Real Gas Confusion

[Dochopeful13]

Hey All I am a little confused on the TBR real gas section. They are saying that when attractive forces are positive that pressure is lower than an ideal gas. That makes sense. However, the very next sentence states that attractive forces makes makes the volume of a real gas lower than an ideal gas. Why would that be?
TBR basically says Intermolecular forces cause the molecules to stick together more, preventing them from spreading out further (ie preventing them from increasing their volume) and applying more pressure to the walls of a container (ie less pressure).

But isn’t volume and pressure opposites? How can real gases a lower pressure and volume then ideal gases. One would think as pressure decreases in real gases their volume would go higher then an ideal gas.


Thanks!

@BerkReviewTeach

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[BerkReviewTeach]

Hey All I am a little confused on the TBR real gas section. They are saying that when attractive forces are positive that pressure is lower than an ideal gas. That makes sense. However, the very next sentence states that attractive forces makes makes the volume of a real gas lower than an ideal gas. Why would that be?
TBR basically says Intermolecular forces cause the molecules to stick together more, preventing them from spreading out further (ie preventing them from increasing their volume) and applying more pressure to the walls of a container (ie less pressure).

But isn’t volume and pressure opposites? How can real gases a lower pressure and volume then ideal gases. One would think as pressure decreases in real gases their volume would go higher then an ideal gas.


Thanks!

@BerkReviewTeach

The concept here is that if you consider the particles in an ideal gas to no interactions, then if they suddenly exhibited attractive interactions, the system would condense (implode slightly), resulting in less collisions with the wall. The observed pressure for the real system exhibiting attractive forces must be lower than the pressure that would be observed with an ideal system. Does that help?

Your reasoning above seems like it is based on PV = nRT, which applies to an ideal gas not a real gas. Deviation from ideal to real cannot be described by the deal gas law.

 

[Dochopeful13]

The concept here is that if you consider the particles in an ideal gas to no interactions, then if they suddenly exhibited attractive interactions, the system would condense (implode slightly), resulting in less collisions with the wall. The observed pressure for the real system exhibiting attractive forces must be lower than the pressure that would be observed with an ideal system. Does that help?

Your reasoning above seems like it is based on PV = nRT, which applies to an ideal gas not a real gas. Deviation from ideal to real cannot be described by the deal gas law.

Thank you. My confusion is with understanding how attractions affect volume of a real gas. According to your book attractions make real gas volume smaller than ideal gas volume. Is that correct?

 

[DrRiker]

So, it really depends on the container of your real gas. If the container is rigid, the volume cannot decrease - only the pressure can decrease with attractive force. In a flexible container, the pressure of your system will attempt to equilibrate with the surrounding environment, and the volume will decrease compared to a system with no attractive forces between gas molecules.

Attractive forces will make the volume smaller as long as the volume can change.

 

[Dochopeful13]

So, it really depends on the container of your real gas. If the container is rigid, the volume cannot decrease - only the pressure can decrease with attractive force. In a flexible container, the pressure of your system will attempt to equilibrate with the surrounding environment, and the volume will decrease compared to a system with no attractive forces between gas molecules.

Attractive forces will make the volume smaller as long as the volume can change.

Thank you for your help. I just wanted to make sure I understand what you are saying. Attractive forces will decrease the pressure of a real gas. In a flexible container the internal pressure of a real gas will try to equilibrate with the atmospheric pressure therefore increasing? As a result the volume will decrease in a flexible container due to the pressure chance? Can you please tell me how and why the gas equilbrates with the atmospheric pressure??

Thank you very much.

 

[DrRiker]

Thank you for your help. I just wanted to make sure I understand what you are saying. Attractive forces will decrease the pressure of a real gas. In a flexible container the internal pressure of a real gas will try to equilibrate with the atmospheric pressure therefore increasing? As a result the volume will decrease in a flexible container due to the pressure chance? Can you please tell me how and why the gas equilbrates with the atmospheric pressure??

Thank you very much.

So consider an ideal gas in maybe a bubble of soap floating through the air. The bubble of soap has a specific diameter corresponding with the volume of gas contained within. The pressure of the ideal gas within the bubble will be the same pressure as the air outside of the bubble. Now, consider that the gas molecules within the bubble magically develop attractive forces. Each gas molecule within the bubble wants to be closer to it's neighbor. That makes the gas kind of want to shrink. If the bubble wasn't flexible, then the pressure within the bubble would decrease. Since the bubble can shrink with the gas, then the bubble will shrink enough to make the pressure within the bubble match the pressure outside the bubble.

Remember that pressure is a force applied over an area. The air outside the bubble pushes on the bubble wall with a certain amount of force. The gas within the bubble pushes against the air with a certain amount of force. When the gas within the bubble starts to have attractive forces, then the gas is attached to itself and exerts less force on the bubble wall. Therefore, the bubble shrinks until the force coming from within the bubble wall matches the force from the air outside the bubble wall

Does this help?