Name: gas mixtures and their partial pressures
Uploaded: 2023-09-18T17:11:17.000Z
Duration: 1 min 11 s
Channel: JoVE

חשבו על גז בתוך מיכל. הלחץ שמפעיל הגז הוא סכום הכוחות המופעלים עקב ההתנגשויות המולקולריות בין המולקולות שלו לבין פני השטח הסובבים אותו.
גז עם מולקולות רבות מפעיל לחץ גבוה יותר מאשר גז עם מספר קטן יותר של מולקולות באותו מיכל. לכן, הלחץ מהמרכיב הבודד נקרא הלחץ החלקי שלו.
שקול תערובת של גזים אידיאליים במיכל בשיווי משקל תרמי. הלחץ הכולל בתוך המיכל שווה לסכום הלחצים החלקיים של מרכיביו. אבחנה זו ידועה כחוק הלחצים החלקיים של דלטון.
אם נזכיר את משוואת הגז האידיאלי ונחליף את הלחצים החלקיים של הגזים הבודדים, ניתן לקבוע את הלחץ הכולל.
הלחץ החלקי של אדים הנמצאים בשיווי משקל עם הפאזה הנוזלית של אותו חומר נקרא לחץ אדים.
היחס בין הלחץ החלקי של אדי מים ללחץ האדים של מים בטמפרטורה ידוע כלחות יחסית.

<ol>
	<li>OpenStax. (2019). פיזיקה אוניברסיטאית כרך 2. [גרסת אינטרנט], עמ' 37 - 38. מקור: <a href="https://openstax.org/books/university-physics-volume-2/pages/2-2-pressure-temperature-and-rms-speed">https://openstax.org/books/university-physics-volume-2/pages/2-2-pressure-temperature-and-rms-speed</a></li>
</ol>

חוק הלחץ החלקי של דלטון

Dalton's Law of Partial Pressure

<div style="direction:rtl">הלחץ החלקי של גז הוא מדד לפעילות התרמודינמית של מולקולות הגז. הלחץ שגז היה יוצר אם היה תופס את הנפח הכולל הזמין נקרא הלחץ החלקי של הגז. אם שני גזים או יותר מעורבבים יחד במיכל, המולקולות נעות באקראי ומתנגשות זו בזו, מה שגורם להן להגיע לשיווי משקל תרמי. כאשר לגזים יש אותה טמפרטורה, למולקולות שלהם יש אותה אנרגיה קינטית ממוצעת. לפיכך, כל גז מציית לחוק הגז האידיאלי בנפרד ומפעיל את אותו לחץ על דפנות המיכל בנפרד. לכן, בתערובת של גזים, הלחץ הכולל במיכל הוא סכום הלחצים החלקיים של הגזים המרכיבים, בהנחה של התנהגות גזים אידיאלית וללא תגובות כימיות בין הרכיבים. חוק זה ידוע כחוק הלחצים החלקיים של דלטון. התיאוריה הוקמה על ידי המדען האנגלי ג'ון דלטון (1766–1844). חוק דלטון עולה בקנה אחד עם העובדה שהלחצים מצטברים, על פי העיקרון של פסקל. בתערובת של גזים אידיאליים בשיווי משקל תרמי, מספר המולקולות של כל גז הוא פרופורציונלי ללחץ החלקי שלו. יישום חשוב נוסף של לחץ חלקי הוא לחץ אדים, שהוא הלחץ החלקי של אד שבו הוא נמצא בשיווי משקל עם הפאזה הנוזלית של אותו חומר. בכל טמפרטורה, הלחץ החלקי של המים באוויר אינו יכול לעלות על לחץ האדים של המים באותה טמפרטורה, שכן בכל פעם שהלחץ החלקי מגיע ללחץ האדים, המים מתעבים מהאוויר. טל הוא דוגמה לעיבוי זה. הטמפרטורה שבה מתרחש עיבוי עבור דגימת אוויר נקראת נקודת הטל. זה נמדד בקלות על ידי קירור איטי של כדור מתכת; נקודת הטל היא הטמפרטורה שבה מופיע לראשונה עיבוי על הכדור.</div>

הלחץ החלקי של גז הוא מדד לפעילות התרמודינמית של מולקולות הגז. הלחץ שגז היה יוצר אם היה כובש ...

Education

JoVE Core

Physics

פיזיקה

Unit 1

יחידה 1

The Kinetic Theory of Gases

התיאוריה הקינטית של הגזים

KEY TERMS AND CONCEPTS

מונחים ומושגים מרכזיים

gas mixtures and their partial pressures

Gas mixtures have a total pressure that comes from the partial pressures of each gas. A partial pressure is the pressure one gas would make if it filled the whole container by itself. This idea helps explain how gases behave when they are mixed together.

When two or more gases share a container, their molecules move randomly and collide with one another. If the gases are at the same temperature, they reach thermal equilibrium. Their molecules then have the same average kinetic energy. In that case, each gas follows the ideal gas law separately and pushes on the walls of the container on its own.

The total pressure in the container is the sum of the partial pressures of the component gases. This result is called Dalton's law of partial pressures. It applies when the gases behave like ideal gases and do not react chemically with one another. The law was developed by the English scientist John Dalton, who lived from 1766 to 1844. It also agrees with Pascal's principle, which says that pressures add up.

Partial pressure also helps describe vapor pressure. Vapor pressure is the partial pressure of a vapor when it is in equilibrium with the liquid form of the same substance. In air, the partial pressure of water cannot rise above the vapor pressure of water at that temperature. If it reaches that value, water condenses out of the air. Dew is one common example of this process.

The temperature where condensation begins in a sample of air is called the dew point. One simple way to measure it is to cool a metal ball slowly. The dew point is the temperature at which condensation first appears on the ball.

Gas Mixtures and Their Partial Pressures

equation of state

Gas Behavior in the Equation of State

The equation of state describes how a gas’s pressure, volume, temperature, and number of moles are related. These macroscopic properties help define the condition, or state, of the matter in a thermodynamics system. The relationship between these quantities can be written as a simple mathematical expression. In some cases, it is too complex to express in a formula. Even then, the link between the quantities still exists. When an equation of state cannot be written mathematically, scientists...

ideal gas equation

Gas Laws Behind the Ideal Equation

The ideal gas equation links pressure, volume, temperature, and amount of gas. It describes a hypothetical gas and is built from four empirical gas laws: Boyle’s law, Charles’s law, Avogadro’s law, and Gay-Lussac’s law. When these proportionalities are combined, they give one constant called the universal gas constant. This gas constant has the same value for all real gases under certain conditions. In chemical calculations, that constant is used in the ideal gas equation. The equation works...

van der waals equation

How Real Gases Deviate from Ideal Behavior

Real gases often deviate from the ideal gas law, especially at low temperatures and high pressures. The van der Waals equation improves the ideal gas model by adding two corrections that account for these real-world effects. One correction accounts for attractive forces between molecules. These forces become stronger at higher densities and lower the measured pressure, so the equation adds a pressure term based on the square of the molar density and a positive constant a. The second...

pv-diagrams

Liquid-Gas Changes on pV Curves

pV curves show how gas pressure changes with volume. A pV diagram is a graph of pressure versus volume for the gas being studied. It helps describe how the substance behaves. When the gas acts like an ideal gas, the ideal gas equation gives the link between pressure and volume. A common pV curve is an isotherm, which is a line drawn at fixed temperature and fixed number of molecules. For an ideal gas, pressure times volume stays constant. That means volume goes down as pressure goes up. The...

kinetic theory of an ideal gas

Avogadro's Number and Gas Particle Motion

Avogadro's number and gas particle motion help explain how gases behave at the particle level. A mole is the amount of a substance that contains as many molecules as there are atoms in exactly 12 grams of carbon-12. This idea came from Amedeo Avogadro, who proposed that equal volumes of gas at the same pressure and temperature contain equal numbers of molecules, no matter what the gas is. Avogadro's number is the number of molecules in one mole. Its value is 6.02 × 10^23 mol−1. This number...

molecular kinetic energy

Gas Pressure and Molecular Motion

Gas pressure comes from the motion of molecules in a container. The word gas came from a Flemish word meaning “chaos,” and J. B. van Helmont first used it to describe vapors. In a gas, molecules move in a continuous and random way. When a molecule hits a container wall, its velocity changes. The part of the velocity parallel to the wall stays the same. The part perpendicular to the wall reverses direction, but its size does not change. If the molecule’s velocity changes in the x-direction, its...

distribution of molecular speeds

Molecular Speed Curves and Vapor Pressure

Molecular speed curves show how the speeds of gas particles are spread out at a given temperature. For a gas with many molecules, this spread is predictable even though each molecule moves randomly in direction and speed. The pattern is called the Maxwell-Boltzmann distribution. Liquids have a similar speed distribution, but it is not exactly the same as in gases. This difference helps explain boiling and vapor pressure. A molecule must reach at least a certain speed, v a, to escape from the...

maxwell-boltzmann distribution: problem solving

maxwell-boltzmann-distribution-problem-solving

Maxwell-Boltzmann Gas Speed Ratio

The Maxwell-Boltzmann distribution describes how molecular speeds are spread in a gas. Even though individual molecules move in random directions, the whole gas has a predictable speed pattern. This distribution is written as f(v), where v is molecular speed. The distribution function is used to find the expected number of particles with speeds between v1 and v2. Because N is dimensionless, f(v) has units of seconds per meter. The relationship can also be written in differential form for speed...

phase diagram

Water Phase Changes on a Pressure Chart

Water phase changes can be mapped on a pressure and temperature chart called a phase diagram. These plots show which phase a substance is in at different conditions. They also reveal where phase transitions happen, because the boundary lines mark the temperatures and pressures where two phases coexist. The water phase diagram shows how pressure affects boiling. As pressure rises, the boiling temperature increases gradually until it reaches 374 °C at 218 atm. A pressure cooker works this way.

mean free path and mean free time

Gas collisions: path length and time

Gas collisions in a cylinder help explain the mean free path and mean free time. Gas molecules move in random motion. They collide with each other and change speed and direction. The mean free path is the average path length between collisions. The mean free path depends on how many molecules are packed into a volume. It varies inversely with density. When more molecules are present, collisions happen more often, so the mean free path becomes shorter. It also depends on molecule size. Larger...

heat capacity: problem-solving

Heat Capacity of Gases by Degrees of Freedom

Heat capacity of gases depends on the gas type and the motion of its molecules. It is the amount of heat energy needed to raise the temperature of a unit mass of gas by one degree Celsius. This thermodynamic property is important in science and industry. To solve heat capacity problems, first identify the gas. A gas's molecular structure affects its heat capacity because the molecules have different degrees of freedom. These values can be found from tables or from empirical equations. Next,...

dalton's law of partial pressure

Gas mixtures have a total pressure that comes from the partial pressures of each gas. A partial pressure is the pressure one gas would make if it filled the whole container by itself. This idea helps explain how gases behave when they are mixed together. When two or more gases share a container, their molecules move randomly and collide with one another. If the gases are at the same temperature, they reach thermal equilibrium. Their molecules then have the same average kinetic energy. In that...

escape velocities of gases

Why Lighter Gases Escape Earth First

Earth’s escape velocity is the speed an object needs to leave gravity behind. Near the top of the atmosphere, about 100 km above Earth, that speed is 11.1 km/s. Gas molecules can escape only if their speeds become high enough to reach that limit. The chance of escape depends on gas temperature and molecular speed. According to kinetic theory, the average kinetic energy of gas molecules is proportional to temperature. As temperature rises, the rms speed, or root-mean-square speed, also rises.

key terms and concepts

Learn how gas mixtures share pressure through partial pressures, how Dalton's law works, and how vapor pressure and dew point explain condensation in air.

Gas Mixtures and Their Partial Pressures | JoVE K12

physics

JoVE Core Physics introduces the theory and application of fundamental physics topics, including kinematics, dynamics, fluid mechanics, statics, waves, and acoustics, with concise and easy-to-understand animated videos. Additionally, the scientist-in-action videos demonstrate the application of related concepts in classical and original research experiments performed in today's laboratories worldwide.

Gas Mixtures and Their Partial Pressures

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