Strange TBR General Chem Q

[SonhosDaVida]

For number 10, can someone explain to me why in a closed system (piston), the volume can change but the pressure does not change if the temperature increases? I thought that temperature increasing will increase kinetic energy, thus increasing the collision frequency and force, thus increasing pressure?

But this is the explanation given.

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[Doogie.Howser]

A piston can move up or down depending on two pressures. The pressure from the outside trying to close the piston is being offset by the pressure inside trying to expand the piston. The piston stays at a 1.0-L volume so the pressure cannot have changed. The pressure changes throughout the process of it moving, but at a 1.0-L volume it hasn't changed.

 

[Flufpot12]

For number 10, can someone explain to me why in a closed system (piston), the volume can change but the pressure does not change if the temperature increases? I thought that temperature increasing will increase kinetic energy, thus increasing the collision frequency and force, thus increasing pressure?

But this is the explanation given.

Wait I don’t think I get it. If the piston stays at a 1.0 L volume then how does the volume change?

Normally I think that the pressure remains constant in Charles’ law because the increase in temperature increases the kinetic energy but since the volume also increases, the pressure essentially remains the same. I don’t understand how the volume changes in a sealed container though. Maybe I’m missing something?

 

[SonhosDaVida]

@Doogie.Howser the explanation states that the volume does change because the pressure isnt changing in the example since internal and external pressure of the piston are equal.

 

[Doogie.Howser]

@Doogie.Howser the explanation states that the volume does change because the pressure isnt changing in the example since internal and external pressure of the piston are equal.

Sorry that I wasn't very clear. When the piston isn't moving, the pressure is constant. Let's make an example:

You have a piston exposed to external air, say 1 atm. The pressure inside the container will also be 1 atm at rest of the piston. In order for the piston to move, the pressure has to change either internally or externally. If the pressure inside the container increases, then the container will expand because the piston will move. But once the volume has expanded enough to make the internal pressure return to 1 atm, it is at equilibrium with the 1 atm external pressure. Therefore, the net change of the pressure will be zero.

 

[SonhosDaVida]

@Doogie.Howser so is it correct to say that because the piston is sealed, thus closed system, that the pressure will not change? But if it is a closed system why does volume show a new change?

 

[SonhosDaVida]

@Doogie.Howser I don't think the pressure equilibrates in this example because it specifically stated that it is a closed container

 

[Doogie.Howser]

@Doogie.Howser so is it correct to say that because the piston is sealed, thus closed system, that the pressure will not change? But if it is a closed system why does volume show a new change?

I think you're making too big of assumptions. Make your understanding of these concepts simpler so that you can apply it to a broader range of topics.

A closed system, now that you mention it, is a system that allows for the transfer of heat and not matter. A closed system doesn't allow you to conclude that the system is not expandable. In fact, a balloon and piston are both closed systems that we know expand with the addition of heat.

Take a balloon to possibly make it simpler, though you could go back and use the piston example if you want. A balloon will stay at a constant volume in say 1 atm of pressure. If you heat the balloon, then the balloon will expand because the pressure increased (Yes, the pressure increases). The expansion of the balloon will decrease the pressure until the internal pressure is back to 1 atm. The pressure changes during expansion/compression, but once the movement is done, the pressure is back to it's original amount (no net change).

 

[SonhosDaVida]

@Doogie.Howser so let me recap the question, a piston and balloon, there is no net change in pressure from beginning to end, but the pressure will try to equilibrate to accomodate the increase in temperature. However, since there is a change in temperature, one value has to change and that is volume since the volume of a piston can be flexible and metal can expand? But the internal pressure of the piston will always try to equilibrate back to equal the external pressure of the piston?thanks msn

 

[PlsLetMeIn21]

I believe the pressure inside the piston must be building up with the rising temperature, which pushes the piston lid out, so the volume expands to more than 1.0 liters. Raising the temperature on a closed system that can expand increases the volume until the pressure inside once again equals the pressure outside. On the molecular level, the particles are moving faster and colliding with the walls more often, causing the pressure to rise and push the walls out. I got rid of A first, because the number of particles does not change. I got rid of B and D, because they both increase with the greater speed (higher temperature.) I got C because it was the only one still standing. Looking back at it now makes sense, because the particles get farther away from one another as the piston expands making mean free path larger.