、存在する化合物の分子構造に依存する、化合物が偏光を回転させる能力を測定します。
グルコースとフルクトースを生成するスクロースの加水分解を考えてみましょう。偏光計は、反応するショ糖溶液を通過する平面偏光の回転度を測定するために使用されます。ショ糖は時計回りの回転を引き起こしますが、グルコースとフルクトースは反時計回りの回転を引き起こします。
設定された時間間隔で光の回転度を測定することにより、ショ糖、グルコース、またはフルクトースの相対濃度を計算し、反応速度を決定できます。
反応速度は、反応物または生成物が特定の波長の光を吸収する能力を利用して、分光光度法を使用して測定することもできます。対象物質の濃度が高いほど、その光吸収率はより強くなります。
例えば、無色の水素ガスは紫色のヨウ素蒸気と反応して無色のヨウ化水素を形成します。ヨウ素蒸気は黄緑色の領域の光を吸収し、紫色の光を反射します。
反応中、分光光度計はサンプルが吸収する光の量を測定し、透過した光を分析します。したがって、反応が進行するにつれて、黄緑色の光吸収率の低下により、ヨウ素蒸気濃度の低下が観察されます。
ランベルトビールの法則を使用すると、さまざまな時点で吸収される光の強度を計算し、濃度の変化に関連付けることができます。
あるいは、反応物または生成物の1つがガスである場合、圧力測定値を使用して、圧力変化を監視することにより反応速度を決定します。
例えば、過酸化水素分解の際には、放出される酸素ガスの圧力を監視するために、圧力計を用いて反応速度を調べます。反応が進行し、より多くの酸素ガスが発生すると、圧力が上昇します。
理想気体方程式を使用して、異なる時点で記録された圧力値が濃度に変換されます。時間の関数としての濃度の変化は、反応速度を決定するために使用されます。
長時間の反応の場合、サンプルまたはアリコートを反応混合物から一定の時間間隔で採取できます。次に、相対濃度は、ガスクロマトグラフィー、質量分析、滴定などの機器技術を使用して測定され、反応速度が計算されます。

反応速度の測定

偏光測定は、化学反応中の光学活性物質の濃度や反応速度を測定する化学反応速度論に応用されています。光学活性物質は、その中を通る直線偏光の偏光面を回転させる（光学回転）性質を持ち、それは物質の分子構造に起因します。通常の単色光は無偏光であり、その伝搬方向に垂直なすべての面で電界の振動を持っています。無偏光の光が偏光板を通過すると、1つの面で振動する直線偏光の光のみが出てきます。
 偏光計は、光の偏光方向や光学活性物質の回転を測定する装置です。平面偏光した光を反応液の入ったチューブに導入することで、系を乱すことなく反応を追うことができます。試料に光学的に不活性な物質が含まれている場合、偏光の向きに変化はありません。分析装置の画面上では同じ強さの光が見え、回転角（ ɑ）は0度となります。
 しかしながら、反応したサンプルに光学活性化合物があると、通過する偏光の面が回転し、出てくる光の明るさが低下します。最大の明るさを観測するためには、分析装置の軸を時計回りまたは反時計回りに回転させる必要があります。分析装置を回転させる必要がある方向は、存在する化合物の性質によって異なります。測定された旋光度は、サンプル中に存在する光学活性物質の濃度に比例します。異なる時点で測定した回転角を分析することで、光学活性化合物の濃度を時間の関数として求めることができます。 
 分光測定
 分光分析などの光学実験技術も、化学反応を可視化し、反応速度に関する定量的情報を確保するために頻繁に使用されます。分光測定では、特定の波長の光が反応サンプルへと照射されます。 サンプル内の分子または化合物（反応物または生成物のいずれか）はいくらかの光を吸収し、残りの透過光を検出器を用いて測定します。 吸収される光の量（吸光度）は、化合物の種類や濃度によって異なります。 たとえば、化合物の濃度が高いほど、その吸光度は大きくなるため、目的化合物の濃度決定に用いることができます。化学反応を伴う系では、定期的な間隔で測定された吸光度から、時間の関数としての反応物または生成物濃度を計算することができます。
 圧力測定
 気相物質を含む反応では、圧力の変化に伴う気体のモル数の変化を定量化することで、反応速度を計算することが出来ます。このために、気体状の反応物または生成物の圧力を測定できる圧力計を用います。反応が進むと、反応物の圧力が下がり、生成物の圧力が上がります。これは圧力計を用いて時間の関数として測定できます。理想気体の法則(気体の濃度はその分圧に比例する)を用いて、化学反応の速度を計算することができます。

偏光測定は、化学反応速度論に応用され、化学反応中の光学活性物質の濃度と反応速度を測定します...

Education

JoVE Core

Chemistry

化学

Chemical Kinetics

化学反応速度論

KEY TERMS AND CONCEPTS

主要用語と概念

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 through a polarizer, it becomes linearly polarized with vibrations in one plane.

In a polarimeter, this plane-polarized light is sent through a tube of reacting solution. The instrument then shows how much the light has been rotated. If the sample contains optically inactive substances, the light keeps the same orientation, the analyzer screen stays equally bright, and the rotation angle is zero degrees. If optically active compounds are present, the light becomes dimmer and the analyzer must be turned clockwise or counter-clockwise to restore maximum brightness. Clockwise rotation is dextrorotatory, and counter-clockwise rotation is levorotatory.

The amount of optical rotation is proportional to the concentration of the optically active substance in the sample. By taking rotation readings at different times, the concentration can be followed as the reaction proceeds. Spectrometry gives similar kinetic information by passing light of a specific wavelength through a reacting sample. Some of the light is absorbed by the reactant or product, and a detector measures the remaining light. Higher concentration gives higher absorbance, so repeated absorbance measurements can be used to find concentration over time.

Pressure measurements are useful for gas-phase reactions. A manometer can be connected to the reaction setup to monitor the pressure of a gaseous reactant or product. As the reaction goes on, reactant pressure may decrease and product pressure may increase. Using the ideal gas law, the concentration of a gas is proportional to its partial pressure, so reaction rate can be calculated from the pressure change.

Tracking Reaction Kinetics with Light and Pressure

tracking-reaction-kinetics-with-light-and-pressure

reaction rate

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

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

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...

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

See how polarimetry, spectrometry, and pressure measurements monitor reaction kinetics by tracking optical rotation, absorbance, and gas pressure changes.

Tracking Reaction Kinetics with Light and Pressure | 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。

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

North Carolina K-12 Science Standard Course of Study

9-12

Matter and its Interactions

PS.Chm.5.1

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.e

Minnesota Academic Standards in Science

3.2.1 9C.3.1.1.2

Alabama Course of Study Science

Matter and Its Interactions: Energy

9-12.C.3.3.d

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.

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Subatomic Particles

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IPC.7.A

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Physical and Chemical Properties of Matter

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IPC.7.B

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Phase Transitions

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Phase Transitions: Vaporization and Condensation

The physical form of a substance changes on changing its temperature. For example, raising the temperature of a liquid causes the liquid to vaporize ...

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IPC.7.C

Emission Spectra

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IPC.7.D

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IPC.7.E

Science concepts. The student knows that relationships exist between the structure and properties of matter. The student is expected to:

Tracking Reaction Kinetics with Light and Pressure

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