Name: pressure-volume work in chemistry
Uploaded: 2020-09-03T13:38:27.000Z
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PS.Chm.7.2

Quantifying Work

Rappelons que l'énergie interne d'un système est définie comme la somme de la chaleur et de l'action. La chaleur est mesurée à l'aide de la calorimétrie, mais comment le travail est-il quantifié? L'action est le résultat d'une force, qu'un objet éprouve-comme en étant tiré, poussé ou soulevé-sur une distance.Ainsi, l'action est égal à la force fois la distance. Si un club de golf frappe une balle de 100 pieds, alors il a fait du travail. L'énergie est transférée du club au ballon.Dans ce cas, le ballon est le système sur lequel le travail est effectué par l'environnement. Dans les réactions chimiques, l'action peut être associé à de multiples changements physiques ou chimiques dans un système. Un type de travail couramment rencontré est le travail pression-volume.Considérez la combustion dans le cylindre d'un moteur. Non seulement la combustion génère-t-elle de la chaleur, mais elle produit également du gaz qui effectue un travail. Lorsque le volume de gaz dans le cylindre se dilate, la pression pousse le piston vers le bas en agissant contre les forces extérieures dans l'environnement.La pression est définie comme une force agissant sur une zone. Ici, la force des gaz en expansion est répartie sur la surface de la base du piston. En d'autres termes, la force est la zone de temps de pression.Ainsi, l'action, qui est la force fois la distance, peut être réécrit comme la pression multipliée par la zone et la distance sur laquelle il agit. Pour le cylindre du moteur, c'est la différence de hauteur que le piston déplace. Considérant que la surface fois la hauteur est le volume d'un cylindre, l'équation pour l'action peut être définie comme la pression fois le changement de volume du système.Cependant, parce que le volume augmente et que le piston est poussé vers le bas, le système effectue effectivement un action sur l'environnement qui, par convention, est une valeur négative. Par conséquent, le travail est défini comme le négatif de la pression P fois delta V ou le changement de volume final et initial, qui s'est produit lors de l'expansion. L'unité d'action pression-volume est généralement définie comme un litre-atmosphère.Cette unité peut être convertie en l'unité conventionnelle d'énergie, Joules, en utilisant le facteur de conversion un litre-atmosphère est égale à 101, 3 Joules.

<ol>
	<li>Schmidt-Rohr, Klaus. « Travail d’expansion sans pression externe et thermodynamique en termes de processus irréversibles quasi-statiques. » Journal of Chemical Education 91, n° 3 (2014) : 402-409.</li>
	<li>Gislason, Eric A., et Norman C. Craig. « Définitions générales du travail et de la chaleur dans les processus thermodynamiques. » Journal of Chemical Education 64, n° 8 (1987) : 660.</li>
	<li>O'Loane, J. Kenneth. « Changements adiabatiques : changements réversibles et irréversibles impliquant uniquement un travail pression-volume. » Journal of Chemical Education 30, no. 4 (1953) : 190. 
	 </li>
</ol>

quantification du travail d'une force

Lorsqu'un système subit une transformation, son énergie interne peut varier et l'énergie peut être transférée du système à l'environnement extérieur, ou de l'environnement au système.  
 
 Le transfert d'énergie se produit par la chaleur et le travail. La relation entre l'énergie interne, la chaleur et le travail est représentée par l'équation : 
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/795/795_Equations-01.png" style="height: 40px;" />
 
 Alors que la chaleur est une fonction d'un changement de température observé, le travail est une fonction d'un changement de volume observé appelé le travail pression-volume. Le travail (w) peut être défini comme une force (F) agissant sur une distance (D). 
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/795/Equations-02.png" style="height: 40px;" />
 
 Le travail pression-volume (ou travail d'expansion) se produit lorsqu'un système repousse l'environnement contre une pression de retenue, ou lorsque l'environnement comprime le système. Un exemple de ceci se produit pendant le fonctionnement d'un moteur à combustion interne. La réaction de combustion de l'essence et de l'oxygène est exothermique. Une partie de cette énergie est cédée sous forme de chaleur, et une partie est réalisée comme un travail en détendant les gaz dans le cylindre, poussant ainsi le piston vers l'extérieur. Les substances impliquées dans la réaction sont le système, et le moteur ainsi que le reste de l'univers sont l'environnement extérieur. Le système perd de l'énergie en chauffant et en effectuant un travail sur l'environnement extérieur, et son énergie interne diminue. 
 
 Lorsque le volume d'un cylindre augmente (c'est-à-dire que le gaz se dilate), il pousse contre une force externe, c'est-à-dire la pression définie comme force par unité de surface. 
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/795/795_Equations-03.png" style="height: 40px;" />
 
 A partir des équations 2 et 3 : 
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/795/795_Equations-04.png" style="height: 40px;" />
 
 Le produit de la surface et de la distance (A × D) est égal à la variation du volume (ΔV) du gaz dans le cylindre. 
 
 Par conséquent, 
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/795/795_Equations-05.png" style="height: 40px;" />
 
 Comme le volume augmente pendant l'expansion, Vfinal > Vinitial, et ΔV est positif. Toutefois, pour une expansion positive (c.-à-d., lorsque le système effectue un travail sur l'environnement extérieur), w doit être négatif et, par conséquent, un signe négatif est ajouté à l'équation.  
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/795/795_Equations-06.png" style="height: 40px;" />
 
 Selon cette équation, le travail pression-volume est la valeur négative de la pression externe (ou de la pression opposée) multiplié par le changement de volume.  
 
 L'unité de travail basée sur cette équation est L·atm. D'autres facteurs de conversion utiles sont :  
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/795/795_Equations-07.png" style="height: 40px;" />
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/795/795_Equations-08.png" style="height: 40px;" />
 
 Ce texte est adapté de <a href="https://openstax.org/books/chemistry-2e/pages/5-3-enthalpy">Openstax, Chimie 2e, Section 5.3 : Enthalpie</a>.

Quantification du travail d'une force

Lorsqu’un système subit un changement, son énergie interne peut changer, et l’énergie peut être transférée du système à l’environnement, ou de l'...

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pressure-volume work in chemistry

Pressure-volume work shows how gas volume changes can transfer energy in chemistry. As a system changes, its internal energy may change too. Energy can move between the system and the surroundings through heat and work.

Internal energy, heat, and work are related by an energy equation. Heat depends on a measured temperature change. Work depends on a measured change in volume called pressure-volume work.

Work can be defined as a force acting through a distance. Pressure-volume work, also called expansion work, happens when a system pushes against a restraining pressure. It can also happen when the surroundings compress the system.

A common example is an internal combustion engine. The combustion of gasoline and oxygen is exothermic, so it gives off energy. Some of that energy is released as heat, and some is used as work that expands the gases in the cylinder and pushes the piston outward. In this situation, the reacting substances are the system, and the engine and the rest of the universe are the surroundings. The system loses energy by heating and by doing work on the surroundings, so its internal energy decreases.

When a gas expands in a cylinder, it pushes against an external force. That force is pressure, which is force per unit area. The area of the piston times the distance it moves equals the change in gas volume, or ΔV. Since the volume increases during expansion, ΔV is positive.

For expansion, the work is written with a negative sign because the system is doing work on the surroundings. Pressure-volume work is the negative of the external pressure multiplied by the change in volume. The unit of this work is L·atm, and it can be converted using other common unit factors.

Pressure-Volume Work in Chemistry

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quantifying work

Pressure-volume work shows how gas volume changes can transfer energy in chemistry. As a system changes, its internal energy may change too. Energy can move between the system and the surroundings through heat and work. Internal energy, heat, and work are related by an energy equation. Heat depends on a measured temperature change. Work depends on a measured change in volume called pressure-volume work. Work can be defined as a force acting through a distance. Pressure-volume work, also...

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Pressure-Volume Work in Chemistry | JoVE K12

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PS.Chm.7.3

Understand the relationship among pressure, temperature, volume, and phase.

Pressure-Volume Work in Chemistry

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