Name: gas pressure in mixtures: partial pressures
Uploaded: 2020-09-03T13:38:26.000Z
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Mixtures of Gases: Dalton's Law of Partial Pressures and Mole Fractions

La pression d'un gaz pur est la somme des collisions moléculaires entre ses particules et les surfaces environnantes. Un échantillon de gaz avec moins de particules dans un volume donné exerce une pression inférieure à celle d'un échantillon avec plus de particules dans le même volume. Mais quelle est la pression d'un mélange de différents gaz?Pour un mélange de gaz à plusieurs composants, la pression est la somme des collisions de toutes les molécules de gaz. On suppose que chaque composant du mélange exerce sa propre pression indépendante des autres gaz présents. La pression de n'importe quel composant individuel est appelée sa pression partielle.La pression totale du mélange gazeux idéal est égale à la somme des pressions partielles de ses composants. Cette observation est la loi de Dalton des pressions partielles. En appliquant la loi du gaz idéal, les pressions partielles des composants individuels du gaz sont substituées par des variables mesurables.Depuis des gaz dans le mélange occupent le même volume et sont à la même température, l'équation peut être simplifiée. La somme des moles des composants individuels est égale au nombre total de moles de tous les composants gazeux, n-total. Par conséquent, la pression totale du mélange gazeux est égale à n-total multiplié par la constante R-T sur V.Le nombre de moles d'un composant divisé par les moles totales dans le mélange est la fraction molaire.Le réarrangement de la fraction molaire pour les moles totales, et la substitution de n-total dans la loi de Dalton des pressions partielles donne des expressions pour la pression totale. Réorganiser de nouveau, la pression partielle d'un gaz dans un mélange est le produit de la fraction molaire et de la pression totale du mélange. Ainsi, dans un mélange gazeux, la pression partielle de tout composant, i, est égale à la fraction molaire de i multipliée par la pression totale.A titre d'exemple de calcul, supposons qu'un récipient rempli de deux gaz, l'hélium et l'argon, soit 40%en volume d'argon. Cela implique que la fraction molaire de l'argon est de 0, 4. Si la pression totale est de 4 atmosphère, quelle est la pression partielle de l'hélium?En utilisant l'équation de la pression partielle d'un gaz, la pression partielle de l'argon est égale à sa fraction molaire multipliée par la pression totale. Ainsi, 0, 4 fois 4 atm donne la pression partielle de l'argon comme 1, 6 atmosphère. Puisque la somme des pressions partielles est égale à la pression totale, l'équation peut être modifié de sorte que la pression partielle d'argon peut être soustraite de la pression totale.Ainsi, la pression partielle de l'hélium est de 2, 4 atmosphère.

mélange de gaz - loi de dalton sur les pressions partielles

À moins que les gaz individuels ne réagissent chimiquement les uns avec les autres, les gaz individuels dans un mélange de gaz n’influencent pas la pression des autres gaz. Chaque gaz d'un mélange exerce la même pression qu'il exercerait s'il se trouvait seul dans le récipient. La pression exercée par chaque gaz individuel dans un mélange est appelée pression partielle. 
 
 Cela signifie que dans un mélange contenant trois gaz différents A, B et C, si PA est la pression partielle du gaz A ; PB est la pression partielle du gaz B ; PC est la pression partielle du gaz C ; alors la pression totale est donnée par l'équation 1 :  
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/1038/1038_Equations-01.png" style="height: 40px;" />
 
 C’est la loi de Dalton sur les pressions partielles : la pression totale d’un mélange de gaz parfaits est égale à la somme des pressions partielles des constituants gazeux. 
 
 Soit nA, nB et nC le nombre de moles de chacun des gaz du mélange. Si chaque gaz obéit à l'équation du gaz parfait, la pression partielle peut être écrite comme suit : 
 
 <img alt="Eq2" src="https://www.jovecore.com/CDNSource/lessons/1038/1038_Equations-02.png" style="height: 40px;" />
 
 Étant donné que tous les gaz sont à la même température et occupent le même volume, leur remplacement dans l'équation 1 donne : 
 
 <img alt="Eq3" src="https://www.jovecore.com/CDNSource/lessons/1038/1038_Equations-03.png" style="height: 40px;" />
 
 L'équation indique qu'à température et volume constants, la pression totale d'un échantillon de gaz est déterminée par le nombre total de moles de gaz présents. 
 
 Pour les mélanges de gaz, il est commode d'introduire une quantité appelée fraction molaire, χ, qui est définie comme le nombre de moles d'une substance particulière dans un mélange divisé par le nombre total de moles de toutes les substances présentes. Mathématiquement, la fraction molaire d'une substance A dans un mélange avec B et C est exprimée comme 
 
 <img alt="Eq4" src="https://www.jovecore.com/CDNSource/lessons/1038/1038_Equations-04.png" style="height: 40px;" />
 
 De même, les fractions molaires de B et C sont; 
 
 <img alt="Eq5" src="https://www.jovecore.com/CDNSource/lessons/1038/1038_Equations-05.png" style="height: 40px;" />
 
 <img alt="Eq6" src="https://www.jovecore.com/CDNSource/lessons/1038/1038_Equations-06.png" style="height: 40px;" />
 
 Lorsqu'on combine l'équation pour la fraction molaire de A et l'équation pour la pression partielle, cela donne : 
 
 <img alt="Eq7" src="https://www.jovecore.com/CDNSource/lessons/1038/1038_Equations-07.png" style="height: 40px;" />
 
 La pression partielle du gaz A est liée à la pression totale du mélange gazeux via sa fraction molaire. 
 
 En d'autres termes, la pression d'un gaz dans un mélange de gaz est le produit de sa fraction molaire et de la pression totale du mélange. 
 
 Ce texte est adapté de <a href="https://openstax.org/books/chemistry-2e/pages/9-3-stoichiometry-of-gaseous-substances-mixtures-and-reactions">Openstax, Chimie 2e, Section 9.3 : Stœchiométrie des substances gazeuses, mélanges gazeux et réactions gazeuses.</a>

Mélange de gaz - Loi de Dalton sur les pressions partielles

À moins que des gaz individuels ne réagissent chimiquement les uns avec les autres, les gaz individuels d’un mélange de gaz n’affectent pas la pression de l’autre. Chaque gaz ...

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gas pressure in mixtures: partial pressures

Gas pressure in mixtures is described by partial pressure and mole fraction. When gases do not chemically react, each gas in the mixture acts as if it were alone in the container. The pressure from one gas is called its partial pressure.

For a mixture of three gases, A, B, and C, the total pressure equals the sum of their partial pressures: PA, PB, and PC. This is Dalton's law of partial pressures. It states that the total pressure of a mixture of ideal gases is the sum of the pressures from each gas.

If each gas follows the ideal-gas equation, its partial pressure can be written in terms of moles, temperature, and volume. Because every gas in the mixture has the same temperature and takes up the same volume, these terms can be combined. At constant temperature and constant volume, the total pressure depends on the total number of moles of gas present.

For gas mixtures, chemists also use mole fraction, x. Mole fraction is the number of moles of one substance divided by the total number of moles in the mixture. For gas A in a mixture with B and C, the mole fractions of A, B, and C are found in this way.

Mole fraction also links directly to partial pressure. The partial pressure of gas A equals its mole fraction times the total pressure of the mixture. In other words, a gas's pressure in a mixture is its share of the total pressure.

Gas Pressure in Mixtures: Partial Pressures

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mixtures of gases: dalton's law of partial pressures and mole fractions

Gas pressure in mixtures is described by partial pressure and mole fraction. When gases do not chemically react, each gas in the mixture acts as if it were alone in the container. The pressure from one gas is called its partial pressure. For a mixture of three gases, A, B, and C, the total pressure equals the sum of their partial pressures: PA, PB, and PC. This is Dalton's law of partial pressures. It states that the total pressure of a mixture of ideal gases is the sum of the pressures from...

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Gas Pressure in Mixtures: Partial Pressures

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