Name: gas particle motion, diffusion, and effusion
Uploaded: 2020-09-03T13:38:27.000Z
Duration: 3 min 48 s
Channel: JoVE

K12 Standards

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NC K-12 Science Standard Course of Study

9-12

Chemistry

Energy

PS.Chm.7.3

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Behavior of Gas Molecules: Molecular Diffusion, Mean Free Path, and Effusion

В закрытом флаконе духов содержится высокая концентрация газообразных ароматических молекул, которые постоянно движутся и беспорядочно сталкиваются. Между тем, воздух за пределами флакона практически не содержит этих молекул. При открытии флакона устанавливается градиент концентрации между областями высокой и низкой концентрации.Молекулы продолжают беспорядочно перемещаться, из области высокой концентрации в область низкой концентрации. Самопроизвольное перемешивание и распространение жидкостей или газов в ответ на градиент концентрации называется молекулярной диффузией. Диффузия медленный процесс.Несмотря на то, что частицы газа движутся с высокой скоростью, многочисленные столкновения вызывают частые изменения скорости и направления. Среднее расстояние, которое проходит частица между столкновениями, известно как длина свободного пробега. Для частицы газа на длину свободного пробега влияет плотность частиц, которая также влияет на давление.По мере увеличения плотности частиц увеличивается и частота столкновений. Таким образом, их длина свободного пробега уменьшается. Точно так же с уменьшением плотности частиц уменьшается частота столкновений, что приводит к увеличению длины свободного пробега.Различные газы смешиваются с разной скоростью в зависимости от скорости частиц газа. Так как среднеквадратичная скорость и молярная масса газа обратно пропорциональны, более легкие газы смешиваются быстрее, чем более тяжелые. Рассмотрим стеклянную трубку, расположенную между резервуарами с равным количеством аммиака и хлороводорода.Когда смешиваемые газы встречаются, они вступают в реакцию и образуют кольцо хлорида аммония. Кольцо расположено ближе к концу трубки с хлористым водородом, потому что более легкие молекулы аммиака прошли дальше по трубке, чем более тяжелые молекулы хлористого водорода, за то же время. Эффузия это еще один процесс, в котором происходит движение молекул газа.Это способность молекул газа проходить через отверстие, диаметр которого намного меньше, чем длина свободного пробега самого газа, в ответ на перепад давления. Вот почему гелиевые шары в конечном итоге сдуваются гелий постепенно истекает через крошечные поры в материале шара. Скорость истечения, как и диффузия, зависит от среднеквадратичной скорости и молярной массы газа.В частности, скорость истечения обратно пропорциональна квадратному корню из молярной массы газа. Следовательно, более тяжелые газы истекают медленнее, чем более легкие. Для любых двух газов отношение скоростей истечения является квадратным корнем из обратного отношения их молярных масс.Это называется законом эффузии Грэма. Рассмотрим два воздушных шара, наполненных до достижения одинакового давления:один наполнен гелием, а другой кислородом. Гелий имеет более низкую молярную массу, чем кислород, о чем свидетельствует плавучесть гелиевого шара в воздухе.Применение закона Грэма к гелию и кислороду предполагает, что гелий истекает в 2, 8 раза быстрее, чем кислород. Таким образом, шар с гелием сдувается быстрее, чем шар с кислородом.

Эффузия и диффузия

Несмотря на то, что молекулы газа путешествуют с огромной скоростью (сотни метров в секунду), они сталкиваются с другими молекулами газа и проходят в разных направлениях, прежде чем достичь нужной цели. При комнатной температуре молекула газа будет испытывать миллиарды столкновений в секунду. Средний свободный путь — это среднее расстояние, которое молекула проходит между столкновениями. Средний свободный путь увеличивается с уменьшением давления; в целом, средний свободный путь для молекулы газа будет в сотни раз больше диаметра молекулы 
 
 В целом, когда проба газа вводится в одну часть закрытого контейнера, ее молекулы очень быстро рассеиваются по всему контейнеру; этот процесс, посредством которого молекулы рассеиваются в пространстве в ответ на различия в концентрации называется диффузией. Газовые атомы или молекулы, конечно, не руководствуются градиентом концентрации; они просто перемещаются случайным образом — области с более высокой концентрацией имеют больше частиц, чем области с более низкой концентрацией, поэтому происходит чистое движение видов из зон с высокой концентрацией в зоны с низкой концентрацией. В закрытой среде диффузия в конечном итоге приведет к равномерной концентрации газа на всем протяжении. Газообразные атомы и молекулы продолжают двигаться, но поскольку их концентрации одинаковы в обеих лампах, скорости передачи между лампами равны (чистая передача молекул не происходит). Количество газа, проходящего через определенную площадь за единицу времени, является скоростью диффузии. 
 
 <img alt=""Eq1"" src=""https://www.jove.com/files/core/CDNSource/lessons/1044/1044_Equations-01.png"" style=""height: 40px;"" />
 
 Скорость диффузии зависит от нескольких факторов: Градиента концентрации (увеличение или уменьшение концентрации с одной точки на другую), площади поверхности, доступной для диффузии, и расстояния, которое должны пройти частицы газа. 
 
 Процесс, связанный с движением газовых частиц, как при диффузии, представляет собой эффузию, утечку молекул газа через маленькую дыру, например, через отверстие в баллоне в вакуум. Несмотря на то, что коэффициенты диффузии и эффузии зависят от молярной массы используемого газа, их скорость не одинакова, однако, коэффициенты их расхода одинаковы. 
 
 Если смесь газов помещается в контейнер с пористыми стенками, газы выходят через небольшие отверстия в стенах. Более легкие газы проходят через небольшие отверстия быстрее (с большей скоростью), чем более тяжелые. В 1832 году Томас Грэм изучил скорости эффузии различных газов и сформулировал закон Грэма об эффузии: Скорость эффузии газа обратно пропорциональна квадратному корню массы его частиц: 
 
 <img alt=""Eq2"" src=""https://www.jove.com/files/core/CDNSource/lessons/1044/1044_Equations-02.png"" style=""height: 40px;"" />
 
 Это означает, что если два газа, А и В, находятся при одинаковой температуре и давлении, отношение их скорости эффузии обратно пропорционально отношению квадратных корней масс их частиц: 
 
 <img alt=""Eq3"" src=""https://www.jove.com/files/core/CDNSource/lessons/1044/1044_Equations-03.png"" style=""height: 40px;"" />
 
 Это соотношение указывает на то, что более легкий газ имеет более высокую скорость эффузии. 
 
 Например, заполненный гелием резиновый баллон сдувается быстрее, чем заполненный воздухом, потому что скорость выдувания через поры резины быстрее для более легких атомов гелия, чем для молекул воздуха. 
 
 Этот текст адаптирован из <a href=""https://openstax.org/books/chemistry-2e/pages/9-4-effusion-and-diffusion-of-gases"">Openstax, Химия 2е изд., раздел 9.4: Эффузия и диффузия газов.</a>"

Хотя газообразные молекулы движутся с огромными скоростями (сотни метров в секунду), они сталкиваются с другими газообразными молекулами и перемещаются во многих...

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KEY TERMS AND CONCEPTS

ОСНОВНЫЕ ТЕРМИНЫ И ПОНЯТИЯ

gas particle motion, diffusion, and effusion

Gas particle motion, diffusion, and effusion explain how gases spread and escape in everyday settings. Gas molecules move at very high speeds, but they collide with other gas molecules and change direction many times before reaching a target. At room temperature, a gas molecule can experience billions of collisions each second.

The mean free path is the average distance a molecule travels between collisions. This distance becomes larger when pressure decreases. In general, the mean free path of a gas molecule is hundreds of times greater than the molecule’s diameter.

Diffusion is the spread of gas particles through space because of differences in concentration. When a gas is added to one part of a closed container, its molecules quickly disperse throughout the container. The molecules move randomly, but the net movement is from higher concentration to lower concentration. In a closed environment, diffusion eventually leads to equal concentrations throughout. The gas particles still move, but there is no net transfer between regions when the concentrations are the same.

The rate of diffusion is the amount of gas that passes through an area in a given time. It depends on the concentration gradient, the available surface area, and the distance the particles must travel. A related process is effusion, which is the escape of gas molecules through a tiny opening, such as a pinhole in a balloon into a vacuum. Diffusion and effusion rates both depend on molar mass, but the rates are not the same.

If a mixture of gases is placed in a container with porous walls, the gases effuse through the small openings. The lighter gases pass through more quickly than the heavier gases. Thomas Graham studied these rates in 1832 and formed Graham’s law of effusion. This law states that the rate of effusion is inversely proportional to the square root of the mass of the particles. For two gases at the same temperature and pressure, the ratio of their effusion rates follows the same relationship. A helium-filled rubber balloon, for example, deflates faster than an air-filled balloon because helium atoms effuse faster through the rubber pores than air molecules.

Gas Particle Motion, Diffusion, and Effusion

definition and measurement of pressure: atmospheric pressure, barometer, and manometer

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Pressure Units and Tools for Gas Measurement

Pressure is the force applied to a given area, and gas pressure comes from gas molecules colliding with surfaces. Each collision is tiny, but many collisions over a small area can create high pressure. Pressure increases when force increases or when the area gets smaller. It decreases when force drops or when the area gets larger. Pressure is measured in pascals, or Pa, in the SI system. One pascal equals 1 newton per square meter, and a newton is a unit of force equal to 1 kg·m/s2. A pascal...

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How Gas Pressure, Volume, and Temperature Change

Gas pressure, volume, temperature, and amount are linked by several gas laws. Scientists found these relationships by studying how a confined gas changes when one variable is held constant and another changes. These patterns apply to an ideal gas, which is a useful model for gases under low pressure and high temperature. Gay-Lussac’s law, also called Amontons’s law, connects pressure and temperature. If gas is trapped in a rigid container, the volume and number of moles stay constant. When the...

applications of the ideal gas law: molar mass, density, and volume

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Ideal Gas Law at STP and Gas Density

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Gas Pressure in Mixtures: Partial Pressures

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Stoichiometry of Gases with the Ideal Gas Law

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Kinetic Molecular Theory and Gas Behavior

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Gas particle motion, diffusion, and effusion explain how gases spread and escape in everyday settings. Gas molecules move at very high speeds, but they collide with other gas molecules and change direction many times before reaching a target. At room temperature, a gas molecule can experience billions of collisions each second. The mean free path is the average distance a molecule travels between collisions. This distance becomes larger when pressure decreases. In general, the mean free path...

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Real gases are gases that do not always follow the ideal gas law, PV = nRT, especially when pressure is high or temperature is low. The ideal gas model assumes gas particles have no attractive or repulsive forces and take up negligible space compared with the container. At room temperature and 1 atm or less, many gases behave close to ideally. When conditions change, the measured relationships between pressure, volume, and temperature can drift away from the ideal prediction. One way to see...

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Explore how gas molecules move, diffuse, and effuse, plus why lighter gases pass through tiny openings faster than heavier gases.

Gas Particle Motion, Diffusion, and Effusion | JoVE K12

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New York State Science Learning Standards

HS. Structure and Properties of Matter

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Vapor Pressure

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Clausius-Clapeyron Equation

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Phase Transitions: Melting and Freezing

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Phase Transitions: Sublimation and Deposition

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Heating and Cooling Curves

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Phase Diagrams

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PS.Chm.7.1

Energy Basics

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First Law of Thermodynamics

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Internal Energy

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Hess's Law

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Standard Enthalpy of Formation

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Enthalpies of Reaction

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PS.Chm.7.2

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Basic Postulates of Kinetic Molecular Theory: Particle Size, Energy, and Collision

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Kinetic Molecular Theory and Gas Laws Explain Properties of Gas Molecules

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Kinetic Molecular Theory: Molecular Velocities, Temperature, and Kinetic Energy

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Although gaseous molecules travel at tremendous speeds (hundreds of meters per second), they collide with other gaseous molecules and travel in many ...

Real Gases: Effects of Intermolecular Forces and Molecular Volume Deriving Van der Waals Equation

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Ideal Gas Law

Source: Laboratory of Dr. Andreas Z&#252;ttel - Swiss Federal Laboratories for Materials Science and Technology
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Understand the relationship among pressure, temperature, volume, and phase.

Gas Particle Motion, Diffusion, and Effusion

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