Pressure/Volume and Work

[Goblue89]

I am confused about work and how it relates to pressure/volume for multiple reasons. However, I do understand that work is the area under the curve of a pressure/volume graph. This confusion relates to the difference between the pressure of a gas in a container and external pressure, such as a piston pressing down on the gas.

1. For a process to be isobaric it occurs at constant pressure. Does this mean that the pressure of the gas in the container is constant or the external pressure if constant?

2. When plotting a graph of pressure vs. volume are we talking about the pressure of the gas or the external pressure? I thought it was referring to the pressure of the gas, however the princeton review gives you the formula
work=-(External Pressure)*(ChangeV)
If the pressure vs volume graph is using the pressure of the gas (which is what I've always been told) shouldn't the equation just be work=-PdeltaV?

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[MT Headed]

(1) It refers to the pressure of your system, presumably the stuff in the container.
(2) It refers to the pressure of your system.

These problems are usually set up with a frictionless horizontal piston, or a massless vertical piston, so that the system equilibrates its pressure with the surroundings by changing the system's volume.

Always remember, W=-PdeltaV only holds true in constant pressure scenarios.

 

[whiteshadodw]

(1) It refers to the pressure of your system, presumably the stuff in the container.
(2) It refers to the pressure of your system.

These problems are usually set up with a frictionless horizontal piston, or a massless vertical piston, so that the system equilibrates its pressure with the surroundings by changing the system's volume.

Always remember, W=-PdeltaV only holds true in constant pressure scenarios.

if you draw these changes on a PV diagram you won't have to remember any formulas. if P is on y axis and V is on x axis, then a isobaric process is a horizonal line. the area under the curve is P*dV = W. conversely, isovolumetric processes end up being vertical lines on a PV diagram. you should be able to find the work done for a "loop" where you have isobaric expansion/compression. then isovolumetric heating/cooling, and then isobaric compression or expansion and again isovolumetric heating/cooling. this will make a "loop" on a PV diagram, and it'll be easy to find the work done.

 

[Goblue89]

Thanks you for the respones! I still do not understand why they would specifically put external pressure in the formula. This cannot be referring to the pressure of the gas.

I was confused by the princeton review book so I want back into my pchem slides and I saw this again! Using the external pressure in the forumla. Do you think there is just 2 different ways to calculate the work done? 1. using the pressure of the gas (system) 2. using the external pressure (surroundings)

Isobaric:​

w= -PexdeltaV

Free expansion:​

​

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no work done against a vacuum,P

ext = 0


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