(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

I know how to do the problem so I don't need help actually solving it, I just don't understand the concepts.

A sample of 4.5g of methane occupies 12.7L at 310k.

a) Calculate the work done when the gas expands isothermally against a constant external pressure of 7.7 kPa until its volume has increased by 2.5L

b) Calculate the work that would be done if the same expansion occurred reversibly.

2. Relevant equations

(1) dw=-pdV

(2) w= nRT ln (V final/V initial)

3. The attempt at a solution

For part a you use equation 1. For part b you use equation 2.

I don't get why part a is not reversible. In my textbook equation 1 is listed with the label "REVERSIBLE EXPANSION WORK". So I use it to calculate the work, but then in part b it's like, now do it it it was reversible, and I just got really confused. If equations 1 and 2 are both reversible equations, when do I use them both?

Is part a not reversible because the pressure is staying constant? I just found another equation that is w=-p(external) (delta)V. I see that equation 1 and the one I just wrote are different somehow. But my book says:

"To achieve reversible expansion we set p(ex) equal to p at each stage of the expansion."

So both processes, reversible and irreversible are isothermal, correct? So the different lies in what is happening with the pressure? How do I tell from the wording of part a that it is not a reversible process?

Thanks.

EDIT: Is the difference that in the irreversible process they just have an arbitrary external pressure that stays constant, but when it is reversible, the external pressure is constant but always equal to the internal pressure?

How would you physically set this up? For the second one you could just have a piston that moves to keep the pressure constant. But what about the first? How can you ever have an external pressure that isn't equal to the internal pressure, without it expanding or contracting to match that pressure.... unless that's the crux of it, that the irreversible process expands or contracts to match the constant pressure, which will be the final pressure of the gas.(or maybe I'm just completely off base here)

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# Homework Help: Reversible vs. Irreversible change (gases)

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