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

Скорость реакции можно изучить, определив изменение концентраций исходных веществ или продуктов реакции как функцию времени. Изменения концентрации можно измерить экспериментальными методами, такими как поляриметрия, спектроскопия или измерение давления. Поляриметрия использует плоско-поляризованный свет в электрическом поле, ориентированным только в одной плоскости.Она измеряет способность соединений вращать поляризованный свет, которая зависит от молекулярной структуры присутствующего соединения. Рассмотрим гидролиз сахарозы, который дает глюкозу и фруктозу. Поляриметр используется для измерения угла вращения плоскополяризованного света, проходящего через реагирующий раствор сахарозы.Сахароза вызывает вращение по часовой стрелке, тогда как глюкоза и фруктоза вызывают вращение против часовой стрелки. Измеряя угол вращения света через заданные интервалы времени, можно рассчитать относительные концентрации сахарозы, глюкозы или фруктозы и определить скорость реакции. Скорости реакции также можно измерить с помощью спектрофотометрических методов, используя способность исходных веществ или продуктов реакции поглощать свет определенных длин волн.Чем выше концентрация интересующего вещества, тем более интенсивным будет его светопоглощение. Например, бесцветный газообразный водород реагирует с фиолетовыми парами йода с образованием бесцветного иодистого водорода. Пары йода поглощают свет в желто-зеленой области и отражают фиолетовый свет.Во время реакции спектрофотометр измеряет количество света, поглощенного образцом, и анализирует прошедший свет. Таким образом, по мере протекания реакции снижение концентрации паров йода наблюдается по уменьшению поглощения желто-зеленого света. Используя закон Бера-Ламберта, можно рассчитать интенсивность света, поглощенного в разные моменты времени, и связать его с изменениями концентрации.В качестве альтернативы, если одно из исходных веществ или продуктов реакции представляет собой газ, для определения скорости реакции используется измерение давления. Например, во время разложения перекиси водорода скорость реакции изучается с помощью манометра, чтобы контролировать давление выделяющегося газообразного кислорода. По мере протекания реакции и выделения большего количества газообразного кислорода давление повышается.Используя уравнение идеального газа, значения давления, записанные в разные моменты времени, преобразуются в концентрации. Изменение концентрации как функция времени используется для определения скорости реакции. Для длительных реакций из реакционной смеси можно брать пробы или аликвоты через равные промежутки времени.Затем относительные концентрации измеряются с помощью инструментальных методов, таких как газовая хроматография, масс-спектрометрия или титрование, для расчета скорости реакции.

Измерение скорости реакции

Поляриметрия находит применение в химической кинетике для измерения концентрации и реакции кинетики оптически активных веществ во время химической реакции. Оптически активные вещества обладают способностью вращать плоскость поляризации проходящего через них света с линейной поляризацией, что называется оптическим вращением. Оптическая активность связана с молекулярной структурой веществ. Нормальный монохроматический свет неполяризован и обладает колебаниями электрического поля во всех возможных плоскостях, перпендикулярных направлению его распространения. Когда неполяризованный свет проходит через поляризатор, возникает линейно поляризованный свет, который поддерживает колебания в одной плоскости. 
 Поляриметрический прибор определяет направление поляризации света или вращения, производимого оптически активным веществом. В поляриметре поляризованный свет вводится в трубку, содержащую раствор-сигнал реакции, и реакция может быть выполнена без нарушения работы системы. Если образец содержит оптически неактивные вещества, ориентация плоскости поляризованного света не изменится. Свет будет виден на экране анализатора с одинаковой интенсивностью, а угол поворота (ɑ) будет равен нулю градусам. 
 Однако наличие оптически активных соединений в реагирующей пробе вызывает вращение плоскости проходящего через поляризованный свет. Свет, выходящее наружу, будет менее ярким. Для наблюдения за максимальной яркостью оси анализатора необходимо повернуть по часовой стрелке (декстраторатриаторное) или против часовой стрелки (левиотаторирующее). Направление вращения анализатора зависит от характера вещества. Измеряемое оптическое вращение пропорционально концентрации оптически активных веществ, присутствующих в образце. Анализируя угол измерения вращения в разных временных точках, концентрации оптически активных соединений можно определить как функцию времени. 
 Спектрометрия 
 Оптические экспериментальные методы, такие как спектрометрия, также часто используются для мониторинга химических реакций и получения количественных данных о кинетике реакции. С помощью спектрометрии свет определенной длины волны проходит через реагирующий образец. Молекулы или соединения (реагент или продукт) внутри образца могут поглощать некоторое количество света, передавая оставшееся количество, которое измеряется детектором. Количество поглощенного света зависит от концентрации интересующей смеси или молекулы. Например, чем выше концентрация вещества, тем больше его поглощение. Из поглощения прибор сможет определить концентрацию интересующей смеси. В реакционной пробе поглощение, измеряемое через определенные промежутки времени, вычисляет концентрацию реагент или продукта в зависимости от времени. 
 Измерения давления 
 При реакциях, связанных с газфазными веществами, за реакционной кинетикой следует количественная оценка изменений в количестве молекул газов в зависимости от изменений давления. Экспериментальные настройки газовой фазовой реакции могут быть подключены к манометру, который может измерять давление газообразного реагент или продукта. По мере развития реакции давление реагенты снижается, и (или) давление продуктов увеличивается. Это может быть измерено манометром в зависимости от времени. Используя идеальный закон о газе, концентрация газа пропорциональна ее частичному давлению, можно рассчитать скорость химической реакции.

Поляриметрия находит применение в химической кинетике для измерения концентрации и кинетики реакции оптически активных веществ во время химического ...

Education

JoVE Core

Chemistry

Химия

Один

Chemical Kinetics

Химическая кинетика

KEY TERMS AND CONCEPTS

ОСНОВНЫЕ ТЕРМИНЫ И ПОНЯТИЯ

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 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.

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

NC 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.

The Atomic Theory of Matter

The earliest recorded discussion of the basic structure of matter comes from ancient Greek philosophers. Leucippus and Democritus argued that all matter ...

Subatomic Particles

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Molar Mass

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Model basic atomic structure and relate an element's atomic structure to its bonding, reactivity, and placement on the Periodic Table.

IPC.7.A

physical-and-chemical-properties-of-matter

Physical and Chemical Properties of Matter

The characteristics that enable us to distinguish one substance from another are called properties.

Physical Properties of Matter

A physical property is ...

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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).



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Alkali Metals

Group 1 elements are soft and shiny metallic solids. They are malleable, ductile, and good conductors of heat and electricity. The melting points of the ...

Halogens

Group 17 elements, known as halogens, are nonmetals. At room temperature, fluorine and chlorine are gases, bromine is a liquid, and iodine a solid. ...

Noble Gases


The elements in group 18 are noble gases (helium, neon, argon, krypton, xenon, and radon). They earned the name &ldquo;noble&rdquo; because they were ...

Use patterns within the Periodic Table to predict the relative physical and chemical properties of elements.

IPC.7.B

Classifying Matter by State

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Classifying Matter by Composition

Matter: Pure Substances and Mixtures
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Phase Transitions

Whether solid, liquid, or gas, a substance&#39;s state depends on the order and arrangement of its particles (atoms, molecules, or ions). Particles in the ...

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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

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Explain how electrons can transition from a high energy level to a low energy state, emitting photons at different frequencies for different energy transitions.

IPC.7.D

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In the macroscopic world, objects that are large enough to be seen by the naked eye follow the rules of classical physics. A billiard ball moving on a ...

The Uncertainty Principle

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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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